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18-03-89/T\TTX02
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 » telesoftware14.adl |
| Filename: | 18-03-89/T\TTX02 |
| Read OK: | ✔ |
| File size: | 859C bytes |
| Load address: | 0000 |
| Exec address: | FFFFFFFF |
File contents
Interfacing with the Acorn Teletext Adaptor - by - Gordon Horsington
--------------------------------------------------------------------
Module 2. The header packet
---------------------------
There are 32 Teletext packets, numbered 0 to 31. Each packet is broadcast
as 45 bytes, and 43 of these bytes are available from the Acorn Teletext
adaptor. Every packet has the format shown in figure 1.
+------+------+-------+------+------+--- ---+
| Clock |Framing| Mag and Row | 40 bytes |
| run-in |code | Addr Group | of data |
+------+------+-------+------+------+--- ---+
1 2 3 4 5 6 - 45 byte number
Figure 1. The data transmitted in a Teletext packet
---------------------------------------------------
The first two bytes of every packet comprise the clock run-in sequence of
alternating bits, beginning 101010... and so on. These two bytes are used
to indicate the presence of a data packet and to establish the timing of
the bits in that packet.
The third byte of every packet is the framing code %11100100. This code
was selected to enable byte synchronisation to be established even if one
bit of the framing code byte has been wrongly received. The framing code
byte can be read from the adaptor as the number &27, %00100111, ie. with
all the bits in the byte reversed.
The magazine and packet numbers are broadcast in the next two bytes, known
as the Magazine and Row Address Group (MRAG). Rows of Teletext data have
been renamed packets but the terms row and packet tend to be used
interchangeably. The Row in MRAG refers to the packet address. Because of
the possible confusion that can be caused by the interchangeable use of
these terms I will use row to refer to the 16 * 64 byte rows of memory in
the Teletext adaptor and packet to refer to the rows of Teletext data
which are stored in the adaptor's memory.
The fourth and fifth byte of every packet, and a number of other bytes in
specific packets, are hamming codes which contain 4 message bits
interleaved with 4 protection bits. Decoding the hamming coded bytes into
4 bit nybbles can correct single errors in the received byte and detect
multiple errors.
The two MRAG bytes are hamming coded and must be decoded before they can
be used. Each byte decodes into a 4 bit nybble. The magazine number is a 3
bit number and the packet number uses 5 bits. When you interface directly
with the adaptor, you have to decode the 3 and 5 bit numbers from these
two nybbles. The 40 bytes of data which follow the magazine and packet
numbers have a variable format which depends upon the packet number.
Packet 0 is known as the Page Header. Packet 0 contains 8 additional
hamming coded bytes which decode to give the page and sub-page numbers and
a series of control bits. It also contains the header information you see
displayed at the top of a Teletext page in the terminal mode. Decoding the
information contained in packet 0 will be dealt with in this module and
using the information will be covered in modules 3 and 4.
Packet numbers 1 to 23 contain the information displayed by the level 1
Teletext terminal. They can also contain redefinable telesoftware format
(RTF) computer software. The decoding of telesoftware will not be covered
in these modules and is best left to the ATS.
Packet numbers greater than 23 are sometimes referred to as 'Ghost rows'
or 'Ghost packets' and they are not normally displayed by level 1 Teletext.
All packets from 0 to 31 can be received by the Acorn Teletext adaptor and
it is interesting to know what the so-called ghost packets contain.
Packets 24 and 25 have the same format as packets 1 to 23 and, when they
are combined with the central 24 bytes in packet 0, they form a page of
1024 bytes which is suitable for telesoftware. Packet 24 can also contain
the colour coded prompts for linked pages used by Fastext and the ATS.
Packet 26 is used to broadcast information which can be used to enhance
the display of higher level decoders. Facilities are included to define
full screen and row colours, pastel colours, accented and special
characters, double height, double width, underline and reduced intensity.
Packet 27 is known as the Page Service Packet and is used to define linked
page addresses. Each packet 27 contains 6 groups of 6 bytes, each of which
specifies a page and sub-page. There are 4 versions of packet 27 so that
up to 24 page numbers can be linked to to any given page. A cyclic
redundancy check (CRC) code is incorporated in packet 27 type 0. This can
be used to verify the data in packets 0 - 25.
Packet 28 is used to redefine the character set. Two versions are used,
one of which is page related and the other magazine related. The data are
broadcast as 13 groups of 3 bytes, each group defining a character. Packet
28 can also be used to redefine pastel colours.
Packet 30 is the first of two page independent packets and contains the
Television Service Data Packet (TSDP). Magazines are referred to as
channels in packets 30 and 31 so that magazines 0 to 7 in packet 30 are
known as page independent channels 0 to 7. There are two formats used for
the TSDP which are known as type 0 and type 2. The TSDP contains
information about the initial page to be grabbed by the decoder, programme
and TV channel labels, time in a standard format, control information and
data for display. Modules 5 and 6 will examine the TSDPs in detail.
Packet 31 is the second page independent packet and magazines 0 to 7 in
packet 31 are usually referred to as page independent channels 8 to 15.
Channels 8, 9, 10 and 11 are used by the BBC for its Datacast service.
Datacast is professional data communications technique which distributes
machine readable information to all parts of the United Kingdom
simultaneously. Datacast is used by organisations such as the London Stock
Exchange and the Post Office and most of the information is encrypted for
security. Packet 31 can be received on the Acorn Teletext adaptor and the
demonstration program associated with module 7 will allow you to look at
some of the data broadcast in this packet. At the time of writing the ITV
network is broadcasting an Air Call test message in packet 31. Neither
Channel 4 nor S4C make any use of packet 31.
In module 1 you were shown how to intercept and recognise the Teletext
generated interrupt. In this module I will show you how to read data from
the adaptor into user memory. The example program HEADER only looks for
packet 0 but the same techniques apply to any packet.
Because all the packets broadcast on Teletext contain at least two hamming
coded bytes it is necessary to include a table of 256 bytes to decode
them. This table (lines 1850 to 2490) will have to be included in almost
every Teletext program which bypasses the ATS or TFS.
A Teletext generated interrupt signals that data are available to be read
from the adaptor and up to 16 packets can be read from the 16 * 64 byte
rows of memory in the adaptor although, at the the time of writing, only 7
of the 16 rows are used. The BBC currently use 6 rows for CEEFAX and 1 for
Datacast but there are plans to use 8 rows for CEEFAX and 2 for Datacast.
Because not all the rows are used it is necessary to be able to detect
which rows contain data. This can be done by looking for the framing code.
Each row can be selected by writing one of the numbers &00 to &0F into the
row register at &FC11 (lines 1180 to 1210). After selecting a row, the
framing code byte can be loaded from the adaptor by reading the data
register at &FC12 (line 1110). If the result of this read is &27 then data
are available. If data are available the program HEADER reads them using
the subroutine readpacket (line 1150 and lines 1380 to 1820).
Before returning from a Teletext interrupt is necessary to clear the
framing code in each row of the adaptor's memory. To do this select each
row in turn and write the number zero into the data register (lines 1220
to 1290). If you do not include lines 1220 to 1290 the program will run
for a while but eventually, and usually quite quickly, the program will
crash. The Teletext interrupt processing routine must also either read
from or write to the status clear register (line 1300) before restoring
the registers and executing the RTI instruction.
Reading and decoding the magazine and packet numbers is done in lines 1390
to 1520 of HEADER and described in more detail in figure 2.
LDY datareg \ read magazine number and bit 0 of packet number
LDA hamtable,Y \ de-ham it into the accumulator
BMI clearflag \ bit 7 set means an irrecoverable error
STA magazine \ save magazine number and bit 0 of packet number
LDY datareg \ read bits 1 to 4 of the packet number into Y
LDA hamtable,Y \ de-ham it into the accumulator
BMI clearflag \ branch if an irrecoverable error
STA packet \ save bits 1 to 4 of the packet number
LDA magazine \ load magazine number and bit 0 of packet number
CMP #&08 \ bit 3 into carry flag
ROL packet \ rotate bits 0 to 3 left and add bit 0
AND #&07 \ isolate the 3 bits of the magazine number
STA magazine \ and store the 3 bit magazine number
Figure 2. Reading and decoding the magazine and packet numbers.
---------------------------------------------------------------
The two bytes which contain the magazine and packet numbers have to be
read and de-hammed before they can be used. If either of the two bytes
generates an irrecoverable error (ie. de-hamms to &FF) then the whole
packet must be rejected because without the information stored in the MRAG
there is no way of knowing the correct identity of the packet.
After reading and de-hamming both bytes the first nybble is loaded into
the accumulator and compared with the number 8. If the content of the
accumulator is equal to or greater than 8 then the carry flag is set. In
this way bit 3 of the first nybble, which is bit 0 of the packet number,
is transferred into the carry flag. The 4 most significant bits of the
packet number are then rotated left and the least significant bit is
placed in bit 0 of the packet number. The accumulator still contains the
first nybble which is ANDed with the number 7 to clear bit 3. The
accumulator then contains the 3 bit magazine number which can be stored
for later use and the previously incomplete 4 bit packet number is now the
correct 5 bit number.
