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OSBITS - An Exploration of the BBC Micro at Machine Level

By Programmer

..........................................................


Part 16: Vectors


With this module we return to the workings of the BBC Micro
operating system.

You may already know the word 'indirection' from BBC BASIC. 
There you have the indirection operators ? and ! which, when
applied to a byte address, return the number held either in
that byte (with ?) or in the 4 byte word starting at that
byte (with !).  Vectors provide a way of intercepting the
action of routines because those routines are indirected
through the vector. 

Here is an example.  Say we have a subroutine at address
'work'.  We would call it by going JSR work.  The following
code might occur at that address.

                      .work
                      JMP(workvec)

You will recall (I hope) that JMP followed by an address in
brackets means that the microprocessor will jump to the
address held in the two bytes starting at the one in the
brackets.  In this case the execution of the program will
jump to the address held in 'workvec' and 'workvec+1'.
[Note that your vector, as with any indirect JMP address,
should never be the last byte of a page, i.e. have FF as the
last byte of the address.  This is because of a bug in the
6502 which stops it interpreting the jump correctly in this
instance.]


'workvec' is a vector.  But what is the point of flying
around the microprocessor's memory like this?  It should
become clearer when you think that the address 'workvec' is
in RAM, and can be modified by you, whereas the original
call address 'work' was maybe in the operating system in ROM
and so could not be changed.

And why might you want to modify the operation of such a
routine?  Well to a certain extent that is up to your
ingenuity but you could add your own * commands by
intercepting the CLI vector or add a new *FX call by
intercepting the OSBYTE vector.  The thing to remember is
that any intercepting will have a semi-permanent effect and
so could modify all programs run on your machine whatever
the language in which they were written.  I say
semi-permanent because all vectors are reset to their
default values when the BREAK key is pressed.

Let's look at the vectors and then go on to discuss an
intercepting application.

In rising address order the vectors are as follows:

    &200    USERV - The User Vector

Certain unrecognised OSWORD calls, with accumulator values
between 224 and 255 pass this way as do *CODE and *LINE. 
This enables you to set up routines of your own which your
programs can call without needing to know where they are, or
indeed in which processor the calling program is. The user
vector only exists in the I/O processor and the program in
this module uses it with *CODE.

    &202     BRKV - The Break Vector

When an interrupt occurs or a BRK is reached in a program
being executed the OS routes program execution through here. 
The language currently in use usually sets up its own
vectors here but you could put in your own routine.  In
general though this is not an area to play around in, so be
careful or at least expect to crash the machine a few times!
Incidentally, this is nothing to do with pressing the BREAK
key.

    &204 and &206   IRQ1V and IRQ2V  -  Interrupt Vectors

The operating system uses these to process interrupts.  We
will come onto these in another module but for completeness
IRQ1 is the route for all interrupts and IRQ2 is the route
for those the OS does not recognise.

    &208     CLIV - The Command Line Interpreter Vector

All * commands pass this way before the OS decides what to
do with them.  OSCLI indirects through here.

    &20A     BYTEV - OSBYTE Indirection Vector
    &20B     WORDV - OSWORD Indirection Vector

OSBYTE/OSWORD calls indirect this way before being
processed.

    &20E     WRCHV - The Write Character Vector

All writes to the screen or printer or wherever pass through
here.

    &210     RDCHV - The Read Character Vector

Characters read from the current input stream pass this way.

    &212     FILEV - File Vector (OSFILE)
    &214     ARGSV - Arguments Vector (OSARGS)
    &216     BGETV - Byte get Vector (OSBGET)
    &218     BPUTV - Byte put Vector (OSBPUT)
    &21A     GBPBV - Block byte vector (OSGBPB)
    &21C     FINDV - Open and Close File (OSFIND)

These vectors pass the commands for the relevant OS routines
for filing systems.

    &21E     FSCV  - Misc Filing Functions

There are further filing functions, such as reading a
catalogue and checking for End of File that are carried out
down this vector.

    &220     EVNTV - Event Vector

Events are a specially packaged form of interrupt.  We will
come on to events next time.

    &222     UPTV  - User Print Vector

You can write your own printer driver software which will be
called when OSBYTE 5,3 (*FX5,3) is carried out.  The address
of your routine should be placed in this vector.

    &224     NETV  - The Network Vector

This is used by the Econet network system.

    &226     VDUV  - VDU Extension Vector

If a non-standard VDU command is made the OS will come down
this vector.  This can happen if you attempt to plot in a
non graphics mode, or by using an unrecognised PLOT number
or by using VDU23,n with n in the range 2 to 31.

    &228     KEYV  - The Keyboard Vector
    &22A     INSV  - Buffer Insert Vector
    &22C     REMV  - Buffer Remove Vector
    &22E     CNPV  - Buffer Count/Purge Vector

These vectors indirect OS routines to control the keyboard
and buffers.

