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OS\BITS/T\OSB24
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.
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| Filename: | OS\BITS/T\OSB24 |
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| Exec address: | 0000 |
File contents
OSBITS - An Exploration of the BBC Micro at Machine Level
By Programmer
.........................................................
Part 24: Sideways ROMs
One of the more unusual, and powerful, parts of the BBC
Micro operating system is the use of sideways ROMs. They
are called sideways because several of them can exist, side
by side, in the same address space in the memory map.
The space between &8000 and &BFFF in the micro's memory is
given over to these sideways ROMs. The operating system can
select which one out of a possible sixteen is actually in
the memory map at any particular moment. The selection is
done by a device with the magic number 74LS161 which
contains a four bit register. This register appears in the
area of memory known as Sheila, at &FE30 up to &FE3F. The
address normally used is &FE30, the other addresses give
identical results. This register can only be written to,
not read.
The four bit register allows ROMs to be selected by numbers
from 0 to 15, hence the maximum of 16. Since the process of
selecting one is known as 'paging it into the memory map',
these ROMS are sometimes known as paged ROMs. On an
unexpanded BBC B there are four sockets on the main printed
circuit board, but many people have added extra sockets to
bring the total up to 16. The paged memory can be RAM as
well as ROM, the OS doesn't care which. With sideways RAM
you can load in what is called a 'ROM Image' from disc and
the RAM will then behave as if it was a sideways ROM. More
on ROM images in a moment.
Usually these sideways ROMs contain machine code programs.
They have the advantage that, apart from memory requirements
for workspace, variables and the like, they use up no user
memory. This is unlike a BASIC program, or a machine code
program you write yourself and then put into user memory
where the program also occupies part of user RAM.
There are basically two types of sideways ROM:
Languages: These are any program that takes over control
of the micro. BASIC is the most obvious, but word
processors, spread sheets, databases and even machine code
monitors generally operate as languages.
Service ROMs: These usually contain programs that carry
out some useful function, or service, and will be called
from within a language, or direct from the OS. Filing
systems like the DFS or ADFS, and toolkit type ROMs
generally are service ROMs.
You might consider a ROM to be half-and-half if its language
functions were on a par with its service ones. The BBC Soft
ATS ROM is like this. The teletext display part of the
program is a language, while the page grabbing functions
operate as service calls.
It is possible for a language ROM to have some service
elements in it. Every ROM except BASIC must respond to
calls for service even if only to select themselves.
Selection of BASIC, with *BASIC, is an exception because the
code to select it is within the OS. Although you can
operate a BBC Micro without BASIC, it is unusual to do so.
The operation of sideways ROMs is both complicated and
simple. The simple part is that, with a few provisos, it is
as easy to write machine code for a sideways ROM as for main
memory. The complicated bit is explaining how the OS keeps
track of the ROMs and knows which one to page into memory at
the appropriate time.
The housekeeping for ROMs is carried out by a series of
service calls. At certain times, such as on power up or
reset, or when an unknown *command is sent to the command
line interpreter, a service call is sent. The first few
bytes of every sideways ROM have to follow a format laid
down by Acorn. This is part of the ROM image I referred to
earlier. It looks like this.
&8000 JMP <language entry point>
&8003 JMP <service entry point>
&8006 ROM type flag
&8007 Offset to start of copyright string
&8008 Version number (in binary)
&8009 Title string
The ROM type flag is made up of the following bits:
bit 7 is set when the ROM has a service entry point, all
ROMs except BASIC will have this bit set.
bit 6 is set if the ROM is a language.
bit 5 is set if the ROM is a language which relocates to
an address other than &8000 when in a second processor.
bit 4 is not used in the BBC micro, it fulfils some
purpose in an Electron.
bits 3 to 0 denote the type of machine code in the ROM.
There are arrangements of these bits specified for 6502
code, 68000 code, Z80 code, 32016 code, 80186 and 80286
code in the BBC Master handbook. If you have a ROM that
contains 6502 machine code and is a service ROM only,
then this byte would be &82.
The ROM type flag is copied, by the OS, into a 16 byte ROM
information table whenever a reset occurs, such as on a
BREAK. OSBYTE 170 (and 171) are used to read the start
address for this table, and the byte for socket X is found X
bytes above that start address. If no ROM is present in a
socket then that socket's position in the ROM information
table holds zero. You can fool the OS into ignoring a ROM
by writing a zero into its position in the ROM information
table; but, as I have already said, this table is rewritten
on a BREAK (reset). In a MASTER micro you can 'unplug' a
ROM in software with an OS command. You might want to
remove a ROM from the machine in order to stop clashes with
commands or over workspace. (More on this in the next
module.)
The copyright string has to start with (C) and is then
usually followed by the name of the copyright holder and the
date. It usually follows the title string in the header.
The title string is the string printed out if the ROM is
selected as a language. Both title and copyright strings
are terminated with nulls (0).
I have written a ROM for this module. The source code for
the ROM is in the program B/osb24 and you need BASIC 2 or
higher (such as HiBASIC) to assemble it. Besides the
ability to assemble code to run at a different address to
the RAM it's assembled in (of which more later), the extra
facilities of later BASICs include some EQU pseudo op-codes
which are as follows:
EQUB - put a byte into memory here
EQUW - put a word (2 bytes) into memory here
EQUD - put a double word (4 bytes) into memory here
EQUS - put a string into memory here
EQUW and EQUD put in numbers such that the most significant
byte is in the highest address. So if EQUD &12345678
appears in your code at address label 'here' you will find
the number in memory like this:
here &78
here+1 &56
here+2 &34
here+3 &12
If you look at lines 380 to 490 of B/osb24 you will find the
code that assembles into the ROM header. These are the
relevant parts
EQUW 0
EQUB 0
JMP service_entry_point
There is no language entry so the first three bytes are
zeros.
