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ATS/T\ATS05
This website contains an archive of files for the Acorn Electron, BBC Micro, Acorn Archimedes, Commodore 16 and Commodore 64 computers, which Dominic Ford has rescued from his private collection of floppy disks and cassettes.
Some of these files were originally commercial releases in the 1980s and 1990s, but they are now widely available online. I assume that copyright over them is no longer being asserted. If you own the copyright and would like files to be removed, please contact me.
| Tape/disk: | Home » CEEFAX disks » telesoftware2.adl |
| Filename: | ATS/T\ATS05 |
| Read OK: | ✔ |
| File size: | 3216 bytes |
| Load address: | 0000 |
| Exec address: | 0000 |
File contents
Interactive programming J.J.Brayshaw 1987
Page linking is a method of giving a teletext decoder details of several
pages that are connected with, or logically follow on from, the currently
selected page. In this way, pages can be "pre-grabbed" in anticipation of
their being requested for display, thus eliminating waiting time. The
links can also be used by intelligent decoders to follow a logical
sequence made up of several teletext pages. The page link information can
be different for every single teletext page, and is decided upon by the
teletext editors. Getting at this information is not as easy as it may
seem at first glance...
__________________________________________________________________________
Teletext Interactive Programming:
Advanced Teletext System. 5
(c) J.J.Brayshaw 1987
Page Links:
Amongst the additional information transmitted with each teletext page
(i.e. other than the displayable text) are a series of 'page links'.
Some of these links are being used to form the new FLOF or FASTEXT system.
This allows a teletext decoder to grab a series of pages the reader is
likely to require, and store them in memory. When the user calls up one of
these pages it can be displayed instantly from memory rather than having
to wait for the page to be re-broadcast.
Although FLOF is a very new idea for teletext televisions, the system has
been available for many years to BBC Micro teletext adapter owners.
FLOF (Full Level One Facilities)has been re-named FASTEXT by at least one
teletext tv set manufacturer. Sets are now being produced that use the
first four links, and allocate them to four colour-coded buttons. Each
page has its own set of associated page links, so after a page has been
requested and displayed, its page link settings are interpreted and
allocated to the coloured buttons. Whether the user requires them or not,
the linked pages will be received when transmitted, then stored in
anticipation of their being requested for display.
The pages pointed to by these links are decided by the teletext editors
and cannot be altered by the user. When another page is requested (be it
one of the linked pages or not), the page memory is cleared then replaced
by the set of pages pointed to by the new display. The teletext editors
may also add an extra row of text to each page explaining where each of
the four links will take the viewer to. This 'prompt' row is row 24, and
cannot be displayed by non-flof teletext sets. It ususally consists simply
Page 1
Interactive programming J.J.Brayshaw 1987
of keywords in colours directly relating to the coloured buttons (red,
green, yellow and blue). This row, however, is for information only and
does not carry the links themselves.
Although FLOF only requires four links to be transmitted, each page can,
in fact, carry up to 24 links. These are currently used for the following
purposes:
Link 0 - Red key FLOF page link
Link 1 - Green key FLOF page link
Link 2 - Yellow key FLOF page link
Link 3 - Blue key FLOF page link
Link 4 - Points to machine-readable page
Link 5 - Points to the index page
Links 6 to 23 - not defined
The page links themselves are carried on packet 27 of each page. (Teletext
rows are more correctly referred to as 'packets'). This packet does not
follow exactly the same rules as all the other rows of a page. Packet 27
is not for display. Not only is there no space on the screen to display
it, there are no meaningful displayable characters in the row.
Normally, sufficient information can be carried on just one packet 27 for
most purposes. The page CRC is included in this packet, as are the first
six page links. Other information is also incorporated which will be
discussed in future modules.
If more than six page links are to be transmitted per page, then
additional packet 27's must be broadcast to carry them. Each packet 27 can
include six page links, and so for the maximum of 24 page links, a total
of four packet 27's will be required. The specification does not allow for
more than this quantity to be broadcast.
It is vital, therefore, that a decoder can identify each different packet
27 uniquely. As with other packets in the page, packet 27 starts with a
byte giving the magazine number, followed by a byte giving the row number
(in this case 27). There then follows a packet 27 identifying byte. It is
very important that this byte is received correctly, therefore it is
transmitted using a code that allows intelligent receivers to check and
even correct simple reception errors. The system of coding used is called
'Hamming', named after the man who invented the principle (Mr. Hamming).
The ATS offers a simple to use routine to decode any Hamming-coded byte.
Once decoded, the identity byte for packet 27 is in the range 0 to 3.
Links 0 to 5 are given in packet 27/0, links 6 to 11 in packet 27/1, links
12 to 17 in packet 27/2 and links 18 to 23 in packet 27/3. We need to
decode the packet 27 identifier to determine which set of links are in
each particular packet.
Page 2
Interactive programming J.J.Brayshaw 1987
Before examining the page link structure, it should be noted that teletext
page numbers are only two digits long, not three. The initial digit of any
requested page is actually the magazine number and not the page number.
Therefore page 701 is page 1 of magazine 7. It also helps to remember that
page and sub-page numbers are actually calculated and quoted in hex. i.e.
page 131 is really page &31 of magazine &1.
The coding of each link is unusually complex. It consists of six bytes,
each Hamming coded for reliability. These bytes are in groups, starting at
the fourth byte of the packet (byte 3). When bytes 3, 4, 5, 6, 7 and 8 are
de-Hammed, they produce 6 four-bit numbers. The first two give the page
number of the link, least significant digit first, each number producing a
single hex digit. For example, if byte 3 de-Hammed to &6, and byte 4
de-Hammed to &1, the page link is pointing to page 16. We do not, as yet,
know which magazine though!
The next four bytes give the sub-page number, but are not so easy to
interpret as they are also used to calculate the magazine number. We need
to ignore some individual bits of two of these bytes. Byte 5 (the first
one of the sub-page coding digits) gives the last digit of the sub-page
number. No alteration, other than de-Hamming, is needed. If, for example,
this decodes to &6, the sub-page number pointed to is ???6 (the first
three digits not yet being known).
The next digit (de-Hammed byte 6) must have the top bit cleared before
being used as the third digit of the sub-code. (We need to look at this
top bit later to help determine the magazine number.) We can achieve this
by ANDing the de-Hammed byte with 7. For example, if this byte de-Hamms to
&1, then the third digit of the sub-page reference is (&1 AND 7), which is
&1. We now have a sub-page reference of ??16.
The next byte (byte 7) needs no such manipulation, and therefore de-Hamms
to be the second sub-page digit. If this de-Hamms to &4 then our example
sub-page coding becomes ?416.
More complications arise in decoding the next byte. In this case, both the
top two bits are used to form the magazine number, and therefore must be
ignored in the sub-page coding. This can be achieved by ANDing the
de-Hammed byte (byte 8) with 3. If the de-Hammed byte is, for example &E,
then this remaining digit in the sub-page code becomes (&E AND 3) which is
&2. Our example sub-page code is now complete as 2416. The link has so far
decoded to page 16, sub-page 2416. All that now remains is to find the
magazine number, and we will have the complete decoded link setting.
Page 3
Interactive programming J.J.Brayshaw 1987
Fig.1: Decoding of link byte structure:
--------------------------------------------------------------------------
Byte: 3 4 5 6 7 8
--------------------------------------------------------------------------
Meaning: Page no. Page no. Sub-page Sub-page Sub-page Sub-page
digit 2 digit 1 digit 4 digit 3 digit 2 digit 1
--------------------------------------------------------------------------
To As As As <byte> As <byte>
decode: deHammed deHammed deHammed AND 7 deHammed AND 3
--------------------------------------------------------------------------
The magazine number is even more complicated to decode! We need to look at
the bits we discarded earlier (the top bit of byte 6, and the top two bits
of byte 8) and the magazine number from which the current page has been
taken.
First, take the three bits in question and form a single three-bit number.
This is done by using bit 3 of byte 6 as the least significant bit of the
new number, bit 2 of byte 8 as the next bit, then bit 3 of byte 8 as the
most significant bit:
Fig 2: Use of bytes 6 and 8:
----------------------------------------------------------------
Packet 27: BYTE 8 BYTE 6
Bits: Bit3 Bit2 Bit1 Bit0 Bit3 Bit2 Bit1 Bit0
| | |
New number: Bit2 Bit1 Bit0
----------------------------------------------------------------
Next, this 3-bit number should be EOR'd with the magazine number of the
current page.
The whole process can be carried out using a single formula such as:
mag%=(((<byte8>AND12)DIV2)OR(<byte6)DIV8))EOR(<current magazine number>)
To find the current page's magazine number, take the left-hand digit of
the currently selected page, or take the first byte from any packet of the
page.
The page link coding is included in packet 27 as chunks of six bytes,
starting at byte 3. Therefore for the second page link, look at bytes 9 to
14; the third - bytes 15 to 20; etc. As each packet 27 contains six page
links, the seventh link (link number six) can be calculated from packet
27/1, bytes 3 to 8, and so on.
Page 4
Interactive programming J.J.Brayshaw 1987
Fig 3: Packet 27 layout:
--------------------------------------------------/ /----------
Byte: 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 ... 39 40 41
--------------------------------------------------/ /----------
Meaning: Mag Row Packet |...........| |................| Link |...|
(27) I.D. Link 0 Link 1 Ctrl CRC
--------------------------------------------------/ /----------
If, after decoding, the page number of the link is &FF, this signifies the
link is not being used. If any of the four packet 27's are not
transmitted, the relevant page links can be assumed also to be not in use.
It is important to remember that you must use *DATA and not *TRANSFER in
order to transfer packet numbers greater than 23 into memory.
As three bits from the sub-page code bytes are used for other purposes, it
can be seen that the maximum value possible for a sub-page code is &3F7F.
If the link's sub-page is decoded as this, it indicates that any sub-page
on the main page number is acceptable. It should not try to find sub-page
3F7F!
Page links are used extensively for telesoftware purposes. Any
telesoftware decoder should follow link 0 from each sub-page to find the
next block in the series. This is not necessarily on the same main page.
If the file is an ordered file, the sub-page reference of link zero should
also be followed, otherwise just the main page reference is used. In this
way, a telesoftware file can start on one page and continue on a different
page!
__________________________________________________________________________
The program accompanying this text will request the teletext channel, page
number and link setting required. When loaded, the page is interrogated
for the appropriate packet 27. The link setting is decoded and presented
on screen. Pressing <SPACE> will call the link decoding routine 24 times,
once for each possible link. All the links will be decoded then shown on
screen. If the message "not given" appears, this indicates the appropriate
packet 27 was found but the link was undefined (i.e. set to &FF). If the
message "not present" is shown, the appropriate packet 27 was not
received.
All 24 links have been set for this week only on page 700, to enable you
to test the link decoding software.