There is also spare vector space between &230 and &235. 
Some of these vectors occur only in the I/O processor (the
BBC Micro itself) and in the case of events the behaviour of
the system is slightly different when you intercept in a
second processor.  In general I think you would probably
want to run your intercepting code in the I/O processor. 
I'll expand on this in a moment.

For detailed information on the register and flag settings
you might expect on intercepting these vectors you should
consult your advanced guide.  We shall, however, go on to
discuss one vector in more detail.

The WRCHV, write character vector, is the route for all the
bytes sent to the VDU drivers when printing text or drawing
graphics or even clearing the screen.  I wrote a routine to
print out the bytes passing down this vector as an aid to
debugging graphics programs, particularly when using a 32016
second processor.

You don't actually have to do anything to get the bytes
flowing through this vector.  They will do so anyway.  To
understand what the byte actually means we must introduce
the concept of the VDU queue and what I shall call main and
tacked-on bytes.

On page 378 of the original BBC Micro User Guide is a table
which summarises the VDU codes.  If you have this book then
it's worth finding this table.  There are three different
general areas into which bytes can fall.  They can be
control codes, values between 0 and 31, they can be
printable ASCII characters, between 32 and 126, and they can
be extra characters when between 128 and 255.  127 is a
little special in that it is an ASCII character (delete) but
is not strictly printable.

When a byte passes to the VDU drivers it is either printed
or is used to control a function.  The values up to 31
control a function and include such things as changing mode,
clearing the screen and plotting.  The allocation of these
functions is not universal.  Although code 13 is generally
used for a carriage return the same is not true of the other
numbers.

The VDU drivers will sometimes expect a main byte to have
one or more other bytes tacked on to the end to complete a
control sequence.  For example code 22 has to be followed by
a second byte because 22 means 'change screen mode' and the
second byte is the new mode.  When that 22 enters the
drivers the VDU queue is set to 1 and the command is not
processed until that second byte has arrived.  The byte with
the longest queue is 23 (VDU 23) which sets up a queue 9
bytes long.  Only control codes can have tacked on bytes.

When we look at what passes down the write character vector
we must keep an eye on the VDU queue to see what the byte we
have is actually for.  Since we look at a byte BEFORE it
reaches the drivers we are actually one step ahead of them
and reading the queue length gives us an incorrect answer. 
Fortunately we only need to know which bytes are main bytes
and they will always arrive at the driver when the queue is
of zero length.

So how do we read the queue length.  OSBYTE 218 will return
this to us if we enter it with 0 in X and 255 (&FF) in Y. 
This is because 218 is one of many OSBYTE calls that will
read or write separate bits of operating system variables.
The number written into the variable is:

               (old value  AND Y) EOR X

So to read a location you set X to 0 and Y to &FF and the
result is in X.  To write you set X to the value you wish to
write and set Y to 0.  The old value of the variable is in X
on exit.

In the case of OSBYTE 218 the variable is actually the 2's
complement negative of the queue length.

B/osb16 deals with three issues raised by intercepting
vectors.  Firstly the OS is vulnerable during the changing
of a vector.  An interrupt could occur when one of the two
bytes has been changed but before the other.  To stop this
the interrupt flag is set which stops maskable interrupts
occurring.  This will be explained more in the Interrupts
module.

Secondly we must make sure the vector changing routine is
executed only once.  More on this later in this module.

Finally we have to put the routine in the I/O processor to
be sure it will work for all cases and all types of second
processor.  Having done this it is obviously going to be
more difficult to switch the intercept routine on and off
but fortunately there is an OS function we can use.

There are two little known built in * commands, *CODE and
*LINE.  *CODE is followed by up to two bytes, separated by
spaces or commas just like *FX.  *LINE is followed by a
string.  When either is executed in whichever processor in
your computer the I/O processor jumps to the contents of the
User Vector at &200.  In this module I have used *CODE but I
will explain both.

The User Vector should be changed to point to some new code
you have just written which is expecting the *CODE or *LINE. 
The module program will illustrate how.  On entry to your
routine from *CODE the X and Y registers will be set as
for the numbers typed in after *CODE.  The accumulator will
contain zero.  With *LINE followed by text and terminated
with a carriage return the OS will put the line of text
somewhere in memory and on entry to your routine A will
contain 1 and X will contain the low byte of the address
where the OS has stored the string and Y will contain the
high byte.

These are particularly useful if you want to be able to
switch your intercept on and off from the keyboard, as we do
here.  In addition *CODE is equivalent to OSBYTE 136
(*FX136).  If there is no user code pointed to by the user
vector calling *CODE or *LINE will generate a 'Bad Command'
message.

The code generated by B/osb16 is in three parts, although
they make up one program.  The first section changes the
user vector to point to our new code.  The second part
enables *CODE to switch the intercept on and off by
modifying the write character vector or restoring it to its
original value.  The final part is the intercept code
itself.  To use this program you will need a printer
connected and ready to go.  I have used the RS423 input
buffer (page &A) as home for this code and I hope that means
it will work with a serial printer as well as a parallel
one, but I only have a parallel printer.  In case of trouble
change the value of code% in line 110 to, say, &C00 if you
have no user defined characters, or &1600 to make use of
some of the disc drive space (not in a Master).