EQUB &82
This is the ROM type flag
EQUB copyright - code%
Then we have the copyright offset byte that holds the offset
of the copyright string from the start of the code.
EQUB 0
The version byte is zero and is followed by the ROM title
and copyright strings. I have incorporated a version string
into the ROM title. The format of the copyright string is
always a zero (null) byte followed by (C). The format has
to be exactly that, otherwise the OS will not recognise that
there is a ROM in the socket. Try deleting the null and
see.
.rom_title EQUS "OSbits Demo ROM "+Version$
.copyright EQUS CHR$0+"(C)BBC "+Date$+CHR$0
.service_entry_point
The service entry point comes next. This is where execution
of the code goes when the operating system sends a service
call. It starts by offering the service to ROM number 15
and then works its way down the ROMS. This is why the ROM
in a high numbered socket has priority over ROMS in lower
numbered sockets. Priority means that, in the case of two
ROMS having the same command in them, the higher numbered
one will actually be executed. Also the code in a higher
numbered ROM will be executed a little faster because the
service call reaches it quicker. This is why AMS suggest
that you put your AMX mouse ROM in the highest possible
socket.
A service call is a jump to the service entry point, and the
numbers in the Accumulator and the X and Y registers carry
the call's parameters.
I'll deal with the X first, since that always holds the
number of the ROM currently being used. Since your code in
a ROM does not know where it is in the machine, and many
actions you will want to carry out will depend on you
knowing the socket number in which your ROM is, you will get
this vital information from the X register at the entry into
your service routine.
The accumulator carries the number of the service call, and
the Y register may carry some parameter for the call. I
will list some of the service calls that the OS is likely to
originate. (Note that, with OSBYTE 143, any program can
originate a service call.) Note that any references to
memory locations are always in the I/O processor (the main
micro) and never in a second processor, should one be
connected.
Service call 0 actually means that the particular service
has been carried out and the original call has been claimed.
A ROM is not usually offered service call zero, but the OS
knows if it detects a zero in the accumulator after a ROM
has been serviced that the operation has been carried out.
You can send a service call 0 with OSBYTE 143 but I can't
think why you might want to since it wouldn't get any
further than the first ROM.
Service call 1 - on a reset ROMS must reserve absolute
static workspace (starting always at &E00 on a B or B+ and
usually for filing systems) on this call. Each ROM that
wants this kind of workspace will compare the number in Y,
which will be the higher byte of the current upper limit of
the workspace , with the value it needs and increase Y if
necessary before exiting. This call must not be claimed,
i.e. the value in the accumulator is to be preserved from
entry to exit.
Service call 2 - on a reset, private dynamic workspace is
reserved with this call. Whereas absolute workspace can be
used by another ROM, subject to housekeeping rules, this
workspace is only for your ROM. You claim it by storing the
value of Y in the ROM workspace table, and then raising Y
by the number of pages of this workspace you need. When you
need to find out where your private workspace starts you can
look in that table, which starts at &DF0, offset by your ROM
number (found from the X register).
Service call 3 is an auto-boot call. When this is detected,
a ROM that wishes to initialise itself on reset looks at the
keyboard (using OSBYTE 120) to see if 'its' key is pressed.
If that is the case, or no key is pressed, then the ROM
should initialise. If another key is pressed it should
ignore the call and pass on the registers unchanged. In
this way you can start in DFS on BREAK by holding down D and
start ADFS by holding down A (or F). If the Y register is
zero then the ROM that claims the call should action the
relevant !BOOT file.
Many ROMS (including the one with this module) use service
call 3 to print out a little message about themselves. This
call is claimed by any filing system that starts up, which
is why your messages on BREAK depend on the socket in which
you put your DFS chip.
Service call 4 - any *command that the OS does not recognise
is offered to the sideways ROMs with this call. Each ROM
looks at the command, which is held at (&F2), Y and
terminated with a carriage return, and if it recognises it
then it should execute the relevant code and claim the
service call. If no ROMs claim the call then the command is
offered to the current filing system. The DFS, for example,
will then try to *RUN a file of that name.
Service call 5 means that an unknown interrupt has occurred.
It will be offered to the ROMS before being passed to the
USER down IRQ2.
Service call 6 means that a BRK (i.e. a software interrupt,
as distinct from pressing the BREAK key) has been detected.
If a ROM wishes to take action it can. This call should not
be claimed.
Service call 7 means that an OSBYTE call not known to the OS
has occurred. The A, X and Y registers of the call can be
found at &EF, &F0 and &F1 respectively.
Service call 8 means that an unknown OSWORD call has
occurred. The A, X and Y registers of the call can be found
at &EF, &F0 and &F1 respectively.
Service call 9 means that *HELP has been detected by the
command line interpreter and ROMS can give whatever helpful
messages they possess. The part of the *HELP string that
follows the *HELP or *H. itself will be pointed to by the
contents of &F2 and &F3 plus the contents of Y. This call
should not be claimed in case two ROMS respond to the same
keyword.
Service call &A (10) is issued when a ROM wants to use the
static workspace. It should issue this call to warn other
ROMs to save their valuable data in their own workspace.
Service calls &B (11) and &C (12) relinquish and claim the
NMI workspace.
Service calls &D (13) and &E (14) relate to the ROM filing
system, or to a Speech Filing System.
Service call &F (15) is issued by a filing system when it
has initialised and sorted out its vectors. Any ROMS that
wish to intercept filing system vectors have to change their
intercepts after this call.
Service call &10 (16) occurs on a reset, and also when
OSBYTE 119 is called. It means that all *SPOOL or *EXEC
files should be closed.