When the magazine and packet numbers have been decoded and stored in user
memory the rest of the 40 data bytes in the row can be read if required.
In HEADER only packet 0 is read and all other packets are rejected (lines
1530 to 1540). You can easily modify this program if you want to look at
any other packets.
The program HEADER and all the Teletext demonstration programs in modules
2 to 7 use a zero page flag byte called grabflag (line 440) to indicate
when data have been transferred into user memory and are available for
display. In HEADER, grabflag can contain either the number &00 or the
number &80. The number &00 means that the data are not ready to print and
the number &80 means that the data are ready to print. The foreground task
does not return control to BASIC if grabflag contains zero (lines 780 to
790) and the background task does not read any more data if bit 7 of
grabflag is set. In this way the machine code foreground task returns
control to BASIC after 300 packets have been stored.
The program HEADER uses a machine code routine to read and store 300
headers in a buffer and, after reading the headers, returns to BASIC to
interpret and display the contents of the buffer. The table in figure 3
shows the format of the header packet. Figure 3 also shows where this
program stores the data it reads with respect to an offset 'head' in the
data buffer 'buffer'.
The byte number in the left hand column of figure 3 shows the order in
which the data are stored in the adaptor. The next column shows whether or
not the data are hamming coded (hc = hamming coded, blank = not coded),
the third column gives a brief description of the byte (or bits from the
byte), the next column shows where the byte (or bits from the byte) are
stored with respect to the offset 'head', and the last column describes
the format in which the program HEADER stores the data (blank = stored as
read).
+-------+----+---------------------------+------------------+-----------+
|byte no|code| description | stored in | format |
+-------+----+---------------------------+------------------+-----------+
| 1 | | clock run-in | (not read) | |
| 2 | | clock run-in | (not read) | |
| 3 | | framing code | (not stored) | |
| 4 | hc | magazine no. in bits 0-2 | head?0 | de-hammed |
| 4 | hc | bit 0 of pack no in bit 3 | (not stored) | |
| 5 | hc | packet number bits 1-4 | (not stored) | |
| 6 | hc | page number, low nybble | head?1 | de-hammed |
| 7 | hc | page number, high nybble | head?2 | de-hammed |
| 8 | hc | sub-page, low nybble | head?3 | de-hammed |
| 9 | hc | sub-page in bits 0 - 2 | head?4 bits 0-2 | de-hammed |
| 9 | hc | erase page control bit | head?4 bit 3 | de-hammed |
| 10 | hc | sub-page | head?5 | de-hammed |
| 11 | hc | subpage,hi nybble,bits 0,1| head?6 bits 0,1 | de-hammed |
| 11 | hc | newsflash control bit | head?6 bit 2 | de-hammed |
| 11 | hc | subtitle control bit | head?6 bit 3 | de-hammed |
| 12 | hc | suppress header cont. bit | head?7 bit 0 | de-hammed |
| 12 | hc | update indicator cont bit | head?7 bit 1 | de-hammed |
| 12 | hc | out of sequence cont. bit | head?7 bit 2 | de-hammed |
| 12 | hc | inhibit display cont. bit | head?7 bit 3 | de-hammed |
| 13 | hc | serial/parallel magazine | head?8 bit 0 | de-hammed |
| 13 | hc | language in bits 1-3 | head?8 bits 1-3 | de-hammed |
| 14-45 | | header display message | head?9 - head?40 | |
+-------+----+---------------------------+------------------+-----------+
Figure 3. The data transmitted in the header packet
---------------------------------------------------
If you compare the information in figure 3 with the format of the header
made available by the ATS then it should be clear that the ATS does some
processing of the data before making them available to the user (see page
41 of the ATS User Guide). When you write a program to bypass the ATS and
interface directly with the Teletext adaptor you have to take the raw data
and process it within your own program. You must not assume that the data
are in the familiar ATS format. If, however, you need to use packet 27 to
find the page links or the CRC bytes you will find that the description of
the last 40 bytes of the packet in the ATS manual will also apply to bytes
6 to 45 of packet 27 taken directly from the adaptor.
The first 5 bytes of the header are in the same format as the first 5
bytes of all Teletext packets. They are therefore read and decoded in the
same way as all packets.
The packet number of the header is zero. Packet zero is the only packet to
contain the page number. All packets (except 30 and 31) received after a
packet zero, which also have the same magazine number as the packet zero,
are assumed to be in the same page. The page changes when a different
packet zero in the same magazine is received.
A sub-page number can be extracted from the de-hammed 8th, 9th, 10th, and
11th bytes of packet zero. The low order nybble is in the eighth byte and
is stored by the program HEADER in head?3. The next (3 bit) nybble is in
bits 0, 1 and 2 of the ninth byte and is stored in head?4. The next nybble
of the sub-page number is in the tenth byte and stored in head?5. The high
order (2 bit) nybble is in bits 0 and 1 of the eleventh byte and stored in
head?6. This strange use of different numbers of bits for the digits in
the sub-page number has its origins in the now largely obsolete time
sub-code.
Originally the sub-page number was designed to reflect the time of day and
it was allocated four hex digits to correspond directly with hours and
minutes. The time has a range from 0000 (midnight) to 2359 (11:59 pm). The
range of BCD digits needed to store the time of day requires 2 bits for
the first digit (0-2), 4 for the second (0-9), 3 for the third (0-5) and 4
for the last (0-9). The sub-page code can therefore be a maximum of &3F7F
and up to 3200 sub-pages can be associated with any page. Not all the
codes between 0 an &3F7F are possible, for example, a sub-page code of
&0080 is not possible, as the third digit cannot exceed 7.
The Alarm clock on CEEFAX page 196 is the only page that is now
transmitted using the sub-page coding as a time-code. Requesting the alarm
at, for example, 17:30, is in fact the same as requesting page 196
sub-page 1730, which is broadcast at exactly 5:30 pm. All the other
cycling sub-pages use the sub-page number as a means to differentiate
between consecutive pages in a rolling sequence.
The control bits broadcast in the header packet will be used in modules 3
and 4 when the design of page grabbers will be considered. There are 8
control bits which are used and 3 unused control bits. The control bits
have the following locations and interpretations:
Erase page.
Broadcast in byte 9, bit 3. (Stored in head?4, bit 3)
If a decoder receives a page containing less than 23 packets it must know
whether these are to be simply overlaid on a previously received page with
the same page number or if the previous text is to be erased before
updating. If the erase page bit is set, it will do the latter. If the
erase-bit is clear it should overlay the information without erasing the
previous page.
Newsflash.
Broadcast in byte 11, bit 2. (Stored in head?6, bit 2)
Newsflashes and subtitles are designed to be mixed with normal TV
pictures and are recognised by Teletext television sets by their control
bits. They are not displayed by the ATS Teletext terminal mode because the
Acorn adaptor cannot mix television and Teletext signals.
Subtitle.
Broadcast in byte 11, bit 3. (Stored in head?6, bit 3)
As with the newsflash pages, subtitles are mixed with the normal TV
picture. Subtitles are not displayed by the ATS Teletext terminal.
Suppress header.
Broadcast in byte 12, bit 0. (Stored in head?7, bit 0)
Normally all the page headers are displayed as they are received. This
ensures the time displayed in the header is kept reasonably accurate, and
the cycling page numbers can be seen. The headers of some pages are not
displayed by Teletext television sets. At the moment the BBC Telesoftware
pages and all pages in the alphabetic range of hexadecimal numbers have
the display of their headers suppressed.
Update indicator.
Broadcast in byte 12, bit 1. (Stored in head?7 bit 1)
The update indicator bit indicates whether the content of the page has
been altered since its last broadcast. When this bit is set the page may
be incomplete, containing only the updated packets of a page.
Out of Sequence.
Broadcast in byte 12, bit 2. (Stored in head?7, bit 2)
This bit is set when a page is broadcast out of strict numerical sequence
in order to give it priority (such as a subtitle) or more frequent
transmission (such as an index page). In order to keep a steady appearance
to the cycling page number display the headers of out of sequence pages
are not displayed. There may be times when a decoder will need to
differentiate between a suppressed header from an in sequence page, and an
unsuppressed header from an out of sequence page, and so the two
conditions are allocated separate control bits.
Inhibit display.
Broadcast in byte 12, bit 3. (Stored in head?7, bit 3)
BBC Telesoftware pages often have the inhibit display bit set to suppress
the display of the Telesoftware file. These files are for computer use
only and may cause some confusion if displayed by a Teletext television
set.
Serial magazine.
Broadcast in byte 13, bit 0. (Stored in head?8, bit 0)
Magazines on Teletext are either serial (BBC 1, BBC 2 and Channel 4) or
parallel (ITV network). The BBC 2 serial transmissions broadcast each page
from magazine 2 followed by each page from magazine 3 then each page from
magazine 7. The parallel transmissions on the ITV network have packets
from the pages in each magazine interleaved with the packets from a
different magazine. Currently ITV use four lines for their national
magazine (100 series pages), and three lines for the regional magazine
(200 series pages).