To modify vectors you first disable interrupts with SEI and
then save the existing contents of the vector somewhere
safe.  Then you put the start address of your piece of code
into the vector (lo byte then hi byte) and finally you
re-enable interrupts again with CLI.  Saving the old vector
contents has two uses.  Firstly you can return them when you
have completely finished with the intercept and secondly you
can chain intercept routines together by jumping to the old
vector contents at the end of your routine.

Lines 250 to 410 save the existing user vector contents in
'old_user' and then replace them with the address of the
code starting at 'user'.  So that a second execution of this
piece of code will not attempt to reset the vectors again
the code checks that the high byte values are different
between the position of the new code ('user) and the
contents of the user vector.  Since the user vector usually
points way up into the OS ROM there should be no problems.

Lines 430 to 920 use *CODE to enable or disable the
intercept.  If the number in the accumulator is not 0 then
we are not responding to a *CODE so the routine branches to
'over_entry' where it jumps to the old vector contents. 
This means that a separate user vector user, like *CODE with
X>1 or *LINE, will still function.

We want X to be 0 or 1 on entry, if it is neither then the
routine also reaches 'over_entry' but if it is either then
we have to see which it is.  A flag, at 'intercept_flag' is
used to make sure that we do not try to reset a vector that
is already reset.  This is an alternative way of stopping
code pointing to itself and locking the machine out.

The intercept itself, starting at line 910, is bracketed
with pushes and pulls to the stack.  The status register,
followed by the X and Y registers is pushed, to be pulled in
the reverse order later.  The accumulator is stored in a
byte labelled 'accustore' for easy access.

Once the registers are saved we direct all output to the
printer using OSBYTE 3 with X=10.  On exit the old X value
is in X and we put that on the stack for use later.

The 'vdu_queue' subroutine returns with the size of the
queue in A and if that is the first character in a set of
bytes then we have to start a new line, print the character
if it is printable, print its hex value (in brackets), and
use spaces to get a neat layout.  All printing in this
routine is carried out using a non-vectored version of
OSWRCH called NVOSWRCH so that the intercepts don't get tied
up in knots.

If we are dealing with a tacked on byte, i.e. the VDU queue
routine returns something other than zero, we continue
printing hex values in brackets across the page.

Finally we restore the printer to its previous state using
OSBYTE 3 and the old value of X from the stack.  Registers
are pulled back from the stack and we head off back down the
old write character vector.  If we were to RTS here rather
than follow the old vector nothing would reach the screen. 
Try it and see.

The 'print_hex' subroutine at line 1460 is exactly like the
one in Module 6 except that to print a nybble I have used a
different method.  It's worth comparing the two particularly
if you feel there is any risk associated with using the
decimal mode on the 6502.  I'll come back to this under
interrupts!  There are two 'tricks' worth remembering here. 
Firstly as we have already masked off the top nybble of the
byte with AND &0F we cannot have anything in the top nybble. 
So if we want to add 48 (&30) we can do this by ORing with
&30 since this does not affect the lower nybble.  The reason
for doing this is that it replaces a CLC and an ADC with one
ORA which is smaller, faster and neater.  The second trick
is that following a BCC where you don't branch the carry
flag has to be set so if we ADC we are adding an extra one. 
Again this saves a CLC and so saves space and time.  (My
thanks to Peter Vince for reminding me of that one.)

The 'vdu_queue' subroutine uses OSBYTE 218 as explained
earlier.  It is simpler to take a 2's complement by EORing
with &FF and adding 1 than by subtracting from zero because
you don't have to find somewhere to store the number while
you LDA #0.

When you have RUN the program there will be a little line
you can copy across in order to *SAVE the machine code. 
This program will only run in the I/O processor, so switch
off your second processor if you have one.  The final
machine code will run from disc by just typing *wcint and
this will automatically load into the I/O processor because
I have set the top 16 bits of the 32 bit addresses in line
2030.  [I adopt a personal policy of using lower case
letters for any machine code program name.]

To try it out type in *wcint and then make sure your printer
is on and ready.  Type *CODE 1 and everything you then enter
will be intercepted.  Type *CODE to stop it (*CODE is
equivalent to *CODE 0).

If you enter:        *CODE 1
                     MODE 4
                     MOVE &321,&123
                     CLS
                     *CODE

you should get the following output.