Service call &11 (17) is used on the BBC A, B and B+ to warn
of a font implosion or explosion, which could affect
workspace kept just above the operating system high water
mark. This call is unused on a Master.
Service call &12 (18) is used by filing systems for
initialisation if files are being transferred between
filing systems.
Service call &15 on a Master is a polling call, made every
centisecond if this facility is enabled with OSBYTE 22.
Service call &21 on a Master is like service call 1 but
offers workspace in private filing system RAM. Ditto for
calls &22 and service call 2. Calls &23 to &26 also relate
to filing systems on the Master.
The Master does not offer service calls 1 and 2 on a soft
reset, instead it offers call &27. Calls &28 and &29 are
used on the Master for CMOS RAM configuration and status.
Service call &2A on a Master informs ROMS of a change of
language.
Service call &2B is a mystery, it is described as 'reserved'
in Acorn's literature but my machine (a B with 1770 DFS)
sends it whereas a normal B (with 8271 DFS) does not.
Finally (at last) calls &FE and &FF are sent at
initialisation of the tube on a reset. With a second
processor present both are sent, but only the former if no
second processor is detected.
One of the things that the OSBITS ROM does is to print out
the service calls as they happen. The calls generated in my
machine are as follows:
Without second processor:
10 0F 01 2B 07 07 02 FE 03 10 0F 0A
With second processor:
10 0F FF 01 2B 07 07 02 FE 11 03 10 0F 0A
Don't worry if you don't understand all those service calls.
I certainly don't understand any of the ones relating to
filing systems, they were never my strong suit. The main
thing is to understand the few you are likely to use if you
write a ROM. These will be mainly numbers 2, 3, 4 and 9.
Language entry is much simpler. If, through service call 4,
a ROM detects its own language name then it simply calls
OSBYTE 142 with its ROM number in the X register. The OS
will then enter the language through the language entry
point and off you go. The OS does not expect a return from
a language.
The ROM assembly code in B/osb24 is intended to give you
some idea of how to write for a service ROM. I don't intend
it to be definitive and there are some good books around and
many magazine articles. Sideways ROMs are a study in
themselves.
I think the best way to proceed is for me to walk you
through the program in this module, section by section, with
an explanation of what is happening. Then I hope you will
feel up to some experimenting. But first a caveat ....
B/osb24 will not run with BASIC 1. I made that decision
partly because of the lack of EQU functions (although these
can be simulated as you well know). More importantly, it is
almost impossible, and not recommended, to assemble code
directly into sideways RAM, particularly if the RAM is in
socket 15. Any faults and your machine will lock out and
you will have to switch it off to cure the fault, losing all
your code. By far the best way is to generate the assembled
code to run in the required space but to be assembled in
another part of the memory. In BASIC 2 et al you can do
this by using values of OPT from 4 to 7 instead of 0 to 3.
Line 310 of this program shows this.
You will then have a program you can save to disc from which
you can load into sideways RAM or, if you have the
technology, blow yourself an EPROM. My machine
configuration has 16K of RAM in ROM socket 15 as part of my
ROM extension board and I can simple *LOAD into that memory.
If you have a B+ or a Master you will probably have sideways
RAM built in and use *SRLOAD to load into it.
If you really want to assemble direct into sideways RAM then
you should change line 310 to read
FOR pass% = 0 TO 2 STEP 2
and the code will be put directly into memory at &8000.
(This will not work on a Master or B+128.) But bear in mind
the possible problems if you should abort the assembly. The
service call JUMP instruction at the start of your ROM image
code might end up pointing to some spurious RAM and your
machine will crash. Because the RAM has highest priority if
it is in socket 15 even a CONTROL/BREAK will not help, you
will have to switch off.
And so to the program.
System variables are set in the first few lines. I have
allocated some workspace in zero page starting at &A8. This
memory is reserved for OS commands during execution, but I
am still preserving the contents of these locations as I use
them. At least you shouldn't find somebody putting their
interrupt code here.
I have defined more OS routine addresses than I have used to
help you if you add any code of your own to this here.
After the ROM header, which I explained earlier, is the
service entry point. Now since I want this ROM to display
the service calls it receives the first thing that happens
is a JSR to a subroutine called 'show_service_calls'. This
is at line 3400 in the listing.
I am using a byte of the OS workspace that is unused by the
OS on a B although there is a OSBYTE call to access it. It
is at &28A and is held as a variable called 'service_flag'
defined in line 250. If this flag is set (anything other
than zero) then the values in the accumulator and in X and Y
are printed out in hex as they go through the ROM. If the
flag is clear (zero) they are not. The default value of the
flag is zero and it can be set by entering *FX 250,1. The
ROM provides a clearer way, which I'll come on to when I
talk about the built-in commands. I think the subroutine is
self-explanatory, but note that it is important to balance
the pushes to and pulls from the stack, which is why there
is a PLA and a PHA together in lines 3460 and 3470.
After going through the service call routine the ROM checks
to see which call it is, and to respond accordingly. This
ROM responds to calls 2 (although I have disabled that
section of the assembler with comments), 3, 4 and 9.
Call 2 allows you to reserve private dynamic workspace.
This is why ROMs sometimes raise the value of PAGE. In this
particular case I have blocked off the relevant bits of the
assembly code using comment back-slashes, if you want some
workspace then remove the back-slashes. The start of your
workspace (a page in this case) is held in &DF0, X where X
is your ROM socket number, passed in X by any service call.
I use service call 3 to print a message in the banner after
a BREAK. This would be a good time to clear some of your
workspace if you needed it reset on a BREAK. Those lines of
the code are again blocked off in the assembler listing in
case you need them. The reason I've blocked those sections
off is that I didn't want to raise PAGE unless it was
needed, and in its present form the ROM doesn't use any
workspace except that in zero page.