Parallel transmission needn't use a fixed number of lines however, and an
intelligent transmission system could give extra lines to the 'other'
magazine if the first would otherwise only transmit 'dummy' headers (page
xFF). However, this needs more complicated transmission software than is
currently available.
When the serial magazine control bit is set, all page headers are
displayed as rolling headers. When this bit is clear only headers from the
current magazine are displayed. This gives the illusion of speed, but it
is just an illusion.
Language
Broadcast in byte 13, bits 1-3. (Stored in head?8, bits 1-3)
These three control bits specify which language the page is in, and hence
which character set is required. Some characters of the standard Latin
Alphabet character set (as used by ASCII) are changed to suit different
languages. The changes for the English language are those characters that
are different in Mode 7 to the other BBC screen modes, the Pound, Divide,
and fraction symbols for example. The allocation is as follows:
Bits Language
321
--- --------
000 English
001 French
010 Swedish/Finnish
011 Czechoslovak
100 German
101 Portuguese/Spanish
110 Italian
111 (reserved)
The way in which a Teletext TV responds to the control bits is pre-
determined and unalterable by the viewer. When the design of page grabbers
is considered in the next two modules you will learn how to use these
control bits so that, for example, you can design a page grabber which
will allow you to see the screens which have the inhibit display bit set.
Chain the program HEADER and select a channel. The program will load and
display 300 header packets and interpret all the control bits for each
header. Press the Shift key to scroll through the display.
Both BBC 1 and BBC 2 broadcast serial magazines but the ITV network
stations broadcast parallel magazines. A Teletext page can be defined as
beginning with, and including, its page header and ending with, and
excluding, the next page header with the same magazine number. All the
intermediate packets with the same magazine number relate to the selected
page. This definition needs to be qualified because the program HEADER
will show you that up to ten headers are broadcast for each page. A more
complete definition of a Teletext page should include the following
qualifications:
1) A header with a page number FF and the same magazine number as a
previous header can be used to indicate the end of a page. For example,
header 1FF indicates the end of any page in magazine 1, 2FF indicates the
end of any page in magazine 2, and so on. The IBA use page number FF
extensively, the BBC use it less often.
2) A header with the same magazine number but a different page number to a
previous header marks the end of one page and, if the page number is not
FF, the start of the next page.
3) All the packets within a page have the same magazine number.
4) Packets 30 and 31 are, by definition, independent of any page.
When the header packets are considered as described above it much easier
to deal with the multiple headers of both serial and parallel magazines
and to understand how the interleaved pages in parallel magazines can be
loaded from the adaptor without mixing the different pages of the parallel
magazines. This will be dealt with in the next module.
10 REM> HEADER
20 MODE7
30 VDU14
40 DIM mcode &300 :REM: space for machine code
50 DIM buffer &3300 :REM: buffer for 300 headers
60 PROCmcode :REM: assemble machine code
70 INPUT"TV channel (1-4) = "answer$
80 channel?0=EVAL("&"+LEFT$(answer$,1))+&1B :REM: TTX enable,
Int enable, AFC
90 IF channel?0 < &1C THEN channel?0 = &1C
100 IF channel?0 > &1F THEN channel?0 = &1F
110 PRINT"Reading TTX - Please wait"
120 CALL mcode :REM: read 300 headers and store in buffer
130 control$=STRING$(60," ")
140 end=buffer+(300*41) :REM: end of buffer
150 FOR head = buffer TO end STEP 41
160 control$=""
170 PRINT;~head?0;~head?2;~head?1;:REM: magazine and page numbers
180 PRINT;~(head?6)AND3;~head?5;~(head?4)AND7;~head?3;
:REM: sub-page number
190 erase=((head?4)AND8)<>0 :REM: erase page
200 news=((head?6)AND4)<>0 :REM: newsflash
210 subtitle=((head?6)AND8)<>0 :REM: subtitle
220 suppress=((head?7)AND1)<>0 :REM: suppress header
230 update=((head?7)AND2)<>0 :REM: update indicator
240 outofseq=((head?7)AND4)<>0 :REM: out of sequence
250 inhibit=((head?7)AND8)<>0 :REM: inhibit display
260 serial=((head?8)AND1)<>0 :REM: serial magazine
270 IF erase control$=control$+" Erase,"
280 IF news control$=control$+" News,"
290 IF subtitle control$=control$+" Sub-T,"
300 IF suppress control$=control$+" Supp,"
310 IF update control$=control$+" Update,"
320 IF outofseq control$=control$+" Out-Seq,"
330 IF inhibit control$=control$+" Inhibit,"
340 IF serial control$=control$+" Serial"
ELSE control$=control$+" Para."