    > [&3E]
    M [&4D]
    O [&4F]
    D [&44]
    E [&45]
      [&20]
    4 [&34]
      [&0A]
      [&0D]
      [&16] [&04]
    > [&3E]
    M [&4D]
    O [&4F]
    V [&56]
    E [&45]
      [&20]
    & [&26]
    3 [&33]
    2 [&32]
    1 [&31]
    , [&2C]
    & [&26]
    1 [&31]
    2 [&32]
    3 [&33]
      [&0A]
      [&0D]
      [&19] [&04] [&21] [&03] [&23] [&01]
    > [&3E]
    C [&43]
    L [&4C]
    S [&53]
      [&0A]
      [&0D]
      [&0C]
    > [&3E]
    * [&2A]
    C [&43]
    O [&4F]
    D [&44]
    E [&45]
      [&0A]
      [&0D]

This is showing how BASIC commands are translated into bytes
to be sent to the VDU drivers.  If you enable the intercept
while running a program you will get a similar, if much more
succinct, result.  The output can be SPOOLED to disc
although you will get extra characters with each byte where
it is printable.

This intercept method can be applied to any vector although
only the RDCH and WRCH carry any significant information in
the accumulator.  The CLI vector passes useful information
in X and Y pointing to strings.  You could modify this
routine to work with other vectors, but don't bother with
the EVENT, BREAK or IRQ vectors as you will simply crash the
machine there.  Those vectors are too sensitive.

Next time ..... EVENTS.  Until then, happy intercepting.