Page 5
00000000 0d 0d 0d 49 6e 74 65 72 61 63 74 69 76 65 20 70 |...Interactive p| 00000010 72 6f 67 72 61 6d 6d 69 6e 67 20 20 20 20 20 20 |rogramming | 00000020 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 | | 00000030 20 20 20 20 20 20 20 20 20 20 20 20 4a 2e 4a 2e | J.J.| 00000040 42 72 61 79 73 68 61 77 20 31 39 38 37 0d 0d 0d |Brayshaw 1987...| 00000050 0d 0d 50 61 67 65 20 6c 69 6e 6b 69 6e 67 20 69 |..Page linking i| 00000060 73 20 61 20 6d 65 74 68 6f 64 20 6f 66 20 67 69 |s a method of gi| 00000070 76 69 6e 67 20 61 20 74 65 6c 65 74 65 78 74 20 |ving a teletext | 00000080 64 65 63 6f 64 65 72 20 64 65 74 61 69 6c 73 20 |decoder details | 00000090 6f 66 20 73 65 76 65 72 61 6c 0d 70 61 67 65 73 |of several.pages| 000000a0 20 74 68 61 74 20 61 72 65 20 63 6f 6e 6e 65 63 | that are connec| 000000b0 74 65 64 20 77 69 74 68 2c 20 6f 72 20 6c 6f 67 |ted with, or log| 000000c0 69 63 61 6c 6c 79 20 66 6f 6c 6c 6f 77 20 6f 6e |ically follow on| 000000d0 20 66 72 6f 6d 2c 20 74 68 65 20 63 75 72 72 65 | from, the curre| 000000e0 6e 74 6c 79 0d 73 65 6c 65 63 74 65 64 20 70 61 |ntly.selected pa| 000000f0 67 65 2e 20 49 6e 20 74 68 69 73 20 77 61 79 2c |ge. In this way,| 00000100 20 70 61 67 65 73 20 63 61 6e 20 62 65 20 22 70 | pages can be "p| 00000110 72 65 2d 67 72 61 62 62 65 64 22 20 69 6e 20 61 |re-grabbed" in a| 00000120 6e 74 69 63 69 70 61 74 69 6f 6e 20 6f 66 0d 74 |nticipation of.t| 00000130 68 65 69 72 20 62 65 69 6e 67 20 72 65 71 75 65 |heir being reque| 00000140 73 74 65 64 20 66 6f 72 20 64 69 73 70 6c 61 79 |sted for display| 00000150 2c 20 74 68 75 73 20 65 6c 69 6d 69 6e 61 74 69 |, thus eliminati| 00000160 6e 67 20 77 61 69 74 69 6e 67 20 74 69 6d 65 2e |ng waiting time.| 00000170 20 54 68 65 0d 6c 69 6e 6b 73 20 63 61 6e 20 61 | The.links can a| 00000180 6c 73 6f 20 62 65 20 75 73 65 64 20 62 79 20 69 |lso be used by i| 00000190 6e 74 65 6c 6c 69 67 65 6e 74 20 64 65 63 6f 64 |ntelligent decod| 000001a0 65 72 73 20 74 6f 20 66 6f 6c 6c 6f 77 20 61 20 |ers to follow a | 000001b0 6c 6f 67 69 63 61 6c 0d 73 65 71 75 65 6e 63 65 |logical.sequence| 000001c0 20 6d 61 64 65 20 75 70 20 6f 66 20 73 65 76 65 | made up of seve| 000001d0 72 61 6c 20 74 65 6c 65 74 65 78 74 20 70 61 67 |ral teletext pag| 000001e0 65 73 2e 20 54 68 65 20 70 61 67 65 20 6c 69 6e |es. The page lin| 000001f0 6b 20 69 6e 66 6f 72 6d 61 74 69 6f 6e 20 63 61 |k information ca| 00000200 6e 0d 62 65 20 64 69 66 66 65 72 65 6e 74 20 66 |n.be different f| 00000210 6f 72 20 65 76 65 72 79 20 73 69 6e 67 6c 65 20 |or every single | 00000220 74 65 6c 65 74 65 78 74 20 70 61 67 65 2c 20 61 |teletext page, a| 00000230 6e 64 20 69 73 20 64 65 63 69 64 65 64 20 75 70 |nd is decided up| 00000240 6f 6e 20 62 79 20 74 68 65 0d 74 65 6c 65 74 65 |on by the.telete| 00000250 78 74 20 65 64 69 74 6f 72 73 2e 20 47 65 74 74 |xt editors. Gett| 00000260 69 6e 67 20 61 74 20 74 68 69 73 20 69 6e 66 6f |ing at this info| 00000270 72 6d 61 74 69 6f 6e 20 69 73 20 6e 6f 74 20 61 |rmation is not a| 00000280 73 20 65 61 73 79 20 61 73 20 69 74 20 6d 61 79 |s easy as it may| 00000290 0d 73 65 65 6d 20 61 74 20 66 69 72 73 74 20 67 |.seem at first g| 000002a0 6c 61 6e 63 65 2e 2e 2e 0d 5f 5f 5f 5f 5f 5f 5f |lance...._______| 000002b0 5f 5f 5f 5f 5f 5f 5f 5f 5f 5f 5f 5f 5f 5f 5f 5f |________________| * 000002f0 5f 5f 5f 0d 0d 0d 54 65 6c 65 74 65 78 74 20 49 |___...Teletext I| 00000300 6e 74 65 72 61 63 74 69 76 65 20 50 72 6f 67 72 |nteractive Progr| 00000310 61 6d 6d 69 6e 67 3a 0d 0d 41 64 76 61 6e 63 65 |amming:..Advance| 00000320 64 20 54 65 6c 65 74 65 78 74 20 53 79 73 74 65 |d Teletext Syste| 00000330 6d 2e 20 20 35 0d 0d 28 63 29 20 4a 2e 4a 2e 42 |m. 5..(c) J.J.B| 00000340 72 61 79 73 68 61 77 20 31 39 38 37 0d 0d 50 61 |rayshaw 1987..Pa| 00000350 67 65 20 4c 69 6e 6b 73 3a 0d 0d 41 6d 6f 6e 67 |ge Links:..Among| 00000360 73 74 20 74 68 65 20 61 64 64 69 74 69 6f 6e 61 |st the additiona| 00000370 6c 20 69 6e 66 6f 72 6d 61 74 69 6f 6e 20 74 72 |l information tr| 00000380 61 6e 73 6d 69 74 74 65 64 20 77 69 74 68 20 65 |ansmitted with e| 00000390 61 63 68 20 74 65 6c 65 74 65 78 74 20 70 61 67 |ach teletext pag| 000003a0 65 0d 28 69 2e 65 2e 20 6f 74 68 65 72 20 74 68 |e.(i.e. other th| 000003b0 61 6e 20 74 68 65 20 64 69 73 70 6c 61 79 61 62 |an the displayab| 000003c0 6c 65 20 74 65 78 74 29 20 61 72 65 20 61 20 73 |le text) are a s| 000003d0 65 72 69 65 73 20 6f 66 20 27 70 61 67 65 20 6c |eries of 'page l| 000003e0 69 6e 6b 73 27 2e 0d 0d 53 6f 6d 65 20 6f 66 20 |inks'...Some of | 000003f0 74 68 65 73 65 20 6c 69 6e 6b 73 20 61 72 65 20 |these links are | 00000400 62 65 69 6e 67 20 75 73 65 64 20 74 6f 20 66 6f |being used to fo| 00000410 72 6d 20 74 68 65 20 6e 65 77 20 46 4c 4f 46 20 |rm the new FLOF | 00000420 6f 72 20 46 41 53 54 45 58 54 20 73 79 73 74 65 |or FASTEXT syste| 00000430 6d 2e 0d 54 68 69 73 20 61 6c 6c 6f 77 73 20 61 |m..This allows a| 00000440 20 74 65 6c 65 74 65 78 74 20 64 65 63 6f 64 65 | teletext decode| 00000450 72 20 74 6f 20 67 72 61 62 20 61 20 73 65 72 69 |r to grab a seri| 00000460 65 73 20 6f 66 20 70 61 67 65 73 20 74 68 65 20 |es of pages the | 00000470 72 65 61 64 65 72 20 69 73 0d 6c 69 6b 65 6c 79 |reader is.likely| 00000480 20 74 6f 20 72 65 71 75 69 72 65 2c 20 61 6e 64 | to require, and| 00000490 20 73 74 6f 72 65 20 74 68 65 6d 20 69 6e 20 6d | store them in m| 000004a0 65 6d 6f 72 79 2e 20 57 68 65 6e 20 74 68 65 20 |emory. When the | 000004b0 75 73 65 72 20 63 61 6c 6c 73 20 75 70 20 6f 6e |user calls up on| 000004c0 65 20 6f 66 0d 74 68 65 73 65 20 70 61 67 65 73 |e of.these pages| 000004d0 20 69 74 20 63 61 6e 20 62 65 20 64 69 73 70 6c | it can be displ| 000004e0 61 79 65 64 20 69 6e 73 74 61 6e 74 6c 79 20 66 |ayed instantly f| 000004f0 72 6f 6d 20 6d 65 6d 6f 72 79 20 72 61 74 68 65 |rom memory rathe| 00000500 72 20 74 68 61 6e 20 68 61 76 69 6e 67 0d 74 6f |r than having.to| 00000510 20 77 61 69 74 20 66 6f 72 20 74 68 65 20 70 61 | wait for the pa| 00000520 67 65 20 74 6f 20 62 65 20 72 65 2d 62 72 6f 61 |ge to be re-broa| 00000530 64 63 61 73 74 2e 0d 0d 41 6c 74 68 6f 75 67 68 |dcast...Although| 00000540 20 46 4c 4f 46 20 69 73 20 61 20 76 65 72 79 20 | FLOF is a very | 00000550 6e 65 77 20 69 64 65 61 20 66 6f 72 20 74 65 6c |new idea for tel| 00000560 65 74 65 78 74 20 74 65 6c 65 76 69 73 69 6f 6e |etext television| 00000570 73 2c 20 74 68 65 20 73 79 73 74 65 6d 20 68 61 |s, the system ha| 00000580 73 0d 62 65 65 6e 20 61 76 61 69 6c 61 62 6c 65 |s.been available| 00000590 20 66 6f 72 20 6d 61 6e 79 20 79 65 61 72 73 20 | for many years | 000005a0 74 6f 20 42 42 43 20 4d 69 63 72 6f 20 74 65 6c |to BBC Micro tel| 000005b0 65 74 65 78 74 20 61 64 61 70 74 65 72 20 6f 77 |etext adapter ow| 000005c0 6e 65 72 73 2e 0d 0d 46 4c 4f 46 20 28 46 75 6c |ners...FLOF (Ful| 000005d0 6c 20 4c 65 76 65 6c 20 4f 6e 65 20 46 61 63 69 |l Level One Faci| 000005e0 6c 69 74 69 65 73 29 68 61 73 20 62 65 65 6e 20 |lities)has been | 000005f0 72 65 2d 6e 61 6d 65 64 20 46 41 53 54 45 58 54 |re-named FASTEXT| 00000600 20 62 79 20 61 74 20 6c 65 61 73 74 20 6f 6e 65 | by at least one| 00000610 0d 74 65 6c 65 74 65 78 74 20 74 76 20 73 65 74 |.teletext tv set| 00000620 20 6d 61 6e 75 66 61 63 74 75 72 65 72 2e 20 53 | manufacturer. S| 00000630 65 74 73 20 61 72 65 20 6e 6f 77 20 62 65 69 6e |ets are now bein| 00000640 67 20 70 72 6f 64 75 63 65 64 20 74 68 61 74 20 |g produced that | 00000650 75 73 65 20 74 68 65 0d 66 69 72 73 74 20 66 6f |use the.first fo| 00000660 75 72 20 6c 69 6e 6b 73 2c 20 61 6e 64 20 61 6c |ur links, and al| 00000670 6c 6f 63 61 74 65 20 74 68 65 6d 20 74 6f 20 66 |locate them to f| 00000680 6f 75 72 20 63 6f 6c 6f 75 72 2d 63 6f 64 65 64 |our colour-coded| 00000690 20 62 75 74 74 6f 6e 73 2e 20 45 61 63 68 0d 70 | buttons. Each.p| 000006a0 61 67 65 20 68 61 73 20 69 74 73 20 6f 77 6e 20 |age has its own | 000006b0 73 65 74 20 6f 66 20 61 73 73 6f 63 69 61 74 65 |set of associate| 000006c0 64 20 70 61 67 65 20 6c 69 6e 6b 73 2c 20 73 6f |d page links, so| 000006d0 20 61 66 74 65 72 20 61 20 70 61 67 65 20 68 61 | after a page ha| 000006e0 73 20 62 65 65 6e 0d 72 65 71 75 65 73 74 65 64 |s been.requested| 000006f0 20 61 6e 64 20 64 69 73 70 6c 61 79 65 64 2c 20 | and displayed, | 00000700 69 74 73 20 70 61 67 65 20 6c 69 6e 6b 20 73 65 |its page link se| 00000710 74 74 69 6e 67 73 20 61 72 65 20 69 6e 74 65 72 |ttings are inter| 00000720 70 72 65 74 65 64 20 61 6e 64 0d 61 6c 6c 6f 63 |preted and.alloc| 00000730 61 74 65 64 20 74 6f 20 74 68 65 20 63 6f 6c 6f |ated to the colo| 00000740 75 72 65 64 20 62 75 74 74 6f 6e 73 2e 20 57 68 |ured buttons. Wh| 00000750 65 74 68 65 72 20 74 68 65 20 75 73 65 72 20 72 |ether the user r| 00000760 65 71 75 69 72 65 73 20 74 68 65 6d 20 6f 72 20 |equires them or | 00000770 6e 6f 74 2c 0d 74 68 65 20 6c 69 6e 6b 65 64 20 |not,.the linked | 00000780 70 61 67 65 73 20 77 69 6c 6c 20 62 65 20 72 65 |pages will be re| 00000790 63 65 69 76 65 64 20 77 68 65 6e 20 74 72 61 6e |ceived when tran| 000007a0 73 6d 69 74 74 65 64 2c 20 74 68 65 6e 20 73 74 |smitted, then st| 000007b0 6f 72 65 64 20 69 6e 0d 61 6e 74 69 63 69 70 61 |ored in.anticipa| 000007c0 74 69 6f 6e 20 6f 66 20 74 68 65 69 72 20 62 65 |tion of their be| 000007d0 69 6e 67 20 72 65 71 75 65 73 74 65 64 20 66 6f |ing requested fo| 000007e0 72 20 64 69 73 70 6c 61 79 2e 0d 0d 54 68 65 20 |r display...The | 000007f0 70 61 67 65 73 20 70 6f 69 6e 74 65 64 20 74 6f |pages pointed to| 00000800 20 62 79 20 74 68 65 73 65 20 6c 69 6e 6b 73 20 | by these links | 00000810 61 72 65 20 64 65 63 69 64 65 64 20 62 79 20 74 |are decided by t| 00000820 68 65 20 74 65 6c 65 74 65 78 74 20 65 64 69 74 |he teletext edit| 00000830 6f 72 73 0d 61 6e 64 20 63 61 6e 6e 6f 74 20 62 |ors.and cannot b| 00000840 65 20 61 6c 74 65 72 65 64 20 62 79 20 74 68 65 |e altered by the| 00000850 20 75 73 65 72 2e 20 57 68 65 6e 20 61 6e 6f 74 | user. When anot| 00000860 68 65 72 20 70 61 67 65 20 69 73 20 72 65 71 75 |her page is requ| 00000870 65 73 74 65 64 20 28 62 65 20 69 74 0d 6f 6e 65 |ested (be it.one| 00000880 20 6f 66 20 74 68 65 20 6c 69 6e 6b 65 64 20 70 | of the linked p| 00000890 61 67 65 73 20 6f 72 20 6e 6f 74 29 2c 20 74 68 |ages or not), th| 000008a0 65 20 70 61 67 65 20 6d 65 6d 6f 72 79 20 69 73 |e page memory is| 000008b0 20 63 6c 65 61 72 65 64 20 74 68 65 6e 20 72 65 | cleared then re| 000008c0 70 6c 61 63 65 64 0d 62 79 20 74 68 65 20 73 65 |placed.by the se| 000008d0 74 20 6f 66 20 70 61 67 65 73 20 70 6f 69 6e 74 |t of pages point| 000008e0 65 64 20 74 6f 20 62 79 20 74 68 65 20 6e 65 77 |ed to by the new| 000008f0 20 64 69 73 70 6c 61 79 2e 20 54 68 65 20 74 65 | display. The te| 00000900 6c 65 74 65 78 74 20 65 64 69 74 6f 72 73 0d 6d |letext editors.m| 00000910 61 79 20 61 6c 73 6f 20 61 64 64 20 61 6e 20 65 |ay also add an e| 00000920 78 74 72 61 20 72 6f 77 20 6f 66 20 74 65 78 74 |xtra row of text| 00000930 20 74 6f 20 65 61 63 68 20 70 61 67 65 20 65 78 | to each page ex| 00000940 70 6c 61 69 6e 69 6e 67 20 77 68 65 72 65 20 65 |plaining where e| 00000950 61 63 68 20 6f 66 0d 74 68 65 20 66 6f 75 72 20 |ach of.the four | 00000960 6c 69 6e 6b 73 20 77 69 6c 6c 20 74 61 6b 65 20 |links will take | 00000970 74 68 65 20 76 69 65 77 65 72 20 74 6f 2e 20 54 |the viewer to. T| 00000980 68 69 73 20 27 70 72 6f 6d 70 74 27 20 72 6f 77 |his 'prompt' row| 00000990 20 69 73 20 72 6f 77 20 32 34 2c 20 61 6e 64 0d | is row 24, and.| 000009a0 63 61 6e 6e 6f 74 20 62 65 20 64 69 73 70 6c 61 |cannot be displa| 000009b0 79 65 64 20 62 79 20 6e 6f 6e 2d 66 6c 6f 66 20 |yed by non-flof | 000009c0 74 65 6c 65 74 65 78 74 20 73 65 74 73 2e 20 49 |teletext sets. I| 000009d0 74 20 75 73 75 73 61 6c 6c 79 20 63 6f 6e 73 69 |t ususally consi| 000009e0 73 74 73 20 73 69 6d 70 6c 79 0d 0d 0d 0d 0d 20 |sts simply..... | 000009f0 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 | | * 00000a10 20 50 61 67 65 20 31 0d 0d 0d 0d 0d 0d 0d 0d 0d | Page 1.........| 00000a20 49 6e 74 65 72 61 63 74 69 76 65 20 70 72 6f 67 |Interactive prog| 00000a30 72 61 6d 6d 69 6e 67 20 20 20 20 20 20 20 20 20 |ramming | 00000a40 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 | | 00000a50 20 20 20 20 20 20 20 20 20 4a 2e 4a 2e 42 72 61 | J.J.Bra| 00000a60 79 73 68 61 77 20 31 39 38 37 0d 0d 0d 0d 0d 6f |yshaw 1987.....o| 00000a70 66 20 6b 65 79 77 6f 72 64 73 20 69 6e 20 63 6f |f keywords in co| 00000a80 6c 6f 75 72 73 20 64 69 72 65 63 74 6c 79 20 72 |lours directly r| 00000a90 65 6c 61 74 69 6e 67 20 74 6f 20 74 68 65 20 63 |elating to the c| 00000aa0 6f 6c 6f 75 72 65 64 20 62 75 74 74 6f 6e 73 20 |oloured buttons | 00000ab0 28 72 65 64 2c 0d 67 72 65 65 6e 2c 20 79 65 6c |(red,.green, yel| 00000ac0 6c 6f 77 20 61 6e 64 20 62 6c 75 65 29 2e 20 54 |low and blue). T| 00000ad0 68 69 73 20 72 6f 77 2c 20 68 6f 77 65 76 65 72 |his row, however| 00000ae0 2c 20 69 73 20 66 6f 72 20 69 6e 66 6f 72 6d 61 |, is for informa| 00000af0 74 69 6f 6e 20 6f 6e 6c 79 20 61 6e 64 0d 64 6f |tion only and.do| 00000b00 65 73 20 6e 6f 74 20 63 61 72 72 79 20 74 68 65 |es not carry the| 00000b10 20 6c 69 6e 6b 73 20 74 68 65 6d 73 65 6c 76 65 | links themselve| 00000b20 73 2e 0d 0d 41 6c 74 68 6f 75 67 68 20 46 4c 4f |s...Although FLO| 00000b30 46 20 6f 6e 6c 79 20 72 65 71 75 69 72 65 73 20 |F only requires | 00000b40 66 6f 75 72 20 6c 69 6e 6b 73 20 74 6f 20 62 65 |four links to be| 00000b50 20 74 72 61 6e 73 6d 69 74 74 65 64 2c 20 65 61 | transmitted, ea| 00000b60 63 68 20 70 61 67 65 20 63 61 6e 2c 0d 69 6e 20 |ch page can,.in | 00000b70 66 61 63 74 2c 20 63 61 72 72 79 20 75 70 20 74 |fact, carry up t| 00000b80 6f 20 32 34 20 6c 69 6e 6b 73 2e 20 54 68 65 73 |o 24 links. Thes| 00000b90 65 20 61 72 65 20 63 75 72 72 65 6e 74 6c 79 20 |e are currently | 00000ba0 75 73 65 64 20 66 6f 72 20 74 68 65 20 66 6f 6c |used for the fol| 00000bb0 6c 6f 77 69 6e 67 0d 70 75 72 70 6f 73 65 73 3a |lowing.purposes:| 00000bc0 0d 0d 4c 69 6e 6b 20 30 20 2d 20 52 65 64 20 6b |..Link 0 - Red k| 00000bd0 65 79 20 46 4c 4f 46 20 70 61 67 65 20 6c 69 6e |ey FLOF page lin| 00000be0 6b 0d 4c 69 6e 6b 20 31 20 2d 20 47 72 65 65 6e |k.Link 1 - Green| 00000bf0 20 6b 65 79 20 46 4c 4f 46 20 70 61 67 65 20 6c | key FLOF page l| 00000c00 69 6e 6b 0d 4c 69 6e 6b 20 32 20 2d 20 59 65 6c |ink.Link 2 - Yel| 00000c10 6c 6f 77 20 6b 65 79 20 46 4c 4f 46 20 70 61 67 |low key FLOF pag| 00000c20 65 20 6c 69 6e 6b 0d 4c 69 6e 6b 20 33 20 2d 20 |e link.Link 3 - | 00000c30 42 6c 75 65 20 6b 65 79 20 46 4c 4f 46 20 70 61 |Blue key FLOF pa| 00000c40 67 65 20 6c 69 6e 6b 0d 4c 69 6e 6b 20 34 20 2d |ge link.Link 4 -| 00000c50 20 50 6f 69 6e 74 73 20 74 6f 20 6d 61 63 68 69 | Points to machi| 00000c60 6e 65 2d 72 65 61 64 61 62 6c 65 20 70 61 67 65 |ne-readable page| 00000c70 0d 4c 69 6e 6b 20 35 20 2d 20 50 6f 69 6e 74 73 |.Link 5 - Points| 00000c80 20 74 6f 20 74 68 65 20 69 6e 64 65 78 20 70 61 | to the index pa| 00000c90 67 65 0d 4c 69 6e 6b 73 20 36 20 74 6f 20 32 33 |ge.Links 6 to 23| 00000ca0 20 2d 20 6e 6f 74 20 64 65 66 69 6e 65 64 0d 0d | - not defined..