350 PRINT control$ :REM: explain the control bits
360 FOR byte=9 TO 40
370 VDU ((head?byte) OR &80):REM: display header
380 NEXT
390 PRINT
400 NEXT
410 VDU15
420 END
430 DEFPROCmcode
440 grabflag=&70 :REM: header grab flag
450 channel=&71 :REM: TV channel
460 workspace=&72 :REM: 2 byte workspace
470 magazine=&74 :REM: magazine number
480 packet=&75 :REM: packet number
490 irq2v=&206 :REM: IRQ2 vector
500 ttxcontrol=&FC10 :REM: TTX control register, write only
510 ttxstatus=&FC10 :REM: TTX status register, read only
520 rowreg=&FC11 :REM: TTX row register, write only
530 datareg=&FC12 :REM: TTX data register, read & write
540 statclr=&FC13 :REM: TTX clear status register, read & write
550 osbyte=&FFF4
560 FOR pass=0 TO 2 STEP 2
570 P%=mcode
580 [ OPT pass
590 LDA #&00
600 STA grabflag \ when grabflag = 0 read and store headers
610 LDX #buffer MOD 256 \ address of the start of the buffer
620 LDY #buffer DIV 256
630 STX workspace \ into zero page for indirect addressing
640 STY workspace+1
650 LDX irq2v \ load secondary interrupt vector
660 LDY irq2v+1
670 STX oldirq2v \ save secondary interrupt vector
680 STY oldirq2v+1
690 LDX #interrupt MOD 256 \ install new interrupt routine
700 LDY #interrupt DIV 256
710 SEI \ disable interrupts when vectors are altered
720 STX irq2v
730 STY irq2v+1
740 CLI \ re-enable interrupts
750 LDA channel \ load (channel number + #&1C)
760 STA ttxcontrol \ enable TTX
770 .mainloop
780 BIT grabflag \ grabflag = &80 when buffer full
790 BMI allover \ branch if buffer full
800 BIT &FF \ poll escape flag
810 BPL mainloop \ loop if escape not pressed
820 LDA #&7E \ decimal 126
830 JSR osbyte \ acknowledge escape
840 .allover
850 LDA #&00
860 STA ttxcontrol \ disable TTX
870 LDX oldirq2v \ load original vector
880 LDY oldirq2v+1
890 SEI \ disable interrupts when vectors are altered
900 STX irq2v \ restore original vector
910 STY irq2v+1
920 CLI \ re-enable interrupts
930 RTS \ return to BASIC
940 .interrupt
950 BIT ttxstatus \ poll TTX hardware
960 BMI ttxinter \ branch if TTX interrupt
970 JMP (oldirq2v) \ not TTX interrupt
980 .ttxinter
990 LDA &FC \ interrupt accumulator save register
1000 PHA \ push interrupt accumulator save register
1010 TXA
1020 PHA \ push X
1030 TYA
1040 PHA \ push Y
1050 BIT grabflag \ are headers still being collected?
1060 BMI clearstatus \ clear status and RTI if finished
1070 CLD \ clear decimal flag
1080 LDY #&00 \ start with row 0
1090 .readttxt
1100 STY rowreg \ try rows 0 to 15
1110 LDA datareg \ load framing code (#&27)
1120 BEQ emptyrow \ if zero try next row
1130 TYA
1140 PHA \ save row number
1150 JSR readpacket \ look for headers and load if found
1160 PLA
1170 TAY \ restore row number
1180 .emptyrow
1190 INY \ increment row number
1200 CPY #&10 \ try rows 0 - 15
1210 BNE readttxt
1220 .clearstatus
1230 LDA #&00
1240 LDY #&0F \ clear 16 rows in adaptor
1250 .clearloop
1260 STY rowreg
1270 STA datareg
1280 DEY
1290 BPL clearloop
1300 STA statclr \ clear status flags before returning
1310 PLA
1320 TAY \ restore Y
1330 PLA
1340 TAX \ restore X
1350 PLA
1360 STA &FC \ restore interrupt accumulator save register
1370 RTI \ return from interrupt
1380 .readpacket
1390 LDY datareg \ read magazine number
1400 LDA hamtable,Y \ de-ham it
1410 BMI return \ stop loading if error
1420 STA magazine \ save magazine number
1430 LDY datareg \ read packet number
1440 LDA hamtable,Y \ de-ham it
1450 BMI return \ stop loading if error
1460 STA packet \ save packet number
1470 LDA magazine \ load magazine number
1480 CMP #&08 \ bit 3 of mag. number is bit 0 of packet
\ number
1490 ROL packet \ 5 bit packet number
1500 AND #&07 \ use only bits 0-2
1510 LDY #&00
1520 STA (workspace),Y \ store magazine number
1530 LDA packet \ header packet number is zero
1540 BNE return \ return if not header
1550 .readheader
1560 INY \ Y = 1-40
1570 LDX datareg \ read data register
1580 LDA hamtable,X \ de-ham it
1590 BMI return \ stop loading if error
1600 STA (workspace),Y \ store de-hammed data
1610 CPY #&08 \ use Y = 1-8 for 8 bytes of hammed data
1620 BCC readheader \ continue reading hammed data
1630 .readmore
1640 INY
1650 LDA datareg \ read data register, not hammed
1660 ORA #&80 \ set bit 7
1670 STA (workspace),Y \ store in buffer
1680 CPY #&28 \ decimal 40
1690 BNE readmore \ go back for more
1700 CLC \ clear carry for addition
1710 LDA workspace
1720 ADC #&29 \ add 41 to point to next part of the buffer
1730 STA workspace
1740 LDA workspace+1
1750 ADC #&00
1760 STA workspace+1
1770 CMP #(buffer+&3300) DIV 256 \ is there room for more data?
1780 BCC return \ branch if there is more room
1790 LDA #&80
1800 STA grabflag \ mark buffer as full
1810 .return
1820 RTS \ return to look at next TTX row
1830 .oldirq2v
1840 EQUW &00
1850 .hamtable
1860 EQUD &0101FF01
1870 EQUD &FF0100FF
1880 EQUD &FF0102FF
1890 EQUD &07FFFF0A
1900 EQUD &FF0100FF
1910 EQUD &00FF0000
1920 EQUD &0BFFFF06
1930 EQUD &FF0300FF
1940 EQUD &FF010CFF
1950 EQUD &07FFFF04
1960 EQUD &07FFFF06
1970 EQUD &070707FF
1980 EQUD &05FFFF06
1990 EQUD &FF0D00FF
2000 EQUD &FF060606
2010 EQUD &07FFFF06
2020 EQUD &FF0102FF
2030 EQUD &09FFFF04
2040 EQUD &02FF0202
2050 EQUD &FF0302FF
2060 EQUD &05FFFF08
2070 EQUD &FF0300FF
2080 EQUD &FF0302FF
2090 EQUD &0303FF03
2100 EQUD &05FFFF04
2110 EQUD &FF040404
2120 EQUD &FF0F02FF
2130 EQUD &07FFFF04
2140 EQUD &050505FF
2150 EQUD &05FFFF04
2160 EQUD &05FFFF06
2170 EQUD &FF030EFF
2180 EQUD &FF010CFF
2190 EQUD &09FFFF0A
2200 EQUD &0BFFFF0A
2210 EQUD &FF0A0A0A
2220 EQUD &0BFFFF08
2230 EQUD &FF0D00FF
2240 EQUD &0B0B0BFF
2250 EQUD &0BFFFF0A
2260 EQUD &0CFF0C0C
2270 EQUD &FF0D0CFF
2280 EQUD &FF0F0CFF
2290 EQUD &07FFFF0A
2300 EQUD &FF0D0CFF
2310 EQUD &0D0DFF0D
2320 EQUD &0BFFFF06
2330 EQUD &FF0D0EFF
2340 EQUD &09FFFF08
2350 EQUD &090909FF
2360 EQUD &FF0F02FF
2370 EQUD &09FFFF0A
2380 EQUD &FF080808
2390 EQUD &09FFFF08
2400 EQUD &0BFFFF08
2410 EQUD &FF030EFF
2420 EQUD &FF0F0CFF
2430 EQUD &09FFFF04
2440 EQUD &0F0FFF0F
2450 EQUD &FF0F0EFF
2460 EQUD &05FFFF08
2470 EQUD &FF0D0EFF
2480 EQUD &FF0F0EFF
2490 EQUD &0EFF0E0E
2500 ]
2510 NEXT
2520 ENDPROC
00000000 49 6e 74 65 72 66 61 63 69 6e 67 20 77 69 74 68 |Interfacing with|
00000010 20 74 68 65 20 41 63 6f 72 6e 20 54 65 6c 65 74 | the Acorn Telet|
00000020 65 78 74 20 41 64 61 70 74 6f 72 20 2d 20 62 79 |ext Adaptor - by|
00000030 20 2d 20 47 6f 72 64 6f 6e 20 48 6f 72 73 69 6e | - Gordon Horsin|