00000000  4f 53 42 49 54 53 20 2d  20 41 6e 20 45 78 70 6c  |OSBITS - An Expl|
00000010  6f 72 61 74 69 6f 6e 20  6f 66 20 74 68 65 20 42  |oration of the B|
00000020  42 43 20 4d 69 63 72 6f  20 61 74 20 4d 61 63 68  |BC Micro at Mach|
00000030  69 6e 65 20 4c 65 76 65  6c 0d 0d 42 79 20 50 72  |ine Level..By Pr|
00000040  6f 67 72 61 6d 6d 65 72  0d 0d 2e 2e 2e 2e 2e 2e  |ogrammer........|
00000050  2e 2e 2e 2e 2e 2e 2e 2e  2e 2e 2e 2e 2e 2e 2e 2e  |................|
*
00000080  2e 2e 2e 2e 0d 0d 0d 50  61 72 74 20 31 36 3a 20  |.......Part 16: |
00000090  56 65 63 74 6f 72 73 0d  0d 0d 57 69 74 68 20 74  |Vectors...With t|
000000a0  68 69 73 20 6d 6f 64 75  6c 65 20 77 65 20 72 65  |his module we re|
000000b0  74 75 72 6e 20 74 6f 20  74 68 65 20 77 6f 72 6b  |turn to the work|
000000c0  69 6e 67 73 20 6f 66 20  74 68 65 20 42 42 43 20  |ings of the BBC |
000000d0  4d 69 63 72 6f 0d 6f 70  65 72 61 74 69 6e 67 20  |Micro.operating |
000000e0  73 79 73 74 65 6d 2e 0d  0d 59 6f 75 20 6d 61 79  |system...You may|
000000f0  20 61 6c 72 65 61 64 79  20 6b 6e 6f 77 20 74 68  | already know th|
00000100  65 20 77 6f 72 64 20 27  69 6e 64 69 72 65 63 74  |e word 'indirect|
00000110  69 6f 6e 27 20 66 72 6f  6d 20 42 42 43 20 42 41  |ion' from BBC BA|
00000120  53 49 43 2e 20 0d 54 68  65 72 65 20 79 6f 75 20  |SIC. .There you |
00000130  68 61 76 65 20 74 68 65  20 69 6e 64 69 72 65 63  |have the indirec|
00000140  74 69 6f 6e 20 6f 70 65  72 61 74 6f 72 73 20 3f  |tion operators ?|
00000150  20 61 6e 64 20 21 20 77  68 69 63 68 2c 20 77 68  | and ! which, wh|
00000160  65 6e 0d 61 70 70 6c 69  65 64 20 74 6f 20 61 20  |en.applied to a |
00000170  62 79 74 65 20 61 64 64  72 65 73 73 2c 20 72 65  |byte address, re|
00000180  74 75 72 6e 20 74 68 65  20 6e 75 6d 62 65 72 20  |turn the number |
00000190  68 65 6c 64 20 65 69 74  68 65 72 20 69 6e 0d 74  |held either in.t|
000001a0  68 61 74 20 62 79 74 65  20 28 77 69 74 68 20 3f  |hat byte (with ?|
000001b0  29 20 6f 72 20 69 6e 20  74 68 65 20 34 20 62 79  |) or in the 4 by|
000001c0  74 65 20 77 6f 72 64 20  73 74 61 72 74 69 6e 67  |te word starting|
000001d0  20 61 74 20 74 68 61 74  0d 62 79 74 65 20 28 77  | at that.byte (w|
000001e0  69 74 68 20 21 29 2e 20  20 56 65 63 74 6f 72 73  |ith !).  Vectors|
000001f0  20 70 72 6f 76 69 64 65  20 61 20 77 61 79 20 6f  | provide a way o|
00000200  66 20 69 6e 74 65 72 63  65 70 74 69 6e 67 20 74  |f intercepting t|
00000210  68 65 0d 61 63 74 69 6f  6e 20 6f 66 20 72 6f 75  |he.action of rou|
00000220  74 69 6e 65 73 20 62 65  63 61 75 73 65 20 74 68  |tines because th|
00000230  6f 73 65 20 72 6f 75 74  69 6e 65 73 20 61 72 65  |ose routines are|
00000240  20 69 6e 64 69 72 65 63  74 65 64 0d 74 68 72 6f  | indirected.thro|
00000250  75 67 68 20 74 68 65 20  76 65 63 74 6f 72 2e 20  |ugh the vector. |
00000260  0d 0d 48 65 72 65 20 69  73 20 61 6e 20 65 78 61  |..Here is an exa|
00000270  6d 70 6c 65 2e 20 20 53  61 79 20 77 65 20 68 61  |mple.  Say we ha|
00000280  76 65 20 61 20 73 75 62  72 6f 75 74 69 6e 65 20  |ve a subroutine |
00000290  61 74 20 61 64 64 72 65  73 73 0d 27 77 6f 72 6b  |at address.'work|
000002a0  27 2e 20 20 57 65 20 77  6f 75 6c 64 20 63 61 6c  |'.  We would cal|
000002b0  6c 20 69 74 20 62 79 20  67 6f 69 6e 67 20 4a 53  |l it by going JS|
000002c0  52 20 77 6f 72 6b 2e 20  20 54 68 65 20 66 6f 6c  |R work.  The fol|
000002d0  6c 6f 77 69 6e 67 0d 63  6f 64 65 20 6d 69 67 68  |lowing.code migh|
000002e0  74 20 6f 63 63 75 72 20  61 74 20 74 68 61 74 20  |t occur at that |
000002f0  61 64 64 72 65 73 73 2e  0d 0d 20 20 20 20 20 20  |address...      |
00000300  20 20 20 20 20 20 20 20  20 20 20 20 20 20 20 20  |                |
00000310  2e 77 6f 72 6b 0d 20 20  20 20 20 20 20 20 20 20  |.work.          |