Using call 3 for banner messages is not really an approved
idea. There is no real harm in it, but a filing system ROM
that claims this call will obviously also stop the banner
message being printed. With the OSBROM (as I have called
it) in socket 15, this is not a problem. You could use call
1 but your message would then appear above the Acorn banner.
You can modify line 650 to read CMP #1 if you want to see
the effect, assuming you leave the comment back-slashes in
place.
Service call 4 is the one for unrecognised commands.
Whenever a *command passes through the command line
interpreter it is passed for action to:
1: The operating system
2: Sideways ROMS with service call 4
3: The current filing system (except for cassette)
in that order. When service call 4 reaches the ROM,
execution jumps to 'commands' at line 2280. After workspace
housekeeping (some of these should perhaps also be blanked
off but no commands actually use the private workspace so,
for the moment, let them stand) we store Y-1 on the stack
for later. (Y-1 so that we can start the loops with an
INY.) Y points to the first letter of the *command which is
held from (&F2),Y onwards, terminated by a carriage
return.
Commands are recognised by being compared with a table of
commands held in the ROM. Each command in the table is
terminated with a null byte. We compare each letter of the
unknown command with each one in the list in turn. If a
match fails then we move on to the next word in the list and
finally we exit altogether with all registers returned to
their entry states. If, during the match, we reach the null
then we have a match. Execution jumps to the address stored
in the two bytes following the relevant null.
Note in line 2580 that I have ANDed the byte in the input
string with &DF. This will make it upper case no matter
what, although this procedure means that the words in the
ROM's list MUST be in upper case and they cannot contain
anything but the letters A-Z.
The commands recognised by this ROM are:
*BEEP - a trivial piece of code which does a VDU7
*SEND <string> prints out the string. This code uses the OS
routines GSINIT and GSREAD to prepare the string and to read
it. GSINIT will cope with quotation marks, spaces before
the word and with escape sequences such as |M in exactly the
same way it does for *KEY programming. By setting the carry
flag on entry to GSINIT we tell it that the string will
terminate either with second quotes or with a carriage
return. If carry was clear then a space would also
terminate. On exit GSINIT sets the zero flag if the string
is null, i.e. of zero length or just a carriage return, and
GSREAD sets the carry flag if we have reached the end of the
string.
*SERVICE enables the service call display. This is
equivalent to a *FX 250,1 and is not reset on BREAK.
*NOSERVICE turns off the service call display and is
equivalent to a *FX 250,0.
You can add any machine code of your own to the ROM by
extending my list. I have left another dummy command,
*WHATSYOURS which just beeps. You can easily change its
name at line 3140 and put your code starting at line 4280.
All command code must return with a JMP exit.
The command list is held from line 3020. The format is
command name first as a string terminated by a null and then
a two byte word holding the address of that particular
routine.
Finally service call 9 is *HELP. It works in two ways of
course. The ROM will respond to both *HELP and to *HELP
OSBITS. When we receive call 9 the character following the
*HELP (or *H.) is at (&F2),Y. If it is a carriage return
then this is a straightforward general HELP call and we
respond by printing out our title and the word we respond
to.
If that character is not a CR then the *HELP has an argument
and we have to then check that against our HELP response
list, which in this case is only one word, OSBITS. This
checking is exactly like that for unknown commands. The
help list is from line 3180.
It is important that service call 9 is not claimed and that
Y is restored to its entry value on exit. It is possible
for more than one ROM to respond to a HELP argument (UTILS
is a common one) and also our ROM must not respond with
anything if the HELP argument does not apply. View 2.1 does
not appear to work properly in this respect as (on my
machine anyway) it announces itself to any help call.
That then is the 'London to Brighton in 4 Minutes' guide to
sideways ROMS. There is much more to them that I can cover
in a module and they are worthy of further study, although
the principles involved are very machine specific.
Finally a couple of programming hints about writing for
Service ROMS. Firstly remember that you will not be able to
write into your address space, so you will have to use
zero-page and main memory workspace a lot. With main memory
you will use a lot of indirect addressing.
To give an error message in a service ROM, you have to copy
your error code - from the BRK right through to the message
and its terminating null - into the bottom of the stack
(from &100 onwards since the stack is upside down), and JMP
to your BRK at &100. You have to do this because the error
code in the language you are calling from will automatically
look in that language's ROM if you execute the BRK in your
ROM. You have to execute the BRK in main memory, and the
stack is a safe place to do so.
From a Service ROM you can address I/O memory reasonably
safely since your code will never execute in a second
processor. Often speed is important, and you can take
advantage of your place in the I/O processor.
That leads me almost neatly onto the topic for the next
module. Acorn often tell us what is legal and what is
illegal. I will expand on that subject next time.