| 00000cb0 54 68 65 20 70 61 67 65 20 6c 69 6e 6b 73 20 74 |The page links t| 00000cc0 68 65 6d 73 65 6c 76 65 73 20 61 72 65 20 63 61 |hemselves are ca| 00000cd0 72 72 69 65 64 20 6f 6e 20 70 61 63 6b 65 74 20 |rried on packet | 00000ce0 32 37 20 6f 66 20 65 61 63 68 20 70 61 67 65 2e |27 of each page.| 00000cf0 20 28 54 65 6c 65 74 65 78 74 0d 72 6f 77 73 20 | (Teletext.rows | 00000d00 61 72 65 20 6d 6f 72 65 20 63 6f 72 72 65 63 74 |are more correct| 00000d10 6c 79 20 72 65 66 65 72 72 65 64 20 74 6f 20 61 |ly referred to a| 00000d20 73 20 27 70 61 63 6b 65 74 73 27 29 2e 20 54 68 |s 'packets'). Th| 00000d30 69 73 20 70 61 63 6b 65 74 20 64 6f 65 73 20 6e |is packet does n| 00000d40 6f 74 0d 66 6f 6c 6c 6f 77 20 65 78 61 63 74 6c |ot.follow exactl| 00000d50 79 20 74 68 65 20 73 61 6d 65 20 72 75 6c 65 73 |y the same rules| 00000d60 20 61 73 20 61 6c 6c 20 74 68 65 20 6f 74 68 65 | as all the othe| 00000d70 72 20 72 6f 77 73 20 6f 66 20 61 20 70 61 67 65 |r rows of a page| 00000d80 2e 20 50 61 63 6b 65 74 20 32 37 0d 69 73 20 6e |. Packet 27.is n| 00000d90 6f 74 20 66 6f 72 20 64 69 73 70 6c 61 79 2e 20 |ot for display. | 00000da0 4e 6f 74 20 6f 6e 6c 79 20 69 73 20 74 68 65 72 |Not only is ther| 00000db0 65 20 6e 6f 20 73 70 61 63 65 20 6f 6e 20 74 68 |e no space on th| 00000dc0 65 20 73 63 72 65 65 6e 20 74 6f 20 64 69 73 70 |e screen to disp| 00000dd0 6c 61 79 0d 69 74 2c 20 74 68 65 72 65 20 61 72 |lay.it, there ar| 00000de0 65 20 6e 6f 20 6d 65 61 6e 69 6e 67 66 75 6c 20 |e no meaningful | 00000df0 64 69 73 70 6c 61 79 61 62 6c 65 20 63 68 61 72 |displayable char| 00000e00 61 63 74 65 72 73 20 69 6e 20 74 68 65 20 72 6f |acters in the ro| 00000e10 77 2e 0d 0d 4e 6f 72 6d 61 6c 6c 79 2c 20 73 75 |w...Normally, su| 00000e20 66 66 69 63 69 65 6e 74 20 69 6e 66 6f 72 6d 61 |fficient informa| 00000e30 74 69 6f 6e 20 63 61 6e 20 62 65 20 63 61 72 72 |tion can be carr| 00000e40 69 65 64 20 6f 6e 20 6a 75 73 74 20 6f 6e 65 20 |ied on just one | 00000e50 70 61 63 6b 65 74 20 32 37 20 66 6f 72 0d 6d 6f |packet 27 for.mo| 00000e60 73 74 20 70 75 72 70 6f 73 65 73 2e 20 54 68 65 |st purposes. The| 00000e70 20 70 61 67 65 20 43 52 43 20 69 73 20 69 6e 63 | page CRC is inc| 00000e80 6c 75 64 65 64 20 69 6e 20 74 68 69 73 20 70 61 |luded in this pa| 00000e90 63 6b 65 74 2c 20 61 73 20 61 72 65 20 74 68 65 |cket, as are the| 00000ea0 20 66 69 72 73 74 0d 73 69 78 20 70 61 67 65 20 | first.six page | 00000eb0 6c 69 6e 6b 73 2e 20 4f 74 68 65 72 20 69 6e 66 |links. Other inf| 00000ec0 6f 72 6d 61 74 69 6f 6e 20 69 73 20 61 6c 73 6f |ormation is also| 00000ed0 20 69 6e 63 6f 72 70 6f 72 61 74 65 64 20 77 68 | incorporated wh| 00000ee0 69 63 68 20 77 69 6c 6c 20 62 65 0d 64 69 73 63 |ich will be.disc| 00000ef0 75 73 73 65 64 20 69 6e 20 66 75 74 75 72 65 20 |ussed in future | 00000f00 6d 6f 64 75 6c 65 73 2e 0d 0d 49 66 20 6d 6f 72 |modules...If mor| 00000f10 65 20 74 68 61 6e 20 73 69 78 20 70 61 67 65 20 |e than six page | 00000f20 6c 69 6e 6b 73 20 61 72 65 20 74 6f 20 62 65 20 |links are to be | 00000f30 74 72 61 6e 73 6d 69 74 74 65 64 20 70 65 72 20 |transmitted per | 00000f40 70 61 67 65 2c 20 74 68 65 6e 0d 61 64 64 69 74 |page, then.addit| 00000f50 69 6f 6e 61 6c 20 70 61 63 6b 65 74 20 32 37 27 |ional packet 27'| 00000f60 73 20 6d 75 73 74 20 62 65 20 62 72 6f 61 64 63 |s must be broadc| 00000f70 61 73 74 20 74 6f 20 63 61 72 72 79 20 74 68 65 |ast to carry the| 00000f80 6d 2e 20 45 61 63 68 20 70 61 63 6b 65 74 20 32 |m. Each packet 2| 00000f90 37 20 63 61 6e 0d 69 6e 63 6c 75 64 65 20 73 69 |7 can.include si| 00000fa0 78 20 70 61 67 65 20 6c 69 6e 6b 73 2c 20 61 6e |x page links, an| 00000fb0 64 20 73 6f 20 66 6f 72 20 74 68 65 20 6d 61 78 |d so for the max| 00000fc0 69 6d 75 6d 20 6f 66 20 32 34 20 70 61 67 65 20 |imum of 24 page | 00000fd0 6c 69 6e 6b 73 2c 20 61 20 74 6f 74 61 6c 0d 6f |links, a total.o| 00000fe0 66 20 66 6f 75 72 20 70 61 63 6b 65 74 20 32 37 |f four packet 27| 00000ff0 27 73 20 77 69 6c 6c 20 62 65 20 72 65 71 75 69 |'s will be requi| 00001000 72 65 64 2e 20 54 68 65 20 73 70 65 63 69 66 69 |red. The specifi| 00001010 63 61 74 69 6f 6e 20 64 6f 65 73 20 6e 6f 74 20 |cation does not | 00001020 61 6c 6c 6f 77 20 66 6f 72 0d 6d 6f 72 65 20 74 |allow for.more t| 00001030 68 61 6e 20 74 68 69 73 20 71 75 61 6e 74 69 74 |han this quantit| 00001040 79 20 74 6f 20 62 65 20 62 72 6f 61 64 63 61 73 |y to be broadcas| 00001050 74 2e 0d 0d 49 74 20 69 73 20 76 69 74 61 6c 2c |t...It is vital,| 00001060 20 74 68 65 72 65 66 6f 72 65 2c 20 74 68 61 74 | therefore, that| 00001070 20 61 20 64 65 63 6f 64 65 72 20 63 61 6e 20 69 | a decoder can i| 00001080 64 65 6e 74 69 66 79 20 65 61 63 68 20 64 69 66 |dentify each dif| 00001090 66 65 72 65 6e 74 20 70 61 63 6b 65 74 0d 32 37 |ferent packet.27| 000010a0 20 75 6e 69 71 75 65 6c 79 2e 20 41 73 20 77 69 | uniquely. As wi| 000010b0 74 68 20 6f 74 68 65 72 20 70 61 63 6b 65 74 73 |th other packets| 000010c0 20 69 6e 20 74 68 65 20 70 61 67 65 2c 20 70 61 | in the page, pa| 000010d0 63 6b 65 74 20 32 37 20 73 74 61 72 74 73 20 77 |cket 27 starts w| 000010e0 69 74 68 20 61 0d 62 79 74 65 20 67 69 76 69 6e |ith a.byte givin| 000010f0 67 20 74 68 65 20 6d 61 67 61 7a 69 6e 65 20 6e |g the magazine n| 00001100 75 6d 62 65 72 2c 20 66 6f 6c 6c 6f 77 65 64 20 |umber, followed | 00001110 62 79 20 61 20 62 79 74 65 20 67 69 76 69 6e 67 |by a byte giving| 00001120 20 74 68 65 20 72 6f 77 20 6e 75 6d 62 65 72 0d | the row number.| 00001130 28 69 6e 20 74 68 69 73 20 63 61 73 65 20 32 37 |(in this case 27| 00001140 29 2e 20 54 68 65 72 65 20 74 68 65 6e 20 66 6f |). There then fo| 00001150 6c 6c 6f 77 73 20 61 20 70 61 63 6b 65 74 20 32 |llows a packet 2| 00001160 37 20 69 64 65 6e 74 69 66 79 69 6e 67 20 62 79 |7 identifying by| 00001170 74 65 2e 20 49 74 20 69 73 0d 76 65 72 79 20 69 |te. It is.very i| 00001180 6d 70 6f 72 74 61 6e 74 20 74 68 61 74 20 74 68 |mportant that th| 00001190 69 73 20 62 79 74 65 20 69 73 20 72 65 63 65 69 |is byte is recei| 000011a0 76 65 64 20 63 6f 72 72 65 63 74 6c 79 2c 20 74 |ved correctly, t| 000011b0 68 65 72 65 66 6f 72 65 20 69 74 20 69 73 0d 74 |herefore it is.t| 000011c0 72 61 6e 73 6d 69 74 74 65 64 20 75 73 69 6e 67 |ransmitted using| 000011d0 20 61 20 63 6f 64 65 20 74 68 61 74 20 61 6c 6c | a code that all| 000011e0 6f 77 73 20 69 6e 74 65 6c 6c 69 67 65 6e 74 20 |ows intelligent | 000011f0 72 65 63 65 69 76 65 72 73 20 74 6f 20 63 68 65 |receivers to che| 00001200 63 6b 20 61 6e 64 0d 65 76 65 6e 20 63 6f 72 72 |ck and.even corr| 00001210 65 63 74 20 73 69 6d 70 6c 65 20 72 65 63 65 70 |ect simple recep| 00001220 74 69 6f 6e 20 65 72 72 6f 72 73 2e 20 54 68 65 |tion errors. The| 00001230 20 73 79 73 74 65 6d 20 6f 66 20 63 6f 64 69 6e | system of codin| 00001240 67 20 75 73 65 64 20 69 73 20 63 61 6c 6c 65 64 |g used is called| 00001250 0d 27 48 61 6d 6d 69 6e 67 27 2c 20 6e 61 6d 65 |.'Hamming', name| 00001260 64 20 61 66 74 65 72 20 74 68 65 20 6d 61 6e 20 |d after the man | 00001270 77 68 6f 20 69 6e 76 65 6e 74 65 64 20 74 68 65 |who invented the| 00001280 20 70 72 69 6e 63 69 70 6c 65 20 28 4d 72 2e 20 | principle (Mr. | 00001290 48 61 6d 6d 69 6e 67 29 2e 0d 0d 54 68 65 20 41 |Hamming)...The A| 000012a0 54 53 20 6f 66 66 65 72 73 20 61 20 73 69 6d 70 |TS offers a simp| 000012b0 6c 65 20 74 6f 20 75 73 65 20 72 6f 75 74 69 6e |le to use routin| 000012c0 65 20 74 6f 20 64 65 63 6f 64 65 20 61 6e 79 20 |e to decode any | 000012d0 48 61 6d 6d 69 6e 67 2d 63 6f 64 65 64 20 62 79 |Hamming-coded by| 000012e0 74 65 2e 0d 4f 6e 63 65 20 64 65 63 6f 64 65 64 |te..Once decoded| 000012f0 2c 20 