00000320  20 20 20 20 20 20 20 20  20 20 20 20 4a 4d 50 28  |            JMP(|
00000330  77 6f 72 6b 76 65 63 29  0d 0d 59 6f 75 20 77 69  |workvec)..You wi|
00000340  6c 6c 20 72 65 63 61 6c  6c 20 28 49 20 68 6f 70  |ll recall (I hop|
00000350  65 29 20 74 68 61 74 20  4a 4d 50 20 66 6f 6c 6c  |e) that JMP foll|
00000360  6f 77 65 64 20 62 79 20  61 6e 20 61 64 64 72 65  |owed by an addre|
00000370  73 73 20 69 6e 0d 62 72  61 63 6b 65 74 73 20 6d  |ss in.brackets m|
00000380  65 61 6e 73 20 74 68 61  74 20 74 68 65 20 6d 69  |eans that the mi|
00000390  63 72 6f 70 72 6f 63 65  73 73 6f 72 20 77 69 6c  |croprocessor wil|
000003a0  6c 20 6a 75 6d 70 20 74  6f 20 74 68 65 0d 61 64  |l jump to the.ad|
000003b0  64 72 65 73 73 20 68 65  6c 64 20 69 6e 20 74 68  |dress held in th|
000003c0  65 20 74 77 6f 20 62 79  74 65 73 20 73 74 61 72  |e two bytes star|
000003d0  74 69 6e 67 20 61 74 20  74 68 65 20 6f 6e 65 20  |ting at the one |
000003e0  69 6e 20 74 68 65 0d 62  72 61 63 6b 65 74 73 2e  |in the.brackets.|
000003f0  20 20 49 6e 20 74 68 69  73 20 63 61 73 65 20 74  |  In this case t|
00000400  68 65 20 65 78 65 63 75  74 69 6f 6e 20 6f 66 20  |he execution of |
00000410  74 68 65 20 70 72 6f 67  72 61 6d 20 77 69 6c 6c  |the program will|
00000420  0d 6a 75 6d 70 20 74 6f  20 74 68 65 20 61 64 64  |.jump to the add|
00000430  72 65 73 73 20 68 65 6c  64 20 69 6e 20 27 77 6f  |ress held in 'wo|
00000440  72 6b 76 65 63 27 20 61  6e 64 20 27 77 6f 72 6b  |rkvec' and 'work|
00000450  76 65 63 2b 31 27 2e 0d  5b 4e 6f 74 65 20 74 68  |vec+1'..[Note th|
00000460  61 74 20 79 6f 75 72 20  76 65 63 74 6f 72 2c 20  |at your vector, |
00000470  61 73 20 77 69 74 68 20  61 6e 79 20 69 6e 64 69  |as with any indi|
00000480  72 65 63 74 20 4a 4d 50  20 61 64 64 72 65 73 73  |rect JMP address|
00000490  2c 0d 73 68 6f 75 6c 64  20 6e 65 76 65 72 20 62  |,.should never b|
000004a0  65 20 74 68 65 20 6c 61  73 74 20 62 79 74 65 20  |e the last byte |
000004b0  6f 66 20 61 20 70 61 67  65 2c 20 69 2e 65 2e 20  |of a page, i.e. |
000004c0  68 61 76 65 20 46 46 20  61 73 20 74 68 65 0d 6c  |have FF as the.l|
000004d0  61 73 74 20 62 79 74 65  20 6f 66 20 74 68 65 20  |ast byte of the |
000004e0  61 64 64 72 65 73 73 2e  20 20 54 68 69 73 20 69  |address.  This i|
000004f0  73 20 62 65 63 61 75 73  65 20 6f 66 20 61 20 62  |s because of a b|
00000500  75 67 20 69 6e 20 74 68  65 0d 36 35 30 32 20 77  |ug in the.6502 w|
00000510  68 69 63 68 20 73 74 6f  70 73 20 69 74 20 69 6e  |hich stops it in|
00000520  74 65 72 70 72 65 74 69  6e 67 20 74 68 65 20 6a  |terpreting the j|
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00004080  20 62 69 74 20 61 64 64  72 65 73 73 65 73 20 69  | bit addresses i|
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000040e0  68 69 6e 65 20 63 6f 64  65 20 70 72 6f 67 72 61  |hine code progra|
000040f0  6d 20 6e 61 6d 65 2e 5d  0d 0d 54 6f 20 74 72 79  |m name.]..To try|
00004100  20 69 74 20 6f 75 74 20  74 79 70 65 20 69 6e 20  | it out type in |
00004110  2a 77 63 69 6e 74 20 61  6e 64 20 74 68 65 6e 20  |*wcint and then |
00004120  6d 61 6b 65 20 73 75 72  65 20 79 6f 75 72 20 70  |make sure your p|
00004130  72 69 6e 74 65 72 0d 69  73 20 6f 6e 20 61 6e 64  |rinter.is on and|
00004140  20 72 65 61 64 79 2e 20  20 54 79 70 65 20 2a 43  | ready.  Type *C|
00004150  4f 44 45 20 31 20 61 6e  64 20 65 76 65 72 79 74  |ODE 1 and everyt|
00004160  68 69 6e 67 20 79 6f 75  20 74 68 65 6e 20 65 6e  |hing you then en|
00004170  74 65 72 0d 77 69 6c 6c  20 62 65 20 69 6e 74 65  |ter.will be inte|
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00004190  43 4f 44 45 20 74 6f 20  73 74 6f 70 20 69 74 20  |CODE to stop it |
000041a0  28 2a 43 4f 44 45 20 69  73 0d 65 71 75 69 76 61  |(*CODE is.equiva|
000041b0  6c 65 6e 74 20 74 6f 20  2a 43 4f 44 45 20 30 29  |lent to *CODE 0)|