00000040 67 74 6f 6e 0d 2d 2d 2d 2d 2d 2d 2d 2d 2d 2d 2d |gton.-----------|
00000050 2d 2d 2d 2d 2d 2d 2d 2d 2d 2d 2d 2d 2d 2d 2d 2d |----------------|
*
00000080 2d 2d 2d 2d 2d 2d 2d 2d 2d 0d 0d 4d 6f 64 75 6c |---------..Modul|
00000090 65 20 32 2e 20 54 68 65 20 68 65 61 64 65 72 20 |e 2. The header |
000000a0 70 61 63 6b 65 74 0d 2d 2d 2d 2d 2d 2d 2d 2d 2d |packet.---------|
000000b0 2d 2d 2d 2d 2d 2d 2d 2d 2d 2d 2d 2d 2d 2d 2d 2d |----------------|
000000c0 2d 2d 0d 0d 54 68 65 72 65 20 61 72 65 20 33 32 |--..There are 32|
000000d0 20 54 65 6c 65 74 65 78 74 20 70 61 63 6b 65 74 | Teletext packet|
000000e0 73 2c 20 6e 75 6d 62 65 72 65 64 20 30 20 74 6f |s, numbered 0 to|
000000f0 20 33 31 2e 20 45 61 63 68 20 70 61 63 6b 65 74 | 31. Each packet|
00000100 20 69 73 20 62 72 6f 61 64 63 61 73 74 0d 61 73 | is broadcast.as|
00000110 20 34 35 20 62 79 74 65 73 2c 20 61 6e 64 20 34 | 45 bytes, and 4|
00000120 33 20 6f 66 20 74 68 65 73 65 20 62 79 74 65 73 |3 of these bytes|
00000130 20 61 72 65 20 61 76 61 69 6c 61 62 6c 65 20 66 | are available f|
00000140 72 6f 6d 20 74 68 65 20 41 63 6f 72 6e 20 54 65 |rom the Acorn Te|
00000150 6c 65 74 65 78 74 0d 61 64 61 70 74 6f 72 2e 20 |letext.adaptor. |
00000160 45 76 65 72 79 20 70 61 63 6b 65 74 20 68 61 73 |Every packet has|
00000170 20 74 68 65 20 66 6f 72 6d 61 74 20 73 68 6f 77 | the format show|
00000180 6e 20 69 6e 20 66 69 67 75 72 65 20 31 2e 0d 0d |n in figure 1...|
00000190 0d 2b 2d 2d 2d 2d 2d 2d 2b 2d 2d 2d 2d 2d 2d 2b |.+------+------+|
000001a0 2d 2d 2d 2d 2d 2d 2d 2b 2d 2d 2d 2d 2d 2d 2b 2d |-------+------+-|
000001b0 2d 2d 2d 2d 2d 2b 2d 2d 2d 20 20 20 20 20 20 2d |-----+--- -|
000001c0 2d 2d 2b 0d 7c 20 20 20 20 43 6c 6f 63 6b 20 20 |--+.| Clock |
000001d0 20 20 7c 46 72 61 6d 69 6e 67 7c 20 4d 61 67 20 | |Framing| Mag |
000001e0 61 6e 64 20 52 6f 77 20 7c 20 20 34 30 20 62 79 |and Row | 40 by|
000001f0 74 65 73 20 20 7c 0d 7c 20 20 20 20 72 75 6e 2d |tes |.| run-|
00000200 69 6e 20 20 20 7c 63 6f 64 65 20 20 20 7c 20 41 |in |code | A|
00000210 64 64 72 20 47 72 6f 75 70 20 20 7c 20 20 6f 66 |ddr Group | of|
00000220 20 64 61 74 61 20 20 20 7c 0d 2b 2d 2d 2d 2d 2d | data |.+-----|
00000230 2d 2b 2d 2d 2d 2d 2d 2d 2b 2d 2d 2d 2d 2d 2d 2d |-+------+-------|
00000240 2b 2d 2d 2d 2d 2d 2d 2b 2d 2d 2d 2d 2d 2d 2b 2d |+------+------+-|
00000250 2d 2d 20 20 20 20 20 20 2d 2d 2d 2b 0d 20 20 20 |-- ---+. |
00000260 31 20 20 20 20 20 20 32 20 20 20 20 20 20 20 33 |1 2 3|
00000270 20 20 20 20 20 20 34 20 20 20 20 20 20 35 20 20 | 4 5 |
00000280 20 20 20 36 20 20 20 20 2d 20 20 20 34 35 20 20 | 6 - 45 |
00000290 62 79 74 65 20 6e 75 6d 62 65 72 0d 0d 46 69 67 |byte number..Fig|
000002a0 75 72 65 20 31 2e 20 54 68 65 20 64 61 74 61 20 |ure 1. The data |
000002b0 74 72 61 6e 73 6d 69 74 74 65 64 20 69 6e 20 61 |transmitted in a|
000002c0 20 54 65 6c 65 74 65 78 74 20 70 61 63 6b 65 74 | Teletext packet|
000002d0 0d 2d 2d 2d 2d 2d 2d 2d 2d 2d 2d 2d 2d 2d 2d 2d |.---------------|
000002e0 2d 2d 2d 2d 2d 2d 2d 2d 2d 2d 2d 2d 2d 2d 2d 2d |----------------|
*
00000300 2d 2d 2d 2d 0d 0d 0d 54 68 65 20 66 69 72 73 74 |----...The first|
00000310 20 74 77 6f 20 62 79 74 65 73 20 6f 66 20 65 76 | two bytes of ev|
00000320 65 72 79 20 70 61 63 6b 65 74 20 63 6f 6d 70 72 |ery packet compr|
00000330 69 73 65 20 74 68 65 20 63 6c 6f 63 6b 20 72 75 |ise the clock ru|
00000340 6e 2d 69 6e 20 73 65 71 75 65 6e 63 65 20 6f 66 |n-in sequence of|
00000350 0d 61 6c 74 65 72 6e 61 74 69 6e 67 20 62 69 74 |.alternating bit|
00000360 73 2c 20 62 65 67 69 6e 6e 69 6e 67 20 31 30 31 |s, beginning 101|
00000370 30 31 30 2e 2e 2e 20 61 6e 64 20 73 6f 20 6f 6e |010... and so on|
00000380 2e 20 54 68 65 73 65 20 74 77 6f 20 62 79 74 65 |. These two byte|
00000390 73 20 61 72 65 20 75 73 65 64 0d 74 6f 20 69 6e |s are used.to in|
000003a0 64 69 63 61 74 65 20 74 68 65 20 70 72 65 73 65 |dicate the prese|
000003b0 6e 63 65 20 6f 66 20 61 20 64 61 74 61 20 70 61 |nce of a data pa|
000003c0 63 6b 65 74 20 61 6e 64 20 74 6f 20 65 73 74 61 |cket and to esta|
000003d0 62 6c 69 73 68 20 74 68 65 20 74 69 6d 69 6e 67 |blish the timing|
000003e0 20 6f 66 0d 74 68 65 20 62 69 74 73 20 69 6e 20 | of.the bits in |
000003f0 74 68 61 74 20 70 61 63 6b 65 74 2e 0d 0d 54 68 |that packet...Th|
00000400 65 20 74 68 69 72 64 20 62 79 74 65 20 6f 66 20 |e third byte of |
00000410 65 76 65 72 79 20 70 61 63 6b 65 74 20 69 73 20 |every packet is |
00000420 74 68 65 20 66 72 61 6d 69 6e 67 20 63 6f 64 65 |the framing code|
00000430 20 25 31 31 31 30 30 31 30 30 2e 20 54 68 69 73 | %11100100. This|
00000440 20 63 6f 64 65 0d 77 61 73 20 73 65 6c 65 63 74 | code.was select|
00000450 65 64 20 74 6f 20 65 6e 61 62 6c 65 20 62 79 74 |ed to enable byt|
00000460 65 20 73 79 6e 63 68 72 6f 6e 69 73 61 74 69 6f |e synchronisatio|
00000470 6e 20 74 6f 20 62 65 20 65 73 74 61 62 6c 69 73 |n to be establis|
00000480 68 65 64 20 65 76 65 6e 20 69 66 20 6f 6e 65 0d |hed even if one.|
00000490 62 69 74 20 6f 66 20 74 68 65 20 66 72 61 6d 69 |bit of the frami|
000004a0 6e 67 20 63 6f 64 65 20 62 79 74 65 20 68 61 73 |ng code byte has|
000004b0 20 62 65 65 6e 20 77 72 6f 6e 67 6c 79 20 72 65 | been wrongly re|
000004c0 63 65 69 76 65 64 2e 20 54 68 65 20 66 72 61 6d |ceived. The fram|
000004d0 69 6e 67 20 63 6f 64 65 0d 62 79 74 65 20 63 61 |ing code.byte ca|
000004e0 6e 20 62 65 20 72 65 61 64 20 66 72 6f 6d 20 74 |n be read from t|
000004f0 68 65 20 61 64 61 70 74 6f 72 20 61 73 20 74 68 |he adaptor as th|
00000500 65 20 6e 75 6d 62 65 72 20 26 32 37 2c 20 25 30 |e number &27, %0|
00000510 30 31 30 30 31 31 31 2c 20 69 65 2e 20 77 69 74 |0100111, ie. wit|
00000520 68 0d 61 6c 6c 20 74 68 65 20 62 69 74 73 20 69 |h.all the bits i|
00000530 6e 20 74 68 65 20 62 79 74 65 20 72 65 76 65 72 |n the byte rever|
00000540 73 65 64 2e 0d 0d 54 68 65 20 6d 61 67 61 7a 69 |sed...The magazi|
00000550 6e 65 20 61 6e 64 20 70 61 63 6b 65 74 20 6e 75 |ne and packet nu|
00000560 6d 62 65 72 73 20 61 72 65 20 62 72 6f 61 64 63 |mbers are broadc|
00000570 61 73 74 20 69 6e 20 74 68 65 20 6e 65 78 74 20 |ast in the next |
00000580 74 77 6f 20 62 79 74 65 73 2c 20 6b 6e 6f 77 6e |two bytes, known|
00000590 0d 61 73 20 74 68 65 20 4d 61 67 61 7a 69 6e 65 |.as the Magazine|
000005a0 20 61 6e 64 20 52 6f 77 20 41 64 64 72 65 73 73 | and Row Address|