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 0d 0d 0d 50 61 72 74 20 32 34 3a 20 53 |......Part 24: S| 00000090 69 64 65 77 61 79 73 20 52 4f 4d 73 0d 0d 0d 4f |ideways ROMs...O| 000000a0 6e 65 20 6f 66 20 74 68 65 20 6d 6f 72 65 20 75 |ne of the more u| 000000b0 6e 75 73 75 61 6c 2c 20 61 6e 64 20 70 6f 77 65 |nusual, and powe| 000000c0 72 66 75 6c 2c 20 70 61 72 74 73 20 6f 66 20 74 |rful, parts of t| 000000d0 68 65 20 42 42 43 0d 4d 69 63 72 6f 20 6f 70 65 |he BBC.Micro ope| 000000e0 72 61 74 69 6e 67 20 73 79 73 74 65 6d 20 69 73 |rating system is| 000000f0 20 74 68 65 20 75 73 65 20 6f 66 20 73 69 64 65 | the use of side| 00000100 77 61 79 73 20 52 4f 4d 73 2e 20 20 54 68 65 79 |ways ROMs. They| 00000110 0d 61 72 65 20 63 61 6c 6c 65 64 20 73 69 64 65 |.are called side| 00000120 77 61 79 73 20 62 65 63 61 75 73 65 20 73 65 76 |ways because sev| 00000130 65 72 61 6c 20 6f 66 20 74 68 65 6d 20 63 61 6e |eral of them can| 00000140 20 65 78 69 73 74 2c 20 73 69 64 65 0d 62 79 20 | exist, side.by | 00000150 73 69 64 65 2c 20 69 6e 20 74 68 65 20 73 61 6d |side, in the sam| 00000160 65 20 61 64 64 72 65 73 73 20 73 70 61 63 65 20 |e address space | 00000170 69 6e 20 74 68 65 20 6d 65 6d 6f 72 79 20 6d 61 |in the memory ma| 00000180 70 2e 0d 0d 54 68 65 20 73 70 61 63 65 20 62 65 |p...The space be| 00000190 74 77 65 65 6e 20 26 38 30 30 30 20 61 6e 64 20 |tween &8000 and | 000001a0 26 42 46 46 46 20 69 6e 20 74 68 65 20 6d 69 63 |&BFFF in the mic| 000001b0 72 6f 27 73 20 6d 65 6d 6f 72 79 20 69 73 0d 67 |ro's memory is.g| 000001c0 69 76 65 6e 20 6f 76 65 72 20 74 6f 20 74 68 65 |iven over to the| 000001d0 73 65 20 73 69 64 65 77 61 79 73 20 52 4f 4d 73 |se sideways ROMs| 000001e0 2e 20 20 54 68 65 20 6f 70 65 72 61 74 69 6e 67 |. 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You | 00005bf0 63 61 6e 20 65 61 73 69 6c 79 20 63 68 61 6e 67 |can easily chang| 00005c00 65 20 69 74 73 0d 6e 61 6d 65 20 61 74 20 6c 69 |e its.name at li| 00005c10 6e 65 20 33 31 34 30 20 61 6e 64 20 70 75 74 20 |ne 3140 and put | 00005c20 79 6f 75 72 20 63 6f 64 65 20 73 74 61 72 74 69 |your code starti| 00005c30 6e 67 20 61 74 20 6c 69 6e 65 20 34 32 38 30 2e |ng at line 4280.| 00005c40 0d 41 6c 6c 20 63 6f 6d 6d 61 6e 64 20 63 6f 64 |.All command cod| 00005c50 65 20 6d 75 73 74 20 72 65 74 75 72 6e 20 77 69 |e must return wi| 00005c60 74 68 20 61 20 4a 4d 50 20 65 78 69 74 2e 0d 0d |th a JMP exit...| 00005c70 54 68 65 20 63 6f 6d 6d 61 6e 64 20 6c 69 73 74 |The command list| 00005c80 20 69 73 20 68 65 6c 64 20 66 72 6f 6d 20 6c 69 | is held from li| 00005c90 6e 65 20 33 30 32 30 2e 20 20 54 68 65 20 66 6f |ne 3020. The fo| 00005ca0 72 6d 61 74 20 69 73 0d 63 6f 6d 6d 61 6e 64 20 |rmat is.command | 00005cb0 6e 61 6d 65 20 66 69 72 73 74 20 61 73 20 61 20 |name first as a | 00005cc0 73 74 72 69 6e 67 20 74 65 72 6d 69 6e 61 74 65 |string terminate| 00005cd0 64 20 62 79 20 61 20 6e 75 6c 6c 20 61 6e 64 20 |d by a null and | 00005ce0 74 68 65 6e 0d 61 20 74 77 6f 20 62 79 74 65 20 |then.a two byte | 00005cf0 77 6f 72 64 20 68 6f 6c 64 69 6e 67 20 74 68 65 |word holding the| 00005d00 20 61 64 64 72 65 73 73 20 6f 66 20 74 68 61 74 | address of that| 00005d10 20 70 61 72 74 69 63 75 6c 61 72 0d 72 6f 75 74 | particular.rout| 00005d20 69 6e 65 2e 0d 0d 46 69 6e 61 6c 6c 79 20 73 65 |ine...Finally se| 00005d30 72 76 69 63 65 20 63 61 6c 6c 20 39 20 69 73 20 |rvice call 9 is | 00005d40 2a 48 45 4c 50 2e 20 20 49 74 20 77 6f 72 6b 73 |*HELP. It works| 00005d50 20 69 6e 20 74 77 6f 20 77 61 79 73 20 6f 66 0d | in two ways of.