000041c0  2e 0d 0d 49 66 20 79 6f  75 20 65 6e 74 65 72 3a  |...If you enter:|
000041d0  20 20 20 20 20 20 20 20  2a 43 4f 44 45 20 31 0d  |        *CODE 1.|
000041e0  20 20 20 20 20 20 20 20  20 20 20 20 20 20 20 20  |                |
000041f0  20 20 20 20 20 4d 4f 44  45 20 34 0d 20 20 20 20  |     MODE 4.    |
00004200  20 20 20 20 20 20 20 20  20 20 20 20 20 20 20 20  |                |
00004210  20 4d 4f 56 45 20 26 33  32 31 2c 26 31 32 33 0d  | MOVE &321,&123.|
00004220  20 20 20 20 20 20 20 20  20 20 20 20 20 20 20 20  |                |
00004230  20 20 20 20 20 43 4c 53  0d 20 20 20 20 20 20 20  |     CLS.       |
00004240  20 20 20 20 20 20 20 20  20 20 20 20 20 20 2a 43  |              *C|
00004250  4f 44 45 0d 0d 79 6f 75  20 73 68 6f 75 6c 64 20  |ODE..you should |
00004260  67 65 74 20 74 68 65 20  66 6f 6c 6c 6f 77 69 6e  |get the followin|
00004270  67 20 6f 75 74 70 75 74  2e 0d 0d 20 20 20 20 3e  |g output...    >|
00004280  20 5b 26 33 45 5d 0d 20  20 20 20 4d 20 5b 26 34  | [&3E].    M [&4|
00004290  44 5d 0d 20 20 20 20 4f  20 5b 26 34 46 5d 0d 20  |D].    O [&4F]. |
000042a0  20 20 20 44 20 5b 26 34  34 5d 0d 20 20 20 20 45  |   D [&44].    E|
000042b0  20 5b 26 34 35 5d 0d 20  20 20 20 20 20 5b 26 32  | [&45].      [&2|
000042c0  30 5d 0d 20 20 20 20 34  20 5b 26 33 34 5d 0d 20  |0].    4 [&34]. |
000042d0  20 20 20 20 20 5b 26 30  41 5d 0d 20 20 20 20 20  |     [&0A].     |
000042e0  20 5b 26 30 44 5d 0d 20  20 20 20 20 20 5b 26 31  | [&0D].      [&1|
000042f0  36 5d 20 5b 26 30 34 5d  0d 20 20 20 20 3e 20 5b  |6] [&04].    > [|
00004300  26 33 45 5d 0d 20 20 20  20 4d 20 5b 26 34 44 5d  |&3E].    M [&4D]|
00004310  0d 20 20 20 20 4f 20 5b  26 34 46 5d 0d 20 20 20  |.    O [&4F].   |
00004320  20 56 20 5b 26 35 36 5d  0d 20 20 20 20 45 20 5b  | V [&56].    E [|
00004330  26 34 35 5d 0d 20 20 20  20 20 20 5b 26 32 30 5d  |&45].      [&20]|
00004340  0d 20 20 20 20 26 20 5b  26 32 36 5d 0d 20 20 20  |.    & [&26].   |
00004350  20 33 20 5b 26 33 33 5d  0d 20 20 20 20 32 20 5b  | 3 [&33].    2 [|
00004360  26 33 32 5d 0d 20 20 20  20 31 20 5b 26 33 31 5d  |&32].    1 [&31]|
00004370  0d 20 20 20 20 2c 20 5b  26 32 43 5d 0d 20 20 20  |.    , [&2C].   |
00004380  20 26 20 5b 26 32 36 5d  0d 20 20 20 20 31 20 5b  | & [&26].    1 [|
00004390  26 33 31 5d 0d 20 20 20  20 32 20 5b 26 33 32 5d  |&31].    2 [&32]|
000043a0  0d 20 20 20 20 33 20 5b  26 33 33 5d 0d 20 20 20  |.    3 [&33].   |
000043b0  20 20 20 5b 26 30 41 5d  0d 20 20 20 20 20 20 5b  |   [&0A].      [|
000043c0  26 30 44 5d 0d 20 20 20  20 20 20 5b 26 31 39 5d  |&0D].      [&19]|
000043d0  20 5b 26 30 34 5d 20 5b  26 32 31 5d 20 5b 26 30  | [&04] [&21] [&0|
000043e0  33 5d 20 5b 26 32 33 5d  20 5b 26 30 31 5d 0d 20  |3] [&23] [&01]. |
000043f0  20 20 20 3e 20 5b 26 33  45 5d 0d 20 20 20 20 43  |   > [&3E].    C|
00004400  20 5b 26 34 33 5d 0d 20  20 20 20 4c 20 5b 26 34  | [&43].    L [&4|
00004410  43 5d 0d 20 20 20 20 53  20 5b 26 35 33 5d 0d 20  |C].    S [&53]. |
00004420  20 20 20 20 20 5b 26 30  41 5d 0d 20 20 20 20 20  |     [&0A].     |
00004430  20 5b 26 30 44 5d 0d 20  20 20 20 20 20 5b 26 30  | [&0D].      [&0|
00004440  43 5d 0d 20 20 20 20 3e  20 5b 26 33 45 5d 0d 20  |C].    > [&3E]. |
00004450  20 20 20 2a 20 5b 26 32  41 5d 0d 20 20 20 20 43  |   * [&2A].    C|
00004460  20 5b 26 34 33 5d 0d 20  20 20 20 4f 20 5b 26 34  | [&43].    O [&4|
00004470  46 5d 0d 20 20 20 20 44  20 5b 26 34 34 5d 0d 20  |F].    D [&44]. |
00004480  20 20 20 45 20 5b 26 34  35 5d 0d 20 20 20 20 20  |   E [&45].     |
00004490  20 5b 26 30 41 5d 0d 20  20 20 20 20 20 5b 26 30  | [&0A].      [&0|
000044a0  44 5d 0d 0d 54 68 69 73  20 69 73 20 73 68 6f 77  |D]..This is show|
000044b0  69 6e 67 20 68 6f 77 20  42 41 53 49 43 20 63 6f  |ing how BASIC co|