000005b0 20 47 72 6f 75 70 20 28 4d 52 41 47 29 2e 20 52 | Group (MRAG). R|
000005c0 6f 77 73 20 6f 66 20 54 65 6c 65 74 65 78 74 20 |ows of Teletext |
000005d0 64 61 74 61 20 68 61 76 65 0d 62 65 65 6e 20 72 |data have.been r|
000005e0 65 6e 61 6d 65 64 20 70 61 63 6b 65 74 73 20 62 |enamed packets b|
000005f0 75 74 20 74 68 65 20 74 65 72 6d 73 20 72 6f 77 |ut the terms row|
00000600 20 61 6e 64 20 70 61 63 6b 65 74 20 74 65 6e 64 | and packet tend|
00000610 20 74 6f 20 62 65 20 75 73 65 64 0d 69 6e 74 65 | to be used.inte|
00000620 72 63 68 61 6e 67 65 61 62 6c 79 2e 20 54 68 65 |rchangeably. The|
00000630 20 52 6f 77 20 69 6e 20 4d 52 41 47 20 72 65 66 | Row in MRAG ref|
00000640 65 72 73 20 74 6f 20 74 68 65 20 70 61 63 6b 65 |ers to the packe|
00000650 74 20 61 64 64 72 65 73 73 2e 20 42 65 63 61 75 |t address. Becau|
00000660 73 65 20 6f 66 0d 74 68 65 20 70 6f 73 73 69 62 |se of.the possib|
00000670 6c 65 20 63 6f 6e 66 75 73 69 6f 6e 20 74 68 61 |le confusion tha|
00000680 74 20 63 61 6e 20 62 65 20 63 61 75 73 65 64 20 |t can be caused |
00000690 62 79 20 74 68 65 20 69 6e 74 65 72 63 68 61 6e |by the interchan|
000006a0 67 65 61 62 6c 65 20 75 73 65 20 6f 66 0d 74 68 |geable use of.th|
000006b0 65 73 65 20 74 65 72 6d 73 20 49 20 77 69 6c 6c |ese terms I will|
000006c0 20 75 73 65 20 72 6f 77 20 74 6f 20 72 65 66 65 | use row to refe|
000006d0 72 20 74 6f 20 74 68 65 20 31 36 20 2a 20 36 34 |r to the 16 * 64|
000006e0 20 62 79 74 65 20 72 6f 77 73 20 6f 66 20 6d 65 | byte rows of me|
000006f0 6d 6f 72 79 20 69 6e 0d 74 68 65 20 54 65 6c 65 |mory in.the Tele|
00000700 74 65 78 74 20 61 64 61 70 74 6f 72 20 61 6e 64 |text adaptor and|
00000710 20 70 61 63 6b 65 74 20 74 6f 20 72 65 66 65 72 | packet to refer|
00000720 20 74 6f 20 74 68 65 20 72 6f 77 73 20 6f 66 20 | to the rows of |
00000730 54 65 6c 65 74 65 78 74 20 64 61 74 61 0d 77 68 |Teletext data.wh|
00000740 69 63 68 20 61 72 65 20 73 74 6f 72 65 64 20 69 |ich are stored i|
00000750 6e 20 74 68 65 20 61 64 61 70 74 6f 72 27 73 20 |n the adaptor's |
00000760 6d 65 6d 6f 72 79 2e 0d 0d 54 68 65 20 66 6f 75 |memory...The fou|
00000770 72 74 68 20 61 6e 64 20 66 69 66 74 68 20 62 79 |rth and fifth by|
00000780 74 65 20 6f 66 20 65 76 65 72 79 20 70 61 63 6b |te of every pack|
00000790 65 74 2c 20 61 6e 64 20 61 20 6e 75 6d 62 65 72 |et, and a number|
000007a0 20 6f 66 20 6f 74 68 65 72 20 62 79 74 65 73 20 | of other bytes |
000007b0 69 6e 0d 73 70 65 63 69 66 69 63 20 70 61 63 6b |in.specific pack|
000007c0 65 74 73 2c 20 61 72 65 20 68 61 6d 6d 69 6e 67 |ets, are hamming|
000007d0 20 63 6f 64 65 73 20 77 68 69 63 68 20 63 6f 6e | codes which con|
000007e0 74 61 69 6e 20 34 20 6d 65 73 73 61 67 65 20 62 |tain 4 message b|
000007f0 69 74 73 0d 69 6e 74 65 72 6c 65 61 76 65 64 20 |its.interleaved |
00000800 77 69 74 68 20 34 20 70 72 6f 74 65 63 74 69 6f |with 4 protectio|
00000810 6e 20 62 69 74 73 2e 20 44 65 63 6f 64 69 6e 67 |n bits. Decoding|
00000820 20 74 68 65 20 68 61 6d 6d 69 6e 67 20 63 6f 64 | the hamming cod|
00000830 65 64 20 62 79 74 65 73 20 69 6e 74 6f 0d 34 20 |ed bytes into.4 |
00000840 62 69 74 20 6e 79 62 62 6c 65 73 20 63 61 6e 20 |bit nybbles can |
00000850 63 6f 72 72 65 63 74 20 73 69 6e 67 6c 65 20 65 |correct single e|
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00000870 65 69 76 65 64 20 62 79 74 65 20 61 6e 64 20 64 |eived byte and d|
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00000890 72 72 6f 72 73 2e 0d 0d 54 68 65 20 74 77 6f 20 |rrors...The two |
000008a0 4d 52 41 47 20 62 79 74 65 73 20 61 72 65 20 68 |MRAG bytes are h|
000008b0 61 6d 6d 69 6e 67 20 63 6f 64 65 64 20 61 6e 64 |amming coded and|
000008c0 20 6d 75 73 74 20 62 65 20 64 65 63 6f 64 65 64 | must be decoded|
000008d0 20 62 65 66 6f 72 65 20 74 68 65 79 20 63 61 6e | before they can|
000008e0 0d 62 65 20 75 73 65 64 2e 20 45 61 63 68 20 62 |.be used. Each b|
000008f0 79 74 65 20 64 65 63 6f 64 65 73 20 69 6e 74 6f |yte decodes into|
00000900 20 61 20 34 20 62 69 74 20 6e 79 62 62 6c 65 2e | a 4 bit nybble.|
00000910 20 54 68 65 20 6d 61 67 61 7a 69 6e 65 20 6e 75 | The magazine nu|
00000920 6d 62 65 72 20 69 73 20 61 20 33 0d 62 69 74 20 |mber is a 3.bit |
00000930 6e 75 6d 62 65 72 20 61 6e 64 20 74 68 65 20 70 |number and the p|
00000940 61 63 6b 65 74 20 6e 75 6d 62 65 72 20 75 73 65 |acket number use|
00000950 73 20 35 20 62 69 74 73 2e 20 57 68 65 6e 20 79 |s 5 bits. When y|
00000960 6f 75 20 69 6e 74 65 72 66 61 63 65 20 64 69 72 |ou interface dir|
00000970 65 63 74 6c 79 0d 77 69 74 68 20 74 68 65 20 61 |ectly.with the a|
00000980 64 61 70 74 6f 72 2c 20 79 6f 75 20 68 61 76 65 |daptor, you have|
00000990 20 74 6f 20 64 65 63 6f 64 65 20 74 68 65 20 33 | to decode the 3|
000009a0 20 61 6e 64 20 35 20 62 69 74 20 6e 75 6d 62 65 | and 5 bit numbe|
000009b0 72 73 20 66 72 6f 6d 20 74 68 65 73 65 0d 74 77 |rs from these.tw|
000009c0 6f 20 6e 79 62 62 6c 65 73 2e 20 54 68 65 20 34 |o nybbles. The 4|
000009d0 30 20 62 79 74 65 73 20 6f 66 20 64 61 74 61 20 |0 bytes of data |
000009e0 77 68 69 63 68 20 66 6f 6c 6c 6f 77 20 74 68 65 |which follow the|
000009f0 20 6d 61 67 61 7a 69 6e 65 20 61 6e 64 20 70 61 | magazine and pa|
00000a00 63 6b 65 74 0d 6e 75 6d 62 65 72 73 20 68 61 76 |cket.numbers hav|
00000a10 65 20 61 20 76 61 72 69 61 62 6c 65 20 66 6f 72 |e a variable for|
00000a20 6d 61 74 20 77 68 69 63 68 20 64 65 70 65 6e 64 |mat which depend|
00000a30 73 20 75 70 6f 6e 20 74 68 65 20 70 61 63 6b 65 |s upon the packe|
00000a40 74 20 6e 75 6d 62 65 72 2e 0d 0d 50 61 63 6b 65 |t number...Packe|
00000a50 74 20 30 20 69 73 20 6b 6e 6f 77 6e 20 61 73 20 |t 0 is known as |
00000a60 74 68 65 20 50 61 67 65 20 48 65 61 64 65 72 2e |the Page Header.|
00000a70 20 50 61 63 6b 65 74 20 30 20 63 6f 6e 74 61 69 | Packet 0 contai|
00000a80 6e 73 20 38 20 61 64 64 69 74 69 6f 6e 61 6c 0d |ns 8 additional.|
00000a90 68 61 6d 6d 69 6e 67 20 63 6f 64 65 64 20 62 79 |hamming coded by|
00000aa0 74 65 73 20 77 68 69 63 68 20 64 65 63 6f 64 65 |tes which decode|
00000ab0 20 74 6f 20 67 69 76 65 20 74 68 65 20 70 61 67 | to give the pag|
00000ac0 65 20 61 6e 64 20 73 75 62 2d 70 61 67 65 20 6e |e and sub-page n|
00000ad0 75 6d 62 65 72 73 20 61 6e 64 0d 61 20 73 65 72 |umbers and.a ser|
00000ae0 69 65 73 20 6f 66 20 63 6f 6e 74 72 6f 6c 20 62 |ies of control b|
00000af0 69 74 73 2e 20 49 74 20 61 6c 73 6f 20 63 6f 6e |its. It also con|