74 68 65 20 69 64 65 6e 74 69 74 79 20 62 |, the identity b| 00001300 79 74 65 20 66 6f 72 20 70 61 63 6b 65 74 20 32 |yte for packet 2| 00001310 37 20 69 73 20 69 6e 20 74 68 65 20 72 61 6e 67 |7 is in the rang| 00001320 65 20 30 20 74 6f 20 33 2e 0d 4c 69 6e 6b 73 20 |e 0 to 3..Links | 00001330 30 20 74 6f 20 35 20 61 72 65 20 67 69 76 65 6e |0 to 5 are given| 00001340 20 69 6e 20 70 61 63 6b 65 74 20 32 37 2f 30 2c | in packet 27/0,| 00001350 20 6c 69 6e 6b 73 20 36 20 74 6f 20 31 31 20 69 | links 6 to 11 i| 00001360 6e 20 70 61 63 6b 65 74 20 32 37 2f 31 2c 20 6c |n packet 27/1, l| 00001370 69 6e 6b 73 0d 31 32 20 74 6f 20 31 37 20 69 6e |inks.12 to 17 in| 00001380 20 70 61 63 6b 65 74 20 32 37 2f 32 20 61 6e 64 | packet 27/2 and| 00001390 20 6c 69 6e 6b 73 20 31 38 20 74 6f 20 32 33 20 | links 18 to 23 | 000013a0 69 6e 20 70 61 63 6b 65 74 20 32 37 2f 33 2e 20 |in packet 27/3. | 000013b0 57 65 20 6e 65 65 64 20 74 6f 0d 64 65 63 6f 64 |We need to.decod| 000013c0 65 20 74 68 65 20 70 61 63 6b 65 74 20 32 37 20 |e the packet 27 | 000013d0 69 64 65 6e 74 69 66 69 65 72 20 74 6f 20 64 65 |identifier to de| 000013e0 74 65 72 6d 69 6e 65 20 77 68 69 63 68 20 73 65 |termine which se| 000013f0 74 20 6f 66 20 6c 69 6e 6b 73 20 61 72 65 20 69 |t of links are i| 00001400 6e 0d 65 61 63 68 20 70 61 72 74 69 63 75 6c 61 |n.each particula| 00001410 72 20 70 61 63 6b 65 74 2e 0d 0d 0d 0d 0d 20 20 |r packet...... | 00001420 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 | | * 00001440 50 61 67 65 20 32 0d 0d 0d 0d 0d 0d 0d 0d 0d 49 |Page 2.........I| 00001450 6e 74 65 72 61 63 74 69 76 65 20 70 72 6f 67 72 |nteractive progr| 00001460 61 6d 6d 69 6e 67 20 20 20 20 20 20 20 20 20 20 |amming | 00001470 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 | | 00001480 20 20 20 20 20 20 20 20 4a 2e 4a 2e 42 72 61 79 | J.J.Bray| 00001490 73 68 61 77 20 31 39 38 37 0d 0d 0d 0d 0d 0d 42 |shaw 1987......B| 000014a0 65 66 6f 72 65 20 65 78 61 6d 69 6e 69 6e 67 20 |efore examining | 000014b0 74 68 65 20 70 61 67 65 20 6c 69 6e 6b 20 73 74 |the page link st| 000014c0 72 75 63 74 75 72 65 2c 20 69 74 20 73 68 6f 75 |ructure, it shou| 000014d0 6c 64 20 62 65 20 6e 6f 74 65 64 20 74 68 61 74 |ld be noted that| 000014e0 20 74 65 6c 65 74 65 78 74 0d 70 61 67 65 20 6e | teletext.page n| 000014f0 75 6d 62 65 72 73 20 61 72 65 20 6f 6e 6c 79 20 |umbers are only | 00001500 74 77 6f 20 64 69 67 69 74 73 20 6c 6f 6e 67 2c |two digits long,| 00001510 20 6e 6f 74 20 74 68 72 65 65 2e 20 54 68 65 20 | not three. The | 00001520 69 6e 69 74 69 61 6c 20 64 69 67 69 74 20 6f 66 |initial digit of| 00001530 20 61 6e 79 0d 72 65 71 75 65 73 74 65 64 20 70 | any.requested p| 00001540 61 67 65 20 69 73 20 61 63 74 75 61 6c 6c 79 20 |age is actually | 00001550 74 68 65 20 6d 61 67 61 7a 69 6e 65 20 6e 75 6d |the magazine num| 00001560 62 65 72 20 61 6e 64 20 6e 6f 74 20 74 68 65 20 |ber and not the | 00001570 70 61 67 65 20 6e 75 6d 62 65 72 2e 0d 54 68 65 |page number..The| 00001580 72 65 66 6f 72 65 20 70 61 67 65 20 37 30 31 20 |refore page 701 | 00001590 69 73 20 70 61 67 65 20 31 20 6f 66 20 6d 61 67 |is page 1 of mag| 000015a0 61 7a 69 6e 65 20 37 2e 20 49 74 20 61 6c 73 6f |azine 7. It also| 000015b0 20 68 65 6c 70 73 20 74 6f 20 72 65 6d 65 6d 62 | helps to rememb| 000015c0 65 72 20 74 68 61 74 0d 70 61 67 65 20 61 6e 64 |er that.page and| 000015d0 20 73 75 62 2d 70 61 67 65 20 6e 75 6d 62 65 72 | sub-page number| 000015e0 73 20 61 72 65 20 61 63 74 75 61 6c 6c 79 20 63 |s are actually c| 000015f0 61 6c 63 75 6c 61 74 65 64 20 61 6e 64 20 71 75 |alculated and qu| 00001600 6f 74 65 64 20 69 6e 20 68 65 78 2e 20 69 2e 65 |oted in hex. i.e| 00001610 2e 0d 70 61 67 65 20 31 33 31 20 69 73 20 72 65 |..page 131 is re| 00001620 61 6c 6c 79 20 70 61 67 65 20 26 33 31 20 6f 66 |ally page &31 of| 00001630 20 6d 61 67 61 7a 69 6e 65 20 26 31 2e 0d 0d 54 | magazine &1...T| 00001640 68 65 20 63 6f 64 69 6e 67 20 6f 66 20 65 61 63 |he coding of eac| 00001650 68 20 6c 69 6e 6b 20 69 73 20 75 6e 75 73 75 61 |h link is unusua| 00001660 6c 6c 79 20 63 6f 6d 70 6c 65 78 2e 20 49 74 20 |lly complex. It | 00001670 63 6f 6e 73 69 73 74 73 20 6f 66 20 73 69 78 20 |consists of six | 00001680 62 79 74 65 73 2c 0d 65 61 63 68 20 48 61 6d 6d |bytes,.each Hamm| 00001690 69 6e 67 20 63 6f 64 65 64 20 66 6f 72 20 72 65 |ing coded for re| 000016a0 6c 69 61 62 69 6c 69 74 79 2e 20 54 68 65 73 65 |liability. These| 000016b0 20 62 79 74 65 73 20 61 72 65 20 69 6e 20 67 72 | bytes are in gr| 000016c0 6f 75 70 73 2c 20 73 74 61 72 74 69 6e 67 20 61 |oups, starting a| 000016d0 74 0d 74 68 65 20 66 6f 75 72 74 68 20 62 79 74 |t.the fourth byt| 000016e0 65 20 6f 66 20 74 68 65 20 70 61 63 6b 65 74 20 |e of the packet | 000016f0 28 62 79 74 65 20 33 29 2e 20 57 68 65 6e 20 62 |(byte 3). When b| 00001700 79 74 65 73 20 33 2c 20 34 2c 20 35 2c 20 36 2c |ytes 3, 4, 5, 6,| 00001710 20 37 20 61 6e 64 20 38 20 61 72 65 0d 64 65 2d | 7 and 8 are.de-| 00001720 48 61 6d 6d 65 64 2c 20 74 68 65 79 20 70 72 6f |Hammed, they pro| 00001730 64 75 63 65 20 36 20 66 6f 75 72 2d 62 69 74 20 |duce 6 four-bit | 00001740 6e 75 6d 62 65 72 73 2e 20 54 68 65 20 66 69 72 |numbers. The fir| 00001750 73 74 20 74 77 6f 20 67 69 76 65 20 74 68 65 20 |st two give the | 00001760 70 61 67 65 0d 6e 75 6d 62 65 72 20 6f 66 20 74 |page.number of t| 00001770 68 65 20 6c 69 6e 6b 2c 20 6c 65 61 73 74 20 73 |he link, least s| 00001780 69 67 6e 69 66 69 63 61 6e 74 20 64 69 67 69 74 |ignificant digit| 00001790 20 66 69 72 73 74 2c 20 65 61 63 68 20 6e 75 6d | first, each num| 000017a0 62 65 72 20 70 72 6f 64 75 63 69 6e 67 20 61 0d |ber producing a.| 000017b0 73 69 6e 67 6c 65 20 68 65 78 20 64 69 67 69 74 |single hex digit| 000017c0 2e 20 46 6f 72 20 65 78 61 6d 70 6c 65 2c 20 69 |. For example, i| 000017d0 66 20 62 79 74 65 20 33 20 64 65 2d 48 61 6d 6d |f byte 3 de-Hamm| 000017e0 65 64 20 74 6f 20 26 36 2c 20 61 6e 64 20 62 79 |ed to &6, and by| 000017f0 74 65 20 34 0d 64 65 2d 48 61 6d 6d 65 64 20 74 |te 4.de-Hammed t| 00001800 6f 20 26 31 2c 20 74 68 65 20 70 61 67 65 20 6c |o &1, the page l| 00001810 69 6e 6b 20 69 73 20 70 6f 69 6e 74 69 6e 67 20 |ink is pointing | 00001820 74 6f 20 70 61 67 65 20 31 36 2e 20 57 65 20 64 |to page 16. We d| 00001830 6f 20 6e 6f 74 2c 20 61 73 20 79 65 74 2c 0d 6b |o not, as yet,.k| 00001840 6e 6f 77 20 77 68 69 63 68 20 6d 61 67 61 7a 69 |now which magazi| 00001850 6e 65 20 74 68 6f 75 67 68 21 0d 0d 54 68 65 20 |ne though!..The | 00001860 6e 65 78 74 20 66 6f 75 72 20 62 79 74 65 73 20 |next four bytes | 00001870 67 69 76 65 20 74 68 65 20 73 75 62 2d 70 61 67 |give the sub-pag| 00001880 65 20 6e 75 6d 62 65 72 2c 20 62 75 74 20 61 72 |e number, but ar| 00001890 65 20 6e 6f 74 20 73 6f 20 65 61 73 79 20 74 6f |e not so easy to| 000018a0 0d 69 6e 74 65 72 70 72 65 74 20 61 73 20 74 68 |.interpret as th| 000018b0 65 79 20 61 72 65 20 61 6c 73 6f 20 75 73 65 64 |ey are also used| 000018c0 20 74 6f 20 63 61 6c 63 75 6c 61 74 65 20 74 68 | to calculate th| 000018d0 65 20 6d 61 67 61 7a 69 6e 65 20 6e 75 6d 62 65 |e magazine numbe| 000018e0 72 2e 20 57 65 20 6e 65 65 64 0d 74 6f 20 69 67 |r. 