| 00005d60 63 6f 75 72 73 65 2e 20 20 54 68 65 20 52 4f 4d |course. The ROM| 00005d70 20 77 69 6c 6c 20 72 65 73 70 6f 6e 64 20 74 6f | will respond to| 00005d80 20 62 6f 74 68 20 2a 48 45 4c 50 20 61 6e 64 20 | both *HELP and | 00005d90 74 6f 20 2a 48 45 4c 50 0d 4f 53 42 49 54 53 2e |to *HELP.OSBITS.| 00005da0 20 20 57 68 65 6e 20 77 65 20 72 65 63 65 69 76 | When we receiv| 00005db0 65 20 63 61 6c 6c 20 39 20 74 68 65 20 63 68 61 |e call 9 the cha| 00005dc0 72 61 63 74 65 72 20 66 6f 6c 6c 6f 77 69 6e 67 |racter following| 00005dd0 20 74 68 65 0d 2a 48 45 4c 50 20 28 6f 72 20 2a | the.*HELP (or *| 00005de0 48 2e 29 20 69 73 20 61 74 20 20 28 26 46 32 29 |H.) is at (&F2)| 00005df0 2c 59 2e 20 20 49 66 20 69 74 20 69 73 20 61 20 |,Y. If it is a | 00005e00 63 61 72 72 69 61 67 65 20 72 65 74 75 72 6e 0d |carriage return.| 00005e10 74 68 65 6e 20 74 68 69 73 20 69 73 20 61 20 73 |then this is a s| 00005e20 74 72 61 69 67 68 74 66 6f 72 77 61 72 64 20 67 |traightforward g| 00005e30 65 6e 65 72 61 6c 20 48 45 4c 50 20 63 61 6c 6c |eneral HELP call| 00005e40 20 61 6e 64 20 77 65 0d 72 65 73 70 6f 6e 64 20 | and we.respond | 00005e50 62 79 20 70 72 69 6e 74 69 6e 67 20 6f 75 74 20 |by printing out | 00005e60 6f 75 72 20 74 69 74 6c 65 20 61 6e 64 20 74 68 |our title and th| 00005e70 65 20 77 6f 72 64 20 77 65 20 72 65 73 70 6f 6e |e word we respon| 00005e80 64 0d 74 6f 2e 0d 0d 49 66 20 74 68 61 74 20 63 |d.to...If that c| 00005e90 68 61 72 61 63 74 65 72 20 69 73 20 6e 6f 74 20 |haracter is not | 00005ea0 61 20 43 52 20 74 68 65 6e 20 74 68 65 20 2a 48 |a CR then the *H| 00005eb0 45 4c 50 20 68 61 73 20 61 6e 20 61 72 67 75 6d |ELP has an argum| 00005ec0 65 6e 74 0d 61 6e 64 20 77 65 20 68 61 76 65 20 |ent.and we have | 00005ed0 74 6f 20 74 68 65 6e 20 63 68 65 63 6b 20 74 68 |to then check th| 00005ee0 61 74 20 61 67 61 69 6e 73 74 20 6f 75 72 20 48 |at against our H| 00005ef0 45 4c 50 20 72 65 73 70 6f 6e 73 65 0d 6c 69 73 |ELP response.lis| 00005f00 74 2c 20 77 68 69 63 68 20 69 6e 20 74 68 69 73 |t, which in this| 00005f10 20 63 61 73 65 20 69 73 20 6f 6e 6c 79 20 6f 6e | case is only on| 00005f20 65 20 77 6f 72 64 2c 20 4f 53 42 49 54 53 2e 20 |e word, OSBITS. | 00005f30 20 54 68 69 73 0d 63 68 65 63 6b 69 6e 67 20 69 | This.checking i| 00005f40 73 20 65 78 61 63 74 6c 79 20 6c 69 6b 65 20 74 |s exactly like t| 00005f50 68 61 74 20 66 6f 72 20 75 6e 6b 6e 6f 77 6e 20 |hat for unknown | 00005f60 63 6f 6d 6d 61 6e 64 73 2e 20 20 54 68 65 0d 68 |commands. The.h| 00005f70 65 6c 70 20 6c 69 73 74 20 69 73 20 66 72 6f 6d |elp list is from| 00005f80 20 6c 69 6e 65 20 33 31 38 30 2e 0d 0d 49 74 20 | line 3180...It | 00005f90 69 73 20 69 6d 70 6f 72 74 61 6e 74 20 74 68 61 |is important tha| 00005fa0 74 20 73 65 72 76 69 63 65 20 63 61 6c 6c 20 39 |t service call 9| 00005fb0 20 69 73 20 6e 6f 74 20 63 6c 61 69 6d 65 64 20 | is not claimed | 00005fc0 61 6e 64 20 74 68 61 74 0d 59 20 69 73 20 72 65 |and that.Y is re| 00005fd0 73 74 6f 72 65 64 20 74 6f 20 69 74 73 20 65 6e |stored to its en| 00005fe0 74 72 79 20 76 61 6c 75 65 20 6f 6e 20 65 78 69 |try value on exi| 00005ff0 74 2e 20 20 49 74 20 69 73 20 70 6f 73 73 69 62 |t. It is possib| 00006000 6c 65 0d 66 6f 72 20 6d 6f 72 65 20 74 68 61 6e |le.for more than| 00006010 20 6f 6e 65 20 52 4f 4d 20 74 6f 20 72 65 73 70 | one ROM to resp| 00006020 6f 6e 64 20 74 6f 20 61 20 48 45 4c 50 20 61 72 |ond to a HELP ar| 00006030 67 75 6d 65 6e 74 20 28 55 54 49 4c 53 0d 69 73 |gument (UTILS.is| 00006040 20 61 20 63 6f 6d 6d 6f 6e 20 6f 6e 65 29 20 61 | a common one) a| 00006050 6e 64 20 61 6c 73 6f 20 6f 75 72 20 52 4f 4d 20 |nd also our ROM | 00006060 6d 75 73 74 20 6e 6f 74 20 72 65 73 70 6f 6e 64 |must not respond| 00006070 20 77 69 74 68 0d 61 6e 79 74 68 69 6e 67 20 69 | with.anything i| 00006080 66 20 74 68 65 20 48 45 4c 50 20 61 