00000b00 74 61 69 6e 73 20 74 68 65 20 68 65 61 64 65 72 |tains the header|
00000b10 20 69 6e 66 6f 72 6d 61 74 69 6f 6e 20 79 6f 75 | information you|
00000b20 20 73 65 65 0d 64 69 73 70 6c 61 79 65 64 20 61 | see.displayed a|
00000b30 74 20 74 68 65 20 74 6f 70 20 6f 66 20 61 20 54 |t the top of a T|
00000b40 65 6c 65 74 65 78 74 20 70 61 67 65 20 69 6e 20 |eletext page in |
00000b50 74 68 65 20 74 65 72 6d 69 6e 61 6c 20 6d 6f 64 |the terminal mod|
00000b60 65 2e 20 44 65 63 6f 64 69 6e 67 20 74 68 65 0d |e. Decoding the.|
00000b70 69 6e 66 6f 72 6d 61 74 69 6f 6e 20 63 6f 6e 74 |information cont|
00000b80 61 69 6e 65 64 20 69 6e 20 70 61 63 6b 65 74 20 |ained in packet |
00000b90 30 20 77 69 6c 6c 20 62 65 20 64 65 61 6c 74 20 |0 will be dealt |
00000ba0 77 69 74 68 20 69 6e 20 74 68 69 73 20 6d 6f 64 |with in this mod|
00000bb0 75 6c 65 20 61 6e 64 0d 75 73 69 6e 67 20 74 68 |ule and.using th|
00000bc0 65 20 69 6e 66 6f 72 6d 61 74 69 6f 6e 20 77 69 |e information wi|
00000bd0 6c 6c 20 62 65 20 63 6f 76 65 72 65 64 20 69 6e |ll be covered in|
00000be0 20 6d 6f 64 75 6c 65 73 20 33 20 61 6e 64 20 34 | modules 3 and 4|
00000bf0 2e 0d 0d 50 61 63 6b 65 74 20 6e 75 6d 62 65 72 |...Packet number|
00000c00 73 20 31 20 74 6f 20 32 33 20 63 6f 6e 74 61 69 |s 1 to 23 contai|
00000c10 6e 20 74 68 65 20 69 6e 66 6f 72 6d 61 74 69 6f |n the informatio|
00000c20 6e 20 64 69 73 70 6c 61 79 65 64 20 62 79 20 74 |n displayed by t|
00000c30 68 65 20 6c 65 76 65 6c 20 31 0d 54 65 6c 65 74 |he level 1.Telet|
00000c40 65 78 74 20 74 65 72 6d 69 6e 61 6c 2e 20 54 68 |ext terminal. Th|
00000c50 65 79 20 63 61 6e 20 61 6c 73 6f 20 63 6f 6e 74 |ey can also cont|
00000c60 61 69 6e 20 72 65 64 65 66 69 6e 61 62 6c 65 20 |ain redefinable |
00000c70 74 65 6c 65 73 6f 66 74 77 61 72 65 20 66 6f 72 |telesoftware for|
00000c80 6d 61 74 0d 28 52 54 46 29 20 63 6f 6d 70 75 74 |mat.(RTF) comput|
00000c90 65 72 20 73 6f 66 74 77 61 72 65 2e 20 54 68 65 |er software. The|
00000ca0 20 64 65 63 6f 64 69 6e 67 20 6f 66 20 74 65 6c | decoding of tel|
00000cb0 65 73 6f 66 74 77 61 72 65 20 77 69 6c 6c 20 6e |esoftware will n|
00000cc0 6f 74 20 62 65 20 63 6f 76 65 72 65 64 0d 69 6e |ot be covered.in|
00000cd0 20 74 68 65 73 65 20 6d 6f 64 75 6c 65 73 20 61 | these modules a|
00000ce0 6e 64 20 69 73 20 62 65 73 74 20 6c 65 66 74 20 |nd is best left |
00000cf0 74 6f 20 74 68 65 20 41 54 53 2e 0d 0d 50 61 63 |to the ATS...Pac|
00000d00 6b 65 74 20 6e 75 6d 62 65 72 73 20 67 72 65 61 |ket numbers grea|
00000d10 74 65 72 20 74 68 61 6e 20 32 33 20 61 72 65 20 |ter than 23 are |
00000d20 73 6f 6d 65 74 69 6d 65 73 20 72 65 66 65 72 72 |sometimes referr|
00000d30 65 64 20 74 6f 20 61 73 20 27 47 68 6f 73 74 20 |ed to as 'Ghost |
00000d40 72 6f 77 73 27 0d 6f 72 20 27 47 68 6f 73 74 20 |rows'.or 'Ghost |
00000d50 70 61 63 6b 65 74 73 27 20 61 6e 64 20 74 68 65 |packets' and the|
00000d60 79 20 61 72 65 20 6e 6f 74 20 6e 6f 72 6d 61 6c |y are not normal|
00000d70 6c 79 20 64 69 73 70 6c 61 79 65 64 20 62 79 20 |ly displayed by |
00000d80 6c 65 76 65 6c 20 31 20 54 65 6c 65 74 65 78 74 |level 1 Teletext|
00000d90 2e 0d 41 6c 6c 20 70 61 63 6b 65 74 73 20 66 72 |..All packets fr|
00000da0 6f 6d 20 30 20 74 6f 20 33 31 20 63 61 6e 20 62 |om 0 to 31 can b|
00000db0 65 20 72 65 63 65 69 76 65 64 20 62 79 20 74 68 |e received by th|
00000dc0 65 20 41 63 6f 72 6e 20 54 65 6c 65 74 65 78 74 |e Acorn Teletext|
00000dd0 20 61 64 61 70 74 6f 72 20 61 6e 64 0d 69 74 20 | adaptor and.it |
00000de0 69 73 20 69 6e 74 65 72 65 73 74 69 6e 67 20 74 |is interesting t|
00000df0 6f 20 6b 6e 6f 77 20 77 68 61 74 20 74 68 65 20 |o know what the |
00000e00 73 6f 2d 63 61 6c 6c 65 64 20 67 68 6f 73 74 20 |so-called ghost |
00000e10 70 61 63 6b 65 74 73 20 63 6f 6e 74 61 69 6e 2e |packets contain.|
00000e20 0d 0d 50 61 63 6b 65 74 73 20 32 34 20 61 6e 64 |..Packets 24 and|
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00006840 20 72 65 67 69 73 74 65 72 2c 20 72 65 61 64 20 | register, read |
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00006bc0 20 43 4c 49 20 20 20 20 20 20 20 20 20 20 20 5c | CLI \|
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00006d30 73 73 65 64 0d 20 20 38 32 30 20 20 20 20 20 20 |ssed. 820 |
00006d40 20 20 20 4c 44 41 20 23 26 37 45 20 20 20 20 20 | LDA #&7E |
00006d50 20 5c 20 64 65 63 69 6d 61 6c 20 31 32 36 0d 20 | \ decimal 126. |
00006d60 20 38 33 30 20 20 20 20 20 20 20 20 20 4a 53 52 | 830 JSR|
00006d70 20 6f 73 62 79 74 65 20 20 20 20 5c 20 61 63 6b | osbyte \ ack|
00006d80 6e 6f 77 6c 65 64 67 65 20 65 73 63 61 70 65 0d |nowledge escape.|
00006d90 20 20 38 34 30 20 2e 61 6c 6c 6f 76 65 72 0d 20 | 840 .allover. |
00006da0 20 38 35 30 20 20 20 20 20 20 20 20 20 4c 44 41 | 850 LDA|
00006db0 20 23 26 30 30 0d 20 20 38 36 30 20 20 20 20 20 | #&00. 860 |
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00006dd0 6f 6c 20 5c 20 64 69 73 61 62 6c 65 20 54 54 58 |ol \ disable TTX|
00006de0 0d 20 20 38 37 30 20 20 20 20 20 20 20 20 20 4c |. 870 L|
00006df0 44 58 20 6f 6c 64 69 72 71 32 76 20 20 5c 20 6c |DX oldirq2v \ l|
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00006e10 74 6f 72 0d 20 20 38 38 30 20 20 20 20 20 20 20 |tor. 880 |
00006e20 20 20 4c 44 59 20 6f 6c 64 69 72 71 32 76 2b 31 | LDY oldirq2v+1|
00006e30 0d 20 20 38 39 30 20 20 20 20 20 20 20 20 20 53 |. 890 S|
00006e40 45 49 20 20 20 20 20 20 20 20 20 20 20 5c 20 64 |EI \ d|
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00006e60 73 20 77 68 65 6e 20 76 65 63 74 6f 72 73 20 61 |s when vectors a|
00006e70 72 65 20 61 6c 74 65 72 65 64 0d 20 20 39 30 30 |re altered. 900|
00006e80 20 20 20 20 20 20 20 20 20 53 54 58 20 69 72 71 | STX irq|
00006e90 32 76 20 20 20 20 20 5c 20 72 65 73 74 6f 72 65 |2v \ restore|
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00006eb0 0d 20 20 39 31 30 20 20 20 20 20 20 20 20 20 53 |. 910 S|
00006ec0 54 59 20 69 72 71 32 76 2b 31 0d 20 20 39 32 30 |TY irq2v+1. 920|
00006ed0 20 20 20 20 20 20 20 20 20 43 4c 49 20 20 20 20 | CLI |
00006ee0 20 20 20 20 20 20 20 5c 20 72 65 2d 65 6e 61 62 | \ re-enab|
00006ef0 6c 65 20 69 6e 74 65 72 72 75 70 74 73 0d 20 20 |le interrupts. |
00006f00 39 33 30 20 20 20 20 20 20 20 20 20 52 54 53 20 |930 RTS |
00006f10 20 20 20 20 20 20 20 20 20 20 5c 20 72 65 74 75 | \ retu|
00006f20 72 6e 20 74 6f 20 42 41 53 49 43 0d 20 20 39 34 |rn to BASIC. 94|