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If, for exam| 000019c0 70 6c 65 2c 0d 74 68 69 73 20 64 65 63 6f 64 65 |ple,.this decode| 000019d0 73 20 74 6f 20 26 36 2c 20 74 68 65 20 73 75 62 |s to &6, the sub| 000019e0 2d 70 61 67 65 20 6e 75 6d 62 65 72 20 70 6f 69 |-page number poi| 000019f0 6e 74 65 64 20 74 6f 20 69 73 20 3f 3f 3f 36 20 |nted to is ???6 | 00001a00 28 74 68 65 20 66 69 72 73 74 0d 74 68 72 65 65 |(the first.three| 00001a10 20 64 69 67 69 74 73 20 6e 6f 74 20 79 65 74 20 | digits not yet | 00001a20 62 65 69 6e 67 20 6b 6e 6f 77 6e 29 2e 0d 0d 54 |being known)...T| 00001a30 68 65 20 6e 65 78 74 20 64 69 67 69 74 20 28 64 |he next digit (d| 00001a40 65 2d 48 61 6d 6d 65 64 20 62 79 74 65 20 36 29 |e-Hammed byte 6)| 00001a50 20 6d 75 73 74 20 68 61 76 65 20 74 68 65 20 74 | must have the t| 00001a60 6f 70 20 62 69 74 20 63 6c 65 61 72 65 64 20 62 |op bit cleared b| 00001a70 65 66 6f 72 65 0d 62 65 69 6e 67 20 75 73 65 64 |efore.being used| 00001a80 20 61 73 20 74 68 65 20 74 68 69 72 64 20 64 69 | as the third di| 00001a90 67 69 74 20 6f 66 20 74 68 65 20 73 75 62 2d 63 |git of the sub-c| 00001aa0 6f 64 65 2e 20 28 57 65 20 6e 65 65 64 20 74 6f |ode. (We need to| 00001ab0 20 6c 6f 6f 6b 20 61 74 20 74 68 69 73 0d 74 6f | look at this.to| 00001ac0 70 20 62 69 74 20 6c 61 74 65 72 20 74 6f 20 68 |p bit later to h| 00001ad0 65 6c 70 20 64 65 74 65 72 6d 69 6e 65 20 74 68 |elp determine th| 00001ae0 65 20 6d 61 67 61 7a 69 6e 65 20 6e 75 6d 62 65 |e magazine numbe| 00001af0 72 2e 29 20 57 65 20 63 61 6e 20 61 63 68 69 65 |r.) We can achie| 00001b00 76 65 20 74 68 69 73 0d 62 79 20 41 4e 44 69 6e |ve this.by ANDin| 00001b10 67 20 74 68 65 20 64 65 2d 48 61 6d 6d 65 64 20 |g the de-Hammed | 00001b20 62 79 74 65 20 77 69 74 68 20 37 2e 20 46 6f 72 |byte with 7. For| 00001b30 20 65 78 61 6d 70 6c 65 2c 20 69 66 20 74 68 69 | example, if thi| 00001b40 73 20 62 79 74 65 20 64 65 2d 48 61 6d 6d 73 20 |s byte de-Hamms | 00001b50 74 6f 0d 26 31 2c 20 74 68 65 6e 20 74 68 65 20 |to.&1, then the | 00001b60 74 68 69 72 64 20 64 69 67 69 74 20 6f 66 20 74 |third digit of t| 00001b70 68 65 20 73 75 62 2d 70 61 67 65 20 72 65 66 65 |he sub-page refe| 00001b80 72 65 6e 63 65 20 69 73 20 28 26 31 20 41 4e 44 |rence is (&1 AND| 00001b90 20 37 29 2c 20 77 68 69 63 68 20 69 73 0d 26 31 | 7), which is.&1| 00001ba0 2e 20 57 65 20 6e 6f 77 20 68 61 76 65 20 61 20 |. We now have a | 00001bb0 73 75 62 2d 70 61 67 65 20 72 65 66 65 72 65 6e |sub-page referen| 00001bc0 63 65 20 6f 66 20 3f 3f 31 36 2e 0d 0d 54 68 65 |ce of ??16...The| 00001bd0 20 6e 65 78 74 20 62 79 74 65 20 28 62 79 74 65 | next byte (byte| 00001be0 20 37 29 20 6e 65 65 64 73 20 6e 6f 20 73 75 63 | 7) needs no suc| 00001bf0 68 20 6d 61 6e 69 70 75 6c 61 74 69 6f 6e 2c 20 |h manipulation, | 00001c00 61 6e 64 20 74 68 65 72 65 66 6f 72 65 20 64 65 |and therefore de| 00001c10 2d 48 61 6d 6d 73 0d 74 6f 20 62 65 20 74 68 65 |-Hamms.to be the| 00001c20 20 73 65 63 6f 6e 64 20 73 75 62 2d 70 61 67 65 | second sub-page| 00001c30 20 64 69 67 69 74 2e 20 49 66 20 74 68 69 73 20 | digit. If this | 00001c40 64 65 2d 48 61 6d 6d 73 20 74 6f 20 26 34 20 74 |de-Hamms to &4 t| 00001c50 68 65 6e 20 6f 75 72 20 65 78 61 6d 70 6c 65 0d |hen our example.| 00001c60 73 75 62 2d 70 61 67 65 20 63 6f 64 69 6e 67 20 |sub-page coding | 00001c70 62 65 63 6f 6d 65 73 20 3f 34 31 36 2e 0d 0d 4d |becomes ?416...M| 00001c80 6f 72 65 20 63 6f 6d 70 6c 69 63 61 74 69 6f 6e |ore complication| 00001c90 73 20 61 72 69 73 65 20 69 6e 20 64 65 63 6f 64 |s arise in decod| 00001ca0 69 6e 67 20 74 68 65 20 6e 65 78 74 20 62 79 74 |ing the next byt| 00001cb0 65 2e 20 49 6e 20 74 68 69 73 20 63 61 73 65 2c |e. In this case,| 00001cc0 20 62 6f 74 68 20 74 68 65 0d 74 6f 70 20 74 77 | both the.top tw| 00001cd0 6f 20 62 69 74 73 20 61 72 65 20 75 73 65 64 20 |o bits are used | 00001ce0 74 6f 20 66 6f 72 6d 20 74 68 65 20 6d 61 67 61 |to form the maga| 00001cf0 7a 69 6e 65 20 6e 75 6d 62 65 72 2c 20 61 6e 64 |zine number, and| 00001d00 20 74 68 65 72 65 66 6f 72 65 20 6d 75 73 74 20 | therefore must | 00001d10 62 65 0d 69 67 6e 6f 72 65 64 20 69 6e 20 74 68 |be.ignored in th| 00001d20 65 20 73 75 62 2d 70 61 67 65 20 63 6f 64 69 6e |e sub-page codin| 00001d30 67 2e 20 54 68 69 73 20 63 61 6e 20 62 65 20 61 |g. This can be a| 00001d40 63 68 69 65 76 65 64 20 62 79 20 41 4e 44 69 6e |chieved by ANDin| 00001d50 67 20 74 68 65 0d 64 65 2d 48 61 6d 6d 65 64 20 |g the.de-Hammed | 00001d60 62 79 74 65 20 28 62 79 74 65 20 38 29 20 77 69 |byte (byte 8) wi| 00001d70 74 68 20 33 2e 20 49 66 20 74 68 65 20 64 65 2d |th 3. If the de-| 00001d80 48 61 6d 6d 65 64 20 62 79 74 65 20 69 73 2c 20 |Hammed byte is, | 00001d90 66 6f 72 20 65 78 61 6d 70 6c 65 20 26 45 2c 0d |for example &E,.| 00001da0 74 68 65 6e 20 74 68 69 73 20 72 65 6d 61 69 6e |then this remain| 00001db0 69 6e 67 20 64 69 67 69 74 20 69 6e 20 74 68 65 |ing digit in the| 00001dc0 20 73 75 62 2d 70 61 67 65 20 63 6f 64 65 20 62 | sub-page code b| 00001dd0 65 63 6f 6d 65 73 20 28 26 45 20 41 4e 44 20 33 |ecomes (&E AND 3| 00001de0 29 20 77 68 69 63 68 20 69 73 0d 26 32 2e 20 4f |) which is.&2. O| 00001df0 75 72 20 65 78 61 6d 70 6c 65 20 73 75 62 2d 70 |ur example sub-p| 00001e00 61 67 65 20 63 6f 64 65 20 69 73 20 6e 6f 77 20 |age code is now | 00001e10 63 6f 6d 70 6c 65 74 65 20 61 73 20 32 34 31 36 |complete as 2416| 00001e20 2e 20 54 68 65 20 6c 69 6e 6b 20 68 61 73 20 73 |. The link has s| 00001e30 6f 20 66 61 72 0d 64 65 63 6f 64 65 64 20 74 6f |o far.decoded to| 00001e40 20 70 61 67 65 20 31 36 2c 20 73 75 62 2d 70 61 | page 16, sub-pa| 00001e50 67 65 20 32 34 31 36 2e 20 41 6c 6c 20 74 68 61 |ge 2416. All tha| 00001e60 74 20 6e 6f 77 20 72 65 6d 61 69 6e 73 20 69 73 |t now remains is| 00001e70 20 74 6f 20 66 69 6e 64 20 74 68 65 0d 6d 61 67 | to find the.mag| 00001e80 61 7a 69 6e 65 20 6e 75 6d 62 65 72 2c 20 61 6e |azine number, an| 00001e90 64 20 77 65 20 77 69 6c 6c 20 68 61 76 65 20 74 |d we will have t| 00001ea0 68 65 20 63 6f 6d 70 6c 65 74 65 20 64 65 63 6f |he complete deco| 00001eb0 64 65 64 20 6c 69 6e 6b 20 73 65 74 74 69 6e 67 |ded link setting| 00001ec0 2e 0d 0d 0d 0d 0d 0d 0d 0d 0d 20 20 20 20 20 20 |.......... | 00001ed0 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 | | 00001ee0 20 20 20 20 20 20 20 20 20 20 20 20 50 61 67 65 | Page| 00001ef0 20 33 0d 0d 0d 0d 0d 0d 0d 0d 0d 49 6e 74 65 72 | 3.........Inter| 00001f00 61 63 74 69 76 65 20 70 72 6f 67 72 61 6d 6d 69 |active programmi| 00001f10 6e 67 20 20 20 20 20 20 20 20 20 20 20 20 20 20 |ng | 00001f20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 | | 00001f30 20 20 20 20 4a 2e 4a 2e 42 72 61 79 73 68 61 77 | J.J.Brayshaw| 00001f40 20 31 39 38 37 0d 0d 0d 0d 0d 46 69 67 2e 31 3a | 1987.....Fig.1:| 00001f50 20 44 65 63 6f 64 69 6e 67 20 6f 66 20 6c 69 6e | Decoding of lin| 00001f60 6b 20 62 79 74 65 20 73 74 72 75 63 74 75 72 65 |k byte structure| 00001f70 3a 0d 0d 2d 2d 2d 2d 2d 2d 2d 2d 2d 2d 2d 2d 2d |:..-------------| 00001f80 2d 2d 2d 2d 2d 2d 2d 2d 2d 2d 2d 2d 2d 2d 2d 2d |----------------| * 00001fb0 2d 2d 2d 2d 2d 2d 2d 2d 2d 2d 2d 2d 2d 0d 42 79 |-------------.By| 00001fc0 74 65 3a 20 20 20 20 20 20 33 20 20 20 20 20 20 |te: 3 | 00001fd0 20 20 20 20 34 20 20 20 20 20 20 20 20 20 20 35 | 4 5| 00001fe0 20 20 20 20 20 20 20 20 20 20 36 20 20 20 20 20 | 6 | 00001ff0 20 20 20 20 20 20 37 20 20 20 20 20 20 20 20 20 | 7 | 00002000 20 38 0d 2d 2d 2d 2d 2d 2d 2d 2d 2d 2d 2d 2d 2d | 8.-------------| 00002010 2d 2d 2d 2d 2d 2d 2d 2d 2d 2d 2d 2d 2d 2d 2d 2d |----------------| * 00002040 2d 2d 2d 2d 2d 2d 2d 2d 2d 2d 2d 2d 2d 0d 4d 65 |-------------.Me| 00002050 61 6e 69 6e 67 3a 20 50 61 67 65 20 6e 6f 2e 20 |aning: Page no. | 00002060 20 20 50 61 67 65 20 6e 6f 2e 20 20 20 20 53 75 | Page no. Su| 00002070 62 2d 70 61 67 65 20 20 20 53 75 62 2d 70 61 67 |b-page Sub-pag| 00002080 65 20 20 20 53 75 62 2d 70 61 67 65 20 20 20 53 |e Sub-page S| 00002090 75 62 2d 70 61 67 65 0d 20 20 20 20 20 20 20 20 |ub-page. | 000020a0 20 64 69 67 69 74 20 32 20 20 20 20 64 69 67 69 | digit 2 digi| 000020b0 74 20 31 20 20 20 20 20 64 69 67 69 74 20 34 20 |t 1 digit 4 | 000020c0 20 20 20 64 69 67 69 74 20 33 20 20 20 20 64 69 | digit 3 di| 000020d0 67 69 74 20 32 20 20 20 20 64 69 67 69 74 20 31 |git 2 digit 1| 000020e0 0d 2d 2d 2d 2d 2d 2d 2d 2d 2d 2d 2d 2d 2d 2d 2d |.