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21_11_87/T\OSB05
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 » telesoftware3.adl |
| Filename: | 21_11_87/T\OSB05 |
| Read OK: | ✔ |
| File size: | 27C0 bytes |
| Load address: | 0000 |
| Exec address: | 0000 |
File contents
OSBITS - An Exploration of the BBC Micro at Machine Level
By Programmer
...........................................................
Part 5: Operating System Calls (I) - OSBYTE and OSWORD
It's worth taking a break from all the mathematical stuff to
look at some of the operating system calls. The OS calls
provide an easy access to operations that would otherwise be
quite difficult. They also help preserve the compatibility
of software as the operating system changes.
OSBYTE first.
OSBYTE is a machine code subroutine that lives in the micro
at address &FFF4. There are 256 possible OSBYTE calls (not
all of them used) and the one you wish to use is selected by
the value in the accumulator when you enter the subroutine.
The X and Y registers carry parameters for the particular
call you are using.
In BASIC you call OSBYTE either by using *FXA,X,Y (for
OSBYTE and *FX calls are the same thing) or by using BASIC's
way of setting A,X and Y which is:
A%=number_of_OSBYTE_call
X%=X
Y%=Y
CALL &FFF4
It's worth noting that because *FX calls are operating
system calls and not actually pert of the BASIC language you
can use them in command mode with other language ROMS like
VIEW and BCPL.
In BASIC2 and above you can also use OSCLI if there is a
variable you wish to include. I won't go into detail with
any of these BASIC methods here, we'll stick to assembler.
To enter the OSBYTE subroutine you have to set the A,X and Y
registers, possibly by using LDA, LDX and LDY like this:
osbyte=&FFF4
LDA #number_of_OSBYTE_call
LDX #X_parameter
LDY #Y_parameter
JSR osbyte
After the JSR you may find that the X and Y registers
contain useful information. In BASIC you would use the USR
function to obtain this but in machine code the bytes are
just sitting in the registers waiting for you.
There is a list of OSBYTE calls in one of your user guides
but I will list a few useful examples here (with
acknowledgments to Bray, Dickens and Holmes 'Advanced User
Guide'):
OSBYTE 2 selects the source from which input is taken,
called the input stream. The input stream is selected by X
and if X=0 the keyboard is selected and the RS423 is
disabled (which means that no carrier would be sent from a
modem for example). If X=1 then the RS423 is selected and
enabled, so you can receive from it. If X=2 then the
keyboard is selected for input but the RS423 is still
enabled.
Once the call has been executed the value in the X register
will be 0 if the previous input was from the keyboard and 1
if the previous input was from the RS423. This can help you
restore previous conditions after you have changed
something.
Also on exit the value in the Accumulator is the same as it
was before the call, we say it has been preserved. The
value in the Y register, and the state of the Carry flag are
undefined.
OSBYTE 117 is an information call. You set the accumulator
to 117 but the values of the X and Y registers are of no
consequence.
After the call the accumulator and Y register have been
preserved and the Carry flag is undefined but the X register
contains useful information about the VDU display status,
bit by bit.
If Bit 0 is set then printer output is enabled by a VDU2
If Bit 1 is set then scrolling is disabled
If Bit 2 is set then paged mode is selected (i.e. CTRL N)
If Bit 3 is set then a text window is in operation
Bit 4 is not used
If Bit 5 is set then VDU5 has been sent and text is