00006f30 30 20 2e 69 6e 74 65 72 72 75 70 74 0d 20 20 39 |0 .interrupt. 9|
00006f40 35 30 20 20 20 20 20 20 20 20 20 42 49 54 20 74 |50 BIT t|
00006f50 74 78 73 74 61 74 75 73 20 5c 20 70 6f 6c 6c 20 |txstatus \ poll |
00006f60 54 54 58 20 68 61 72 64 77 61 72 65 0d 20 20 39 |TTX hardware. 9|
00006f70 36 30 20 20 20 20 20 20 20 20 20 42 4d 49 20 74 |60 BMI t|
00006f80 74 78 69 6e 74 65 72 20 20 5c 20 62 72 61 6e 63 |txinter \ branc|
00006f90 68 20 69 66 20 54 54 58 20 69 6e 74 65 72 72 75 |h if TTX interru|
00006fa0 70 74 0d 20 20 39 37 30 20 20 20 20 20 20 20 20 |pt. 970 |
00006fb0 20 4a 4d 50 20 28 6f 6c 64 69 72 71 32 76 29 20 | JMP (oldirq2v) |
00006fc0 5c 20 6e 6f 74 20 54 54 58 20 69 6e 74 65 72 72 |\ not TTX interr|
00006fd0 75 70 74 0d 20 20 39 38 30 20 2e 74 74 78 69 6e |upt. 980 .ttxin|
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000080e0 20 20 20 45 51 55 44 20 26 30 35 46 46 46 46 30 | EQUD &05FFFF0|
000080f0 34 20 0d 20 32 31 31 30 20 20 20 20 20 20 20 20 |4 . 2110 |
00008100 20 45 51 55 44 20 26 46 46 30 34 30 34 30 34 20 | EQUD &FF040404 |
00008110 0d 20 32 31 32 30 20 20 20 20 20 20 20 20 20 45 |. 2120 E|
00008120 51 55 44 20 26 46 46 30 46 30 32 46 46 20 0d 20 |QUD &FF0F02FF . |
00008130 32 31 33 30 20 20 20 20 20 20 20 20 20 45 51 55 |2130 EQU|
00008140 44 20 26 30 37 46 46 46 46 30 34 0d 20 32 31 34 |D &07FFFF04. 214|
00008150 30 20 20 20 20 20 20 20 20 20 45 51 55 44 20 26 |0 EQUD &|
00008160 30 35 30 35 30 35 46 46 20 0d 20 32 31 35 30 20 |050505FF . 2150 |
00008170 20 20 20 20 20 20 20 20 45 51 55 44 20 26 30 35 | EQUD &05|
00008180 46 46 46 46 30 34 20 0d 20 32 31 36 30 20 20 20 |FFFF04 . 2160 |
00008190 20 20 20 20 20 20 45 51 55 44 20 26 30 35 46 46 | EQUD &05FF|
000081a0 46 46 30 36 20 0d 20 32 31 37 30 20 20 20 20 20 |FF06 . 2170 |
000081b0 20 20 20 20 45 51 55 44 20 26 46 46 30 33 30 45 | EQUD &FF030E|
000081c0 46 46 0d 20 32 31 38 30 20 20 20 20 20 20 20 20 |FF. 2180 |
000081d0 20 45 51 55 44 20 26 46 46 30 31 30 43 46 46 20 | EQUD &FF010CFF |
000081e0 0d 20 32 31 39 30 20 20 20 20 20 20 20 20 20 45 |. 2190 E|
000081f0 51 55 44 20 26 30 39 46 46 46 46 30 41 20 0d 20 |QUD &09FFFF0A . |
00008200 32 32 30 30 20 20 20 20 20 20 20 20 20 45 51 55 |2200 EQU|
00008210 44 20 26 30 42 46 46 46 46 30 41 20 0d 20 32 32 |D &0BFFFF0A . 22|
00008220 31 30 20 20 20 20 20 20 20 20 20 45 51 55 44 20 |10 EQUD |
00008230 26 46 46 30 41 30 41 30 41 0d 20 32 32 32 30 20 |&FF0A0A0A. 2220 |
00008240 20 20 20 20 20 20 20 20 45 51 55 44 20 26 30 42 | EQUD &0B|
00008250 46 46 46 46 30 38 20 0d 20 32 32 33 30 20 20 20 |FFFF08 . 2230 |
00008260 20 20 20 20 20 20 45 51 55 44 20 26 46 46 30 44 | EQUD &FF0D|
00008270 30 30 46 46 20 0d 20 32 32 34 30 20 20 20 20 20 |00FF . 2240 |
00008280 20 20 20 20 45 51 55 44 20 26 30 42 30 42 30 42 | EQUD &0B0B0B|
00008290 46 46 20 0d 20 32 32 35 30 20 20 20 20 20 20 20 |FF . 2250 |
000082a0 20 20 45 51 55 44 20 26 30 42 46 46 46 46 30 41 | EQUD &0BFFFF0A|
000082b0 0d 20 32 32 36 30 20 20 20 20 20 20 20 20 20 45 |. 2260 E|
000082c0 51 55 44 20 26 30 43 46 46 30 43 30 43 20 0d 20 |QUD &0CFF0C0C . |
000082d0 32 32 37 30 20 20 20 20 20 20 20 20 20 45 51 55 |2270 EQU|
000082e0 44 20 26 46 46 30 44 30 43 46 46 20 0d 20 32 32 |D &FF0D0CFF . 22|
000082f0 38 30 20 20 20 20 20 20 20 20 20 45 51 55 44 20 |80 EQUD |
00008300 26 46 46 30 46 30 43 46 46 20 0d 20 32 32 39 30 |&FF0F0CFF . 2290|
00008310 20 20 20 20 20 20 20 20 20 45 51 55 44 20 26 30 | EQUD &0|
00008320 37 46 46 46 46 30 41 0d 20 32 33 30 30 20 20 20 |7FFFF0A. 2300 |
00008330 20 20 20 20 20 20 45 51 55 44 20 26 46 46 30 44 | EQUD &FF0D|
00008340 30 43 46 46 20 0d 20 32 33 31 30 20 20 20 20 20 |0CFF . 2310 |
00008350 20 20 20 20 45 51 55 44 20 26 30 44 30 44 46 46 | EQUD &0D0DFF|
00008360 30 44 20 0d 20 32 33 32 30 20 20 20 20 20 20 20 |0D . 2320 |
00008370 20 20 45 51 55 44 20 26 30 42 46 46 46 46 30 36 | EQUD &0BFFFF06|
00008380 20 0d 20 32 33 33 30 20 20 20 20 20 20 20 20 20 | . 2330 |
00008390 45 51 55 44 20 26 46 46 30 44 30 45 46 46 0d 20 |EQUD &FF0D0EFF. |
000083a0 32 33 34 30 20 20 20 20 20 20 20 20 20 45 51 55 |2340 EQU|
000083b0 44 20 26 30 39 46 46 46 46 30 38 20 0d 20 32 33 |D &09FFFF08 . 23|
000083c0 35 30 20 20 20 20 20 20 20 20 20 45 51 55 44 20 |50 EQUD |
000083d0 26 30 39 30 39 30 39 46 46 20 0d 20 32 33 36 30 |&090909FF . 2360|
000083e0 20 20 20 20 20 20 20 20 20 45 51 55 44 20 26 46 | EQUD &F|
000083f0 46 30 46 30 32 46 46 20 0d 20 32 33 37 30 20 20 |F0F02FF . 2370 |
00008400 20 20 20 20 20 20 20 45 51 55 44 20 26 30 39 46 | EQUD &09F|
00008410 46 46 46 30 41 0d 20 32 33 38 30 20 20 20 20 20 |FFF0A. 2380 |
00008420 20 20 20 20 45 51 55 44 20 26 46 46 30 38 30 38 | EQUD &FF0808|
00008430 30 38 20 0d 20 32 33 39 30 20 20 20 20 20 20 20 |08 . 2390 |
00008440 20 20 45 51 55 44 20 26 30 39 46 46 46 46 30 38 | EQUD &09FFFF08|
00008450 20 0d 20 32 34 30 30 20 20 20 20 20 20 20 20 20 | . 2400 |
00008460 45 51 55 44 20 26 30 42 46 46 46 46 30 38 20 0d |EQUD &0BFFFF08 .|
00008470 20 32 34 31 30 20 20 20 20 20 20 20 20 20 45 51 | 2410 EQ|
00008480 55 44 20 26 46 46 30 33 30 45 46 46 0d 20 32 34 |UD &FF030EFF. 24|
00008490 32 30 20 20 20 20 20 20 20 20 20 45 51 55 44 20 |20 EQUD |
000084a0 26 46 46 30 46 30 43 46 46 20 0d 20 32 34 33 30 |&FF0F0CFF . 2430|
000084b0 20 20 20 20 20 20 20 20 20 45 51 55 44 20 26 30 | EQUD &0|
000084c0 39 46 46 46 46 30 34 20 0d 20 32 34 34 30 20 20 |9FFFF04 . 2440 |
000084d0 20 20 20 20 20 20 20 45 51 55 44 20 26 30 46 30 | EQUD &0F0|
000084e0 46 46 46 30 46 20 0d 20 32 34 35 30 20 20 20 20 |FFF0F . 2450 |
000084f0 20 20 20 20 20 45 51 55 44 20 26 46 46 30 46 30 | EQUD &FF0F0|
00008500 45 46 46 0d 20 32 34 36 30 20 20 20 20 20 20 20 |EFF. 2460 |
00008510 20 20 45 51 55 44 20 26 30 35 46 46 46 46 30 38 | EQUD &05FFFF08|
00008520 20 0d 20 32 34 37 30 20 20 20 20 20 20 20 20 20 | . 2470 |
00008530 45 51 55 44 20 26 46 46 30 44 30 45 46 46 20 0d |EQUD &FF0D0EFF .|
00008540 20 32 34 38 30 20 20 20 20 20 20 20 20 20 45 51 | 2480 EQ|
00008550 55 44 20 26 46 46 30 46 30 45 46 46 20 0d 20 32 |UD &FF0F0EFF . 2|
00008560 34 39 30 20 20 20 20 20 20 20 20 20 45 51 55 44 |490 EQUD|
00008570 20 26 30 45 46 46 30 45 30 45 0d 20 32 35 30 30 | &0EFF0E0E. 2500|
00008580 20 5d 0d 20 32 35 31 30 20 4e 45 58 54 0d 20 32 | ]. 2510 NEXT. 2|
00008590 35 32 30 20 45 4e 44 50 52 4f 43 0d |520 ENDPROC.|
0000859c 
.