72 67 75 6d |f the HELP argum| 00006090 65 6e 74 20 64 6f 65 73 20 6e 6f 74 20 61 70 70 |ent does not app| 000060a0 6c 79 2e 20 20 56 69 65 77 20 32 2e 31 20 64 6f |ly. View 2.1 do| 000060b0 65 73 0d 6e 6f 74 20 61 70 70 65 61 72 20 74 6f |es.not appear to| 000060c0 20 77 6f 72 6b 20 70 72 6f 70 65 72 6c 79 20 69 | work properly i| 000060d0 6e 20 74 68 69 73 20 72 65 73 70 65 63 74 20 61 |n this respect a| 000060e0 73 20 28 6f 6e 20 6d 79 0d 6d 61 63 68 69 6e 65 |s (on my.machine| 000060f0 20 61 6e 79 77 61 79 29 20 69 74 20 61 6e 6e 6f | anyway) it anno| 00006100 75 6e 63 65 73 20 69 74 73 65 6c 66 20 74 6f 20 |unces itself to | 00006110 61 6e 79 20 68 65 6c 70 20 63 61 6c 6c 2e 0d 0d |any help call...| 00006120 54 68 61 74 20 74 68 65 6e 20 69 73 20 74 68 65 |That then is the| 00006130 20 27 4c 6f 6e 64 6f 6e 20 74 6f 20 42 72 69 67 | 'London to Brig| 00006140 68 74 6f 6e 20 69 6e 20 34 20 4d 69 6e 75 74 65 |hton in 4 Minute| 00006150 73 27 20 67 75 69 64 65 20 74 6f 0d 73 69 64 65 |s' guide to.side| 00006160 77 61 79 73 20 52 4f 4d 53 2e 20 20 54 68 65 72 |ways ROMS. Ther| 00006170 65 20 69 73 20 6d 75 63 68 20 6d 6f 72 65 20 74 |e is much more t| 00006180 6f 20 74 68 65 6d 20 74 68 61 74 20 49 20 63 61 |o them that I ca| 00006190 6e 20 63 6f 76 65 72 0d 69 6e 20 61 20 6d 6f 64 |n cover.in a mod| 000061a0 75 6c 65 20 61 6e 64 20 74 68 65 79 20 61 72 65 |ule and they are| 000061b0 20 77 6f 72 74 68 79 20 6f 66 20 66 75 72 74 68 | worthy of furth| 000061c0 65 72 20 73 74 75 64 79 2c 20 61 6c 74 68 6f 75 |er study, althou| 000061d0 67 68 0d 74 68 65 20 70 72 69 6e 63 69 70 6c 65 |gh.the principle| 000061e0 73 20 69 6e 76 6f 6c 76 65 64 20 61 72 65 20 76 |s involved are v| 000061f0 65 72 79 20 6d 61 63 68 69 6e 65 20 73 70 65 63 |ery machine spec| 00006200 69 66 69 63 2e 0d 0d 46 69 6e 61 6c 6c 79 20 61 |ific...Finally a| 00006210 20 63 6f 75 70 6c 65 20 6f 66 20 70 72 6f 67 72 | couple of progr| 00006220 61 6d 6d 69 6e 67 20 68 69 6e 74 73 20 61 62 6f |amming hints abo| 00006230 75 74 20 77 72 69 74 69 6e 67 20 66 6f 72 0d 53 |ut writing for.S| 00006240 65 72 76 69 63 65 20 52 4f 4d 53 2e 20 20 46 69 |ervice ROMS. Fi| 00006250 72 73 74 6c 79 20 72 65 6d 65 6d 62 65 72 20 74 |rstly remember t| 00006260 68 61 74 20 79 6f 75 20 77 69 6c 6c 20 6e 6f 74 |hat you will not| 00006270 20 62 65 20 61 62 6c 65 20 74 6f 0d 77 72 69 74 | be able to.writ| 00006280 65 20 69 6e 74 6f 20 79 6f 75 72 20 61 64 64 72 |e into your addr| 00006290 65 73 73 20 73 70 61 63 65 2c 20 73 6f 20 79 6f |ess space, so yo| 000062a0 75 20 77 69 6c 6c 20 68 61 76 65 20 74 6f 20 75 |u will have to u| 000062b0 73 65 0d 7a 65 72 6f 2d 70 61 67 65 20 61 6e 64 |se.zero-page and| 000062c0 20 6d 61 69 6e 20 6d 65 6d 6f 72 79 20 77 6f 72 | main memory wor| 000062d0 6b 73 70 61 63 65 20 61 20 6c 6f 74 2e 20 20 57 |kspace a lot. W| 000062e0 69 74 68 20 6d 61 69 6e 20 6d 65 6d 6f 72 79 0d |ith main memory.| 000062f0 79 6f 75 20 77 69 6c 6c 20 75 73 65 20 61 20 6c |you will use a l| 00006300 6f 74 20 6f 66 20 69 6e 64 69 72 65 63 74 20 61 |ot of indirect a| 00006310 64 64 72 65 73 73 69 6e 67 2e 0d 0d 54 6f 20 67 |ddressing...To g| 00006320 69 76 65 20 61 6e 20 65 72 72 6f 72 20 6d 65 73 |ive an error mes| 00006330 73 61 67 65 20 69 6e 20 61 20 73 65 72 76 69 63 |sage in a servic| 00006340 65 20 52 4f 4d 2c 20 79 6f 75 20 68 61 76 65 20 |e ROM, you have | 00006350 74 6f 20 63 6f 70 79 0d 79 6f 75 72 20 65 72 72 |to copy.your err| 00006360 6f 72 20 63 6f 64 65 20 2d 20 66 72 6f 6d 20 74 |or code - from t| 00006370 68 65 20 42 52 4b 20 72 69 67 68 74 20 74 68 72 |he BRK right thr| 00006380 6f 75 67 68 20 74 6f 20 74 68 65 20 6d 65 73 73 |ough to the mess| 00006390 61 67 65 0d 61 6e 64 20 69 74 73 20 74 65 72 6d |age.and its term| 000063a0 69 6e 61 74 69 6e 67 20 6e 75 6c 6c 20 2d 20 69 |inating null - i| 000063b0 6e 74 6f 20 74 68 65 20 62 6f 74 74 6f 6d 20 6f |nto the bottom o| 000063c0 66 20 74 68 65 20 73 74 61 63 6b 0d 28 66 72 6f |f the stack.