---------------| 000020f0 2d 2d 2d 2d 2d 2d 2d 2d 2d 2d 2d 2d 2d 2d 2d 2d |----------------| * 00002120 2d 2d 2d 2d 2d 2d 2d 2d 2d 2d 2d 0d 54 6f 20 20 |-----------.To | 00002130 20 20 20 20 20 41 73 20 20 20 20 20 20 20 20 20 | As | 00002140 41 73 20 20 20 20 20 20 20 20 20 20 41 73 20 20 |As As | 00002150 20 20 20 20 20 20 20 20 3c 62 79 74 65 3e 20 20 | <byte> | 00002160 20 20 41 73 20 20 20 20 20 20 20 20 20 20 3c 62 | As <b| 00002170 79 74 65 3e 0d 64 65 63 6f 64 65 3a 20 20 64 65 |yte>.decode: de| 00002180 48 61 6d 6d 65 64 20 20 20 64 65 48 61 6d 6d 65 |Hammed deHamme| 00002190 64 20 20 20 20 64 65 48 61 6d 6d 65 64 20 20 20 |d deHammed | 000021a0 20 20 41 4e 44 20 37 20 20 20 20 64 65 48 61 6d | AND 7 deHam| 000021b0 6d 65 64 20 20 20 20 20 41 4e 44 20 33 0d 2d 2d |med AND 3.--| 000021c0 2d 2d 2d 2d 2d 2d 2d 2d 2d 2d 2d 2d 2d 2d 2d 2d |----------------| * 00002200 2d 2d 2d 2d 2d 2d 2d 2d 0d 0d 54 68 65 20 6d 61 |--------..The ma| 00002210 67 61 7a 69 6e 65 20 6e 75 6d 62 65 72 20 69 73 |gazine number is| 00002220 20 65 76 65 6e 20 6d 6f 72 65 20 63 6f 6d 70 6c | even more compl| 00002230 69 63 61 74 65 64 20 74 6f 20 64 65 63 6f 64 65 |icated to decode| 00002240 21 20 57 65 20 6e 65 65 64 20 74 6f 20 6c 6f 6f |! We need to loo| 00002250 6b 20 61 74 0d 74 68 65 20 62 69 74 73 20 77 65 |k at.the bits we| 00002260 20 64 69 73 63 61 72 64 65 64 20 65 61 72 6c 69 | discarded earli| 00002270 65 72 20 28 74 68 65 20 74 6f 70 20 62 69 74 20 |er (the top bit | 00002280 6f 66 20 62 79 74 65 20 36 2c 20 61 6e 64 20 74 |of byte 6, and t| 00002290 68 65 20 74 6f 70 20 74 77 6f 20 62 69 74 73 0d |he top two bits.| 000022a0 6f 66 20 62 79 74 65 20 38 29 20 61 6e 64 20 74 |of byte 8) and t| 000022b0 68 65 20 6d 61 67 61 7a 69 6e 65 20 6e 75 6d 62 |he magazine numb| 000022c0 65 72 20 66 72 6f 6d 20 77 68 69 63 68 20 74 68 |er from which th| 000022d0 65 20 63 75 72 72 65 6e 74 20 70 61 67 65 20 68 |e current page h| 000022e0 61 73 20 62 65 65 6e 0d 74 61 6b 65 6e 2e 0d 0d |as been.taken...| 000022f0 46 69 72 73 74 2c 20 74 61 6b 65 20 74 68 65 20 |First, take the | 00002300 74 68 72 65 65 20 62 69 74 73 20 69 6e 20 71 75 |three bits in qu| 00002310 65 73 74 69 6f 6e 20 61 6e 64 20 66 6f 72 6d 20 |estion and form | 00002320 61 20 73 69 6e 67 6c 65 20 74 68 72 65 65 2d 62 |a single three-b| 00002330 69 74 20 6e 75 6d 62 65 72 2e 0d 54 68 69 73 20 |it number..This | 00002340 69 73 20 64 6f 6e 65 20 62 79 20 75 73 69 6e 67 |is done by using| 00002350 20 62 69 74 20 33 20 6f 66 20 62 79 74 65 20 36 | bit 3 of byte 6| 00002360 20 61 73 20 74 68 65 20 6c 65 61 73 74 20 73 69 | as the least si| 00002370 67 6e 69 66 69 63 61 6e 74 20 62 69 74 20 6f 66 |gnificant bit of| 00002380 20 74 68 65 0d 6e 65 77 20 6e 75 6d 62 65 72 2c | the.new number,| 00002390 20 62 69 74 20 32 20 6f 66 20 62 79 74 65 20 38 | bit 2 of byte 8| 000023a0 20 61 73 20 74 68 65 20 6e 65 78 74 20 62 69 74 | as the next bit| 000023b0 2c 20 74 68 65 6e 20 62 69 74 20 33 20 6f 66 20 |, then bit 3 of | 000023c0 62 79 74 65 20 38 20 61 73 20 74 68 65 0d 6d 6f |byte 8 as the.mo| 000023d0 73 74 20 73 69 67 6e 69 66 69 63 61 6e 74 20 62 |st significant b| 000023e0 69 74 3a 0d 0d 46 69 67 20 32 3a 20 55 73 65 20 |it:..Fig 2: Use | 000023f0 6f 66 20 62 79 74 65 73 20 36 20 61 6e 64 20 38 |of bytes 6 and 8| 00002400 3a 0d 20 20 20 2d 2d 2d 2d 2d 2d 2d 2d 2d 2d 2d |:. -----------| 00002410 2d 2d 2d 2d 2d 2d 2d 2d 2d 2d 2d 2d 2d 2d 2d 2d |----------------| * 00002440 2d 2d 2d 2d 2d 0d 20 20 20 50 61 63 6b 65 74 20 |-----. Packet | 00002450 32 37 3a 20 20 20 20 20 20 20 20 20 20 20 20 20 |27: | 00002460 20 20 42 59 54 45 20 38 20 20 20 20 20 20 20 20 | BYTE 8 | 00002470 20 20 20 20 20 20 20 20 20 20 20 42 59 54 45 20 | BYTE | 00002480 36 0d 20 20 20 42 69 74 73 3a 20 20 20 20 20 20 |6. Bits: | 00002490 20 20 20 20 20 20 20 20 42 69 74 33 20 42 69 74 | Bit3 Bit| 000024a0 32 20 42 69 74 31 20 42 69 74 30 20 20 20 20 20 |2 Bit1 Bit0 | 000024b0 20 42 69 74 33 20 42 69 74 32 20 42 69 74 31 20 | Bit3 Bit2 Bit1 | 000024c0 42 69 74 30 0d 20 20 20 20 20 20 20 20 20 20 20 |Bit0. | 000024d0 20 20 20 20 20 20 20 20 20 20 20 20 7c 20 20 20 | | | 000024e0 20 7c 20 20 20 20 20 20 20 20 20 20 20 20 20 20 | | | 000024f0 20 20 20 20 20 7c 0d 20 20 20 4e 65 77 20 6e 75 | |. 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Byt| 00002920 65 3a 20 20 20 20 20 30 20 20 20 31 20 20 20 20 |e: 0 1 | 00002930 20 32 20 20 20 20 20 33 20 34 20 35 20 36 20 37 | 2 3 4 5 6 7| 00002940 20 38 20 20 20 20 39 20 31 30 20 31 31 20 31 32 | 8 9 10 11 12| 00002950 20 31 33 20 31 34 20 20 2e 2e 2e 20 20 33 39 20 | 13 14 ... 39 | 00002960 20 34 30 20 34 31 0d 20 20 20 20 20 20 20 20 20 | 40 41. | 00002970 2d 2d 2d 2d 2d 2d 2d 2d 2d 2d 2d 2d 2d 2d 2d 2d |----------------| * 000029a0 2d 2d 2f 20 20 20 2f 2d 2d 2d 2d 2d 2d 2d 2d 2d |--/ /---------| 000029b0 2d 0d 20 4d 65 61 6e 69 6e 67 3a 20 4d 61 67 20 |-. Meaning: Mag | 000029c0 52 6f 77 20 20 50 61 63 6b 65 74 20 7c 2e 2e 2e |Row Packet |...| 000029d0 2e 2e 2e 2e 2e 2e 2e 2e 7c 20 20 7c 2e 2e 2e 2e |........| |....| 000029e0 2e 2e 2e 2e 2e 2e 2e 2e 2e 2e 2e 2e 7c 20 20 20 |............| | 000029f0 20 20 4c 69 6e 6b 20 7c 2e 2e 2e 7c 0d 20 20 20 | Link |...|. | 00002a00 20 20 20 20 20 20 20 20 20 20 20 28 32 37 29 20 | (27) | 00002a10 20 49 2e 44 2e 20 20 20 20 20 4c 69 6e 6b 20 30 | I.D. Link 0| 00002a20 20 20 20 20 20 20 20 20 20 20 20 4c 69 6e 6b 20 | Link | 00002a30 31 20 20 20 20 20 20 20 20 20 20 20 20 43 74 72 |1 Ctr| 00002a40 6c 20 20 43 52 43 0d 20 20 20 20 20 20 20 20 20 |l CRC. | 00002a50 2d 2d 2d 2d 2d 2d 2d 2d 2d 2d 2d 2d 2d 2d 2d 2d |----------------| * 00002a80 2d 2d 2f 20 20 20 2f 2d 2d 2d 2d 2d 2d 2d 2d 2d |--/ /---------| 00002a90 2d 0d 0d 49 66 2c 20 61 66 74 65 72 20 64 65 63 |-..If, after dec| 00002aa0 6f 64 69 6e 67 2c 20 74 68 65 20 70 61 67 65 20 |oding, the page | 00002ab0 6e 75 6d 62 65 72 20 6f 66 20 74 68 65 20 6c 69 |number of the li| 00002ac0 6e 6b 20 69 73 20 26 46 46 2c 20 74 68 69 73 20 |nk is &FF, this | 00002ad0 73 69 67 6e 69 66 69 65 73 20 74 68 65 0d 6c 69 |signifies the.li| 00002ae0 6e 6b 20 69 73 20 6e 6f 74 20 62 65 69 6e 67 20 |nk is not being | 00002af0 75 73 65 64 2e 20 49 66 20 61 6e 79 20 6f 66 20 |used. If any of | 00002b00 74 68 65 20 66 6f 75 72 20 70 61 63 6b 65 74 20 |the four packet | 00002b10 32 37 27 73 20 61 72 65 20 6e 6f 74 0d 74 72 61 |27's are not.tra| 00002b20 6e 73 6d 69 74 74 65 64 2c 20 74 68 65 20 72 65 |nsmitted, the re| 00002b30 6c 65 76 61 6e 74 20 70 61 67 65 20 6c 69 6e 6b |levant page link| 00002b40 73 20 63 61 6e 20 62 65 20 61 73 73 75 6d 65 64 |s can be assumed| 00002b50 20 61 6c 73 6f 20 74 6f 20 62 65 20 6e 6f 74 20 | also to be not | 00002b60 69 6e 20 75 73 65 2e 0d 0d 49 74 20 69 73 20 69 |in use...It is i| 00002b70 6d 70 6f 72 74 61 6e 74 20 74 6f 20 72 65 6d 65 |mportant to reme| 00002b80 6d 62 65 72 20 74 68 61 74 20 79 6f 75 20 6d 75 |mber that you mu| 00002b90 73 74 20 75 73 65 20 2a 44 41 54 41 20 61 6e 64 |st use *DATA and| 00002ba0 20 6e 6f 74 20 2a 54 52 41 4e 53 46 45 52 20 69 | not *TRANSFER i| 00002bb0 6e 0d 6f 72 64 65 72 20 74 6f 20 74 72 61 6e 73 |n.order to trans| 00002bc0 66 65 72 20 70 61 63 6b 65 74 20 6e 75 6d 62 65 |fer packet numbe| 00002bd0 72 73 20 67 72 65 61 74 65 72 20 74 68 61 6e 20 |rs greater than | 00002be0 32 33 20 69 6e 74 6f 20 6d 65 6d 6f 72 79 2e 0d |23 into memory..