000044c0  6d 6d 61 6e 64 73 20 61  72 65 20 74 72 61 6e 73  |mmands are trans|
000044d0  6c 61 74 65 64 20 69 6e  74 6f 20 62 79 74 65 73  |lated into bytes|
000044e0  0d 74 6f 20 62 65 20 73  65 6e 74 20 74 6f 20 74  |.to be sent to t|
000044f0  68 65 20 56 44 55 20 64  72 69 76 65 72 73 2e 20  |he VDU drivers. |
00004500  20 49 66 20 79 6f 75 20  65 6e 61 62 6c 65 20 74  | If you enable t|
00004510  68 65 20 69 6e 74 65 72  63 65 70 74 0d 77 68 69  |he intercept.whi|
00004520  6c 65 20 72 75 6e 6e 69  6e 67 20 61 20 70 72 6f  |le running a pro|
00004530  67 72 61 6d 20 79 6f 75  20 77 69 6c 6c 20 67 65  |gram you will ge|
00004540  74 20 61 20 73 69 6d 69  6c 61 72 2c 20 69 66 20  |t a similar, if |
00004550  6d 75 63 68 20 6d 6f 72  65 0d 73 75 63 63 69 6e  |much more.succin|
00004560  63 74 2c 20 72 65 73 75  6c 74 2e 20 20 54 68 65  |ct, result.  The|
00004570  20 6f 75 74 70 75 74 20  63 61 6e 20 62 65 20 53  | output can be S|
00004580  50 4f 4f 4c 45 44 20 74  6f 20 64 69 73 63 0d 61  |POOLED to disc.a|
00004590  6c 74 68 6f 75 67 68 20  79 6f 75 20 77 69 6c 6c  |lthough you will|
000045a0  20 67 65 74 20 65 78 74  72 61 20 63 68 61 72 61  | get extra chara|
000045b0  63 74 65 72 73 20 77 69  74 68 20 65 61 63 68 20  |cters with each |
000045c0  62 79 74 65 20 77 68 65  72 65 0d 69 74 20 69 73  |byte where.it is|
000045d0  20 70 72 69 6e 74 61 62  6c 65 2e 0d 0d 54 68 69  | printable...Thi|
000045e0  73 20 69 6e 74 65 72 63  65 70 74 20 6d 65 74 68  |s intercept meth|
000045f0  6f 64 20 63 61 6e 20 62  65 20 61 70 70 6c 69 65  |od can be applie|
00004600  64 20 74 6f 20 61 6e 79  20 76 65 63 74 6f 72 20  |d to any vector |
00004610  61 6c 74 68 6f 75 67 68  0d 6f 6e 6c 79 20 74 68  |although.only th|
00004620  65 20 52 44 43 48 20 61  6e 64 20 57 52 43 48 20  |e RDCH and WRCH |
00004630  63 61 72 72 79 20 61 6e  79 20 73 69 67 6e 69 66  |carry any signif|
00004640  69 63 61 6e 74 20 69 6e  66 6f 72 6d 61 74 69 6f  |icant informatio|
00004650  6e 20 69 6e 0d 74 68 65  20 61 63 63 75 6d 75 6c  |n in.the accumul|
00004660  61 74 6f 72 2e 20 20 54  68 65 20 43 4c 49 20 76  |ator.  The CLI v|
00004670  65 63 74 6f 72 20 70 61  73 73 65 73 20 75 73 65  |ector passes use|
00004680  66 75 6c 20 69 6e 66 6f  72 6d 61 74 69 6f 6e 0d  |ful information.|
00004690  69 6e 20 58 20 61 6e 64  20 59 20 70 6f 69 6e 74  |in X and Y point|
000046a0  69 6e 67 20 74 6f 20 73  74 72 69 6e 67 73 2e 20  |ing to strings. |
000046b0  20 59 6f 75 20 63 6f 75  6c 64 20 6d 6f 64 69 66  | You could modif|
000046c0  79 20 74 68 69 73 0d 72  6f 75 74 69 6e 65 20 74  |y this.routine t|
000046d0  6f 20 77 6f 72 6b 20 77  69 74 68 20 6f 74 68 65  |o work with othe|
000046e0  72 20 76 65 63 74 6f 72  73 2c 20 62 75 74 20 64  |r vectors, but d|
000046f0  6f 6e 27 74 20 62 6f 74  68 65 72 20 77 69 74 68  |on't bother with|
00004700  0d 74 68 65 20 45 56 45  4e 54 2c 20 42 52 45 41  |.the EVENT, BREA|
00004710  4b 20 6f 72 20 49 52 51  20 76 65 63 74 6f 72 73  |K or IRQ vectors|
00004720  20 61 73 20 79 6f 75 20  77 69 6c 6c 20 73 69 6d  | as you will sim|
00004730  70 6c 79 20 63 72 61 73  68 20 74 68 65 0d 6d 61  |ply crash the.ma|
00004740  63 68 69 6e 65 20 74 68  65 72 65 2e 20 20 54 68  |chine there.  Th|
00004750  6f 73 65 20 76 65 63 74  6f 72 73 20 61 72 65 20  |ose vectors are |
00004760  74 6f 6f 20 73 65 6e 73  69 74 69 76 65 2e 0d 0d  |too sensitive...|
00004770  4e 65 78 74 20 74 69 6d  65 20 2e 2e 2e 2e 2e 20  |Next time ..... |
00004780  45 56 45 4e 54 53 2e 20  20 55 6e 74 69 6c 20 74  |EVENTS.  Until t|
00004790  68 65 6e 2c 20 68 61 70  70 79 20 69 6e 74 65 72  |hen, happy inter|
000047a0  63 65 70 74 69 6e 67 2e  0d                       |cepting..|
000047a9
OS\BITS/T\OSB16.m0
OS\BITS/T\OSB16.m1
OS\BITS/T\OSB16.m2
OS\BITS/T\OSB16.m4
OS\BITS/T\OSB16.m5

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