(fro| 000063d0 6d 20 26 31 30 30 20 6f 6e 77 61 72 64 73 20 73 |m &100 onwards s| 000063e0 69 6e 63 65 20 74 68 65 20 73 74 61 63 6b 20 69 |ince the stack i| 000063f0 73 20 75 70 73 69 64 65 20 64 6f 77 6e 29 2c 20 |s upside down), | 00006400 61 6e 64 20 4a 4d 50 0d 74 6f 20 79 6f 75 72 20 |and JMP.to your | 00006410 42 52 4b 20 61 74 20 26 31 30 30 2e 20 20 59 6f |BRK at &100. Yo| 00006420 75 20 68 61 76 65 20 74 6f 20 64 6f 20 74 68 69 |u have to do thi| 00006430 73 20 62 65 63 61 75 73 65 20 74 68 65 20 65 72 |s because the er| 00006440 72 6f 72 0d 63 6f 64 65 20 69 6e 20 74 68 65 20 |ror.code in the | 00006450 6c 61 6e 67 75 61 67 65 20 79 6f 75 20 61 72 65 |language you are| 00006460 20 63 61 6c 6c 69 6e 67 20 66 72 6f 6d 20 77 69 | calling from wi| 00006470 6c 6c 20 61 75 74 6f 6d 61 74 69 63 61 6c 6c 79 |ll automatically| 00006480 0d 6c 6f 6f 6b 20 69 6e 20 74 68 61 74 20 6c 61 |.look in that la| 00006490 6e 67 75 61 67 65 27 73 20 52 4f 4d 20 69 66 20 |nguage's ROM if | 000064a0 79 6f 75 20 65 78 65 63 75 74 65 20 74 68 65 20 |you execute the | 000064b0 42 52 4b 20 69 6e 20 79 6f 75 72 0d 52 4f 4d 2e |BRK in your.ROM.| 000064c0 20 20 59 6f 75 20 68 61 76 65 20 74 6f 20 65 78 | You have to ex| 000064d0 65 63 75 74 65 20 74 68 65 20 42 52 4b 20 69 6e |ecute the BRK in| 000064e0 20 6d 61 69 6e 20 6d 65 6d 6f 72 79 2c 20 61 6e | main memory, an| 000064f0 64 20 74 68 65 0d 73 74 61 63 6b 20 69 73 20 61 |d the.stack is a| 00006500 20 73 61 66 65 20 70 6c 61 63 65 20 74 6f 20 64 | safe place to d| 00006510 6f 20 73 6f 2e 0d 0d 46 72 6f 6d 20 61 20 53 65 |o so...From a Se| 00006520 72 76 69 63 65 20 52 4f 4d 20 79 6f 75 20 63 61 |rvice ROM you ca| 00006530 6e 20 61 64 64 72 65 73 73 20 49 2f 4f 20 6d 65 |n address I/O me| 00006540 6d 6f 72 79 20 72 65 61 73 6f 6e 61 62 6c 79 0d |mory reasonably.| 00006550 73 61 66 65 6c 79 20 73 69 6e 63 65 20 79 6f 75 |safely since you| 00006560 72 20 63 6f 64 65 20 77 69 6c 6c 20 6e 65 76 65 |r code will neve| 00006570 72 20 65 78 65 63 75 74 65 20 69 6e 20 61 20 73 |r execute in a s| 00006580 65 63 6f 6e 64 0d 70 72 6f 63 65 73 73 6f 72 2e |econd.processor.| 00006590 20 20 4f 66 74 65 6e 20 73 70 65 65 64 20 69 73 | Often speed is| 000065a0 20 69 6d 70 6f 72 74 61 6e 74 2c 20 61 6e 64 20 | important, and | 000065b0 79 6f 75 20 63 61 6e 20 74 61 6b 65 0d 61 64 76 |you can take.adv| 000065c0 61 6e 74 61 67 65 20 6f 66 20 79 6f 75 72 20 70 |antage of your p| 000065d0 6c 61 63 65 20 69 6e 20 74 68 65 20 49 2f 4f 20 |lace in the I/O | 000065e0 70 72 6f 63 65 73 73 6f 72 2e 0d 0d 54 68 61 74 |processor...That| 000065f0 20 6c 65 61 64 73 20 6d 65 20 61 6c 6d 6f 73 74 | leads me almost| 00006600 20 6e 65 61 74 6c 79 20 6f 6e 74 6f 20 74 68 65 | neatly onto the| 00006610 20 74 6f 70 69 63 20 66 6f 72 20 74 68 65 20 6e | topic for the n| 00006620 65 78 74 0d 6d 6f 64 75 6c 65 2e 20 20 41 63 6f |ext.module. Aco| 00006630 72 6e 20 6f 66 74 65 6e 20 74 65 6c 6c 20 75 73 |rn often tell us| 00006640 20 77 68 61 74 20 69 73 20 6c 65 67 61 6c 20 61 | what is legal a| 00006650 6e 64 20 77 68 61 74 20 69 73 0d 69 6c 6c 65 67 |nd what is.illeg| 00006660 61 6c 2e 20 49 20 77 69 6c 6c 20 65 78 70 61 6e |al. I will expan| 00006670 64 20 6f 6e 20 74 68 61 74 20 73 75 62 6a 65 63 |d on that subjec| 00006680 74 20 6e 65 78 74 20 74 69 6d 65 2e 0d |t next time..| 0000668d
OS\BITS/T\OSB24.m0
OS\BITS/T\OSB24.m1
OS\BITS/T\OSB24.m2
OS\BITS/T\OSB24.m4
OS\BITS/T\OSB24.m5
.