| 00002bf0 0d 41 73 20 74 68 72 65 65 20 62 69 74 73 20 66 |.As three bits f| 00002c00 72 6f 6d 20 74 68 65 20 73 75 62 2d 70 61 67 65 |rom the sub-page| 00002c10 20 63 6f 64 65 20 62 79 74 65 73 20 61 72 65 20 | code bytes are | 00002c20 75 73 65 64 20 66 6f 72 20 6f 74 68 65 72 20 70 |used for other p| 00002c30 75 72 70 6f 73 65 73 2c 20 69 74 0d 63 61 6e 20 |urposes, it.can | 00002c40 62 65 20 73 65 65 6e 20 74 68 61 74 20 74 68 65 |be seen that the| 00002c50 20 6d 61 78 69 6d 75 6d 20 76 61 6c 75 65 20 70 | maximum value p| 00002c60 6f 73 73 69 62 6c 65 20 66 6f 72 20 61 20 73 75 |ossible for a su| 00002c70 62 2d 70 61 67 65 20 63 6f 64 65 20 69 73 20 26 |b-page code is &| 00002c80 33 46 37 46 2e 0d 49 66 20 74 68 65 20 6c 69 6e |3F7F..If the lin| 00002c90 6b 27 73 20 73 75 62 2d 70 61 67 65 20 69 73 20 |k's sub-page is | 00002ca0 64 65 63 6f 64 65 64 20 61 73 20 74 68 69 73 2c |decoded as this,| 00002cb0 20 69 74 20 69 6e 64 69 63 61 74 65 73 20 74 68 | it indicates th| 00002cc0 61 74 20 61 6e 79 20 73 75 62 2d 70 61 67 65 0d |at any sub-page.| 00002cd0 6f 6e 20 74 68 65 20 6d 61 69 6e 20 70 61 67 65 |on the main page| 00002ce0 20 6e 75 6d 62 65 72 20 69 73 20 61 63 63 65 70 | number is accep| 00002cf0 74 61 62 6c 65 2e 20 49 74 20 73 68 6f 75 6c 64 |table. It should| 00002d00 20 6e 6f 74 20 74 72 79 20 74 6f 20 66 69 6e 64 | not try to find| 00002d10 20 73 75 62 2d 70 61 67 65 0d 33 46 37 46 21 0d | sub-page.3F7F!.| 00002d20 0d 50 61 67 65 20 6c 69 6e 6b 73 20 61 72 65 20 |.Page links are | 00002d30 75 73 65 64 20 65 78 74 65 6e 73 69 76 65 6c 79 |used extensively| 00002d40 20 66 6f 72 20 74 65 6c 65 73 6f 66 74 77 61 72 | for telesoftwar| 00002d50 65 20 70 75 72 70 6f 73 65 73 2e 20 41 6e 79 0d |e purposes. Any.| 00002d60 74 65 6c 65 73 6f 66 74 77 61 72 65 20 64 65 63 |telesoftware dec| 00002d70 6f 64 65 72 20 73 68 6f 75 6c 64 20 66 6f 6c 6c |oder should foll| 00002d80 6f 77 20 6c 69 6e 6b 20 30 20 66 72 6f 6d 20 65 |ow link 0 from e| 00002d90 61 63 68 20 73 75 62 2d 70 61 67 65 20 74 6f 20 |ach sub-page to | 00002da0 66 69 6e 64 20 74 68 65 0d 6e 65 78 74 20 62 6c |find the.next bl| 00002db0 6f 63 6b 20 69 6e 20 74 68 65 20 73 65 72 69 65 |ock in the serie| 00002dc0 73 2e 20 54 68 69 73 20 69 73 20 6e 6f 74 20 6e |s. This is not n| 00002dd0 65 63 65 73 73 61 72 69 6c 79 20 6f 6e 20 74 68 |ecessarily on th| 00002de0 65 20 73 61 6d 65 20 6d 61 69 6e 20 70 61 67 65 |e same main page| 00002df0 2e 0d 49 66 20 74 68 65 20 66 69 6c 65 20 69 73 |..If the file is| 00002e00 20 61 6e 20 6f 72 64 65 72 65 64 20 66 69 6c 65 | an ordered file| 00002e10 2c 20 74 68 65 20 73 75 62 2d 70 61 67 65 20 72 |, the sub-page r| 00002e20 65 66 65 72 65 6e 63 65 20 6f 66 20 6c 69 6e 6b |eference of link| 00002e30 20 7a 65 72 6f 20 73 68 6f 75 6c 64 0d 61 6c 73 | zero should.als| 00002e40 6f 20 62 65 20 66 6f 6c 6c 6f 77 65 64 2c 20 6f |o be followed, o| 00002e50 74 68 65 72 77 69 73 65 20 6a 75 73 74 20 74 68 |therwise just th| 00002e60 65 20 6d 61 69 6e 20 70 61 67 65 20 72 65 66 65 |e main page refe| 00002e70 72 65 6e 63 65 20 69 73 20 75 73 65 64 2e 20 49 |rence is used. I| 00002e80 6e 20 74 68 69 73 0d 77 61 79 2c 20 61 20 74 65 |n this.way, a te| 00002e90 6c 65 73 6f 66 74 77 61 72 65 20 66 69 6c 65 20 |lesoftware file | 00002ea0 63 61 6e 20 73 74 61 72 74 20 6f 6e 20 6f 6e 65 |can start on one| 00002eb0 20 70 61 67 65 20 61 6e 64 20 63 6f 6e 74 69 6e | page and contin| 00002ec0 75 65 20 6f 6e 20 61 20 64 69 66 66 65 72 65 6e |ue on a differen| 00002ed0 74 0d 70 61 67 65 21 0d 5f 5f 5f 5f 5f 5f 5f 5f |t.page!.________| 00002ee0 5f 5f 5f 5f 5f 5f 5f 5f 5f 5f 5f 5f 5f 5f 5f 5f |________________| * 00002f20 5f 5f 0d 0d 54 68 65 20 70 72 6f 67 72 61 6d 20 |__..The program | 00002f30 61 63 63 6f 6d 70 61 6e 79 69 6e 67 20 74 68 69 |accompanying thi| 00002f40 73 20 74 65 78 74 20 77 69 6c 6c 20 72 65 71 75 |s text will requ| 00002f50 65 73 74 20 74 68 65 20 74 65 6c 65 74 65 78 74 |est the teletext| 00002f60 20 63 68 61 6e 6e 65 6c 2c 20 70 61 67 65 0d 6e | channel, page.n| 00002f70 75 6d 62 65 72 20 61 6e 64 20 6c 69 6e 6b 20 73 |umber and link s| 00002f80 65 74 74 69 6e 67 20 72 65 71 75 69 72 65 64 2e |etting required.| 00002f90 20 57 68 65 6e 20 6c 6f 61 64 65 64 2c 20 74 68 | When loaded, th| 00002fa0 65 20 70 61 67 65 20 69 73 20 69 6e 74 65 72 72 |e page is interr| 00002fb0 6f 67 61 74 65 64 0d 66 6f 72 20 74 68 65 20 61 |ogated.for the a| 00002fc0 70 70 72 6f 70 72 69 61 74 65 20 70 61 63 6b 65 |ppropriate packe| 00002fd0 74 20 32 37 2e 20 54 68 65 20 6c 69 6e 6b 20 73 |t 27. The link s| 00002fe0 65 74 74 69 6e 67 20 69 73 20 64 65 63 6f 64 65 |etting is decode| 00002ff0 64 20 61 6e 64 20 70 72 65 73 65 6e 74 65 64 0d |d and presented.| 00003000 6f 6e 20 73 63 72 65 65 6e 2e 20 50 72 65 73 73 |on screen. Press| 00003010 69 6e 67 20 3c 53 50 41 43 45 3e 20 77 69 6c 6c |ing <SPACE> will| 00003020 20 63 61 6c 6c 20 74 68 65 20 6c 69 6e 6b 20 64 | call the link d| 00003030 65 63 6f 64 69 6e 67 20 72 6f 75 74 69 6e 65 20 |ecoding routine | 00003040 32 34 20 74 69 6d 65 73 2c 0d 6f 6e 63 65 20 66 |24 times,.once f| 00003050 6f 72 20 65 61 63 68 20 70 6f 73 73 69 62 6c 65 |or each possible| 00003060 20 6c 69 6e 6b 2e 20 41 6c 6c 20 74 68 65 20 6c | link. All the l| 00003070 69 6e 6b 73 20 77 69 6c 6c 20 62 65 20 64 65 63 |inks will be dec| 00003080 6f 64 65 64 20 74 68 65 6e 20 73 68 6f 77 6e 20 |oded then shown | 00003090 6f 6e 0d 73 63 72 65 65 6e 2e 20 49 66 20 74 68 |on.screen. If th| 000030a0 65 20 6d 65 73 73 61 67 65 20 22 6e 6f 74 20 67 |e message "not g| 000030b0 69 76 65 6e 22 20 61 70 70 65 61 72 73 2c 20 74 |iven" appears, t| 000030c0 68 69 73 20 69 6e 64 69 63 61 74 65 73 20 74 68 |his indicates th| 000030d0 65 20 61 70 70 72 6f 70 72 69 61 74 65 0d 70 61 |e appropriate.pa| 000030e0 63 6b 65 74 20 32 37 20 77 61 73 20 66 6f 75 6e |cket 27 was foun| 000030f0 64 20 62 75 74 20 74 68 65 20 6c 69 6e 6b 20 77 |d but the link w| 00003100 61 73 20 75 6e 64 65 66 69 6e 65 64 20 28 69 2e |as undefined (i.| 00003110 65 2e 20 73 65 74 20 74 6f 20 26 46 46 29 2e 20 |e. set to &FF). | 00003120 49 66 20 74 68 65 0d 6d 65 73 73 61 67 65 20 22 |If the.message "| 00003130 6e 6f 74 20 70 72 65 73 65 6e 74 22 20 69 73 20 |not present" is | 00003140 73 68 6f 77 6e 2c 20 74 68 65 20 61 70 70 72 6f |shown, the appro| 00003150 70 72 69 61 74 65 20 70 61 63 6b 65 74 20 32 37 |priate packet 27| 00003160 20 77 61 73 20 6e 6f 74 0d 72 65 63 65 69 76 65 | was not.receive| 00003170 64 2e 0d 0d 41 6c 6c 20 32 34 20 6c 69 6e 6b 73 |d...All 24 links| 00003180 20 68 61 76 65 20 62 65 65 6e 20 73 65 74 20 66 | have been set f| 00003190 6f 72 20 74 68 69 73 20 77 65 65 6b 20 6f 6e 6c |or this week onl| 000031a0 79 20 6f 6e 20 70 61 67 65 20 37 30 30 2c 20 74 |y on page 700, t| 000031b0 6f 20 65 6e 61 62 6c 65 20 79 6f 75 0d 74 6f 20 |o enable you.to | 000031c0 74 65 73 74 20 74 68 65 20 6c 69 6e 6b 20 64 65 |test the link de| 000031d0 63 6f 64 69 6e 67 20 73 6f 66 74 77 61 72 65 2e |coding software.| 000031e0 0d 0d 0d 0d 0d 0d 0d 0d 0d 0d 0d 20 20 20 20 20 |........... | 000031f0 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 | | 00003200 20 20 20 20 20 20 20 20 20 20 20 20 20 50 61 67 | Pag| 00003210 65 20 35 0d 0d 0d |e 5...| 00003216
ATS/T\ATS05.m0
ATS/T\ATS05.m1
ATS/T\ATS05.m2
ATS/T\ATS05.m4
ATS/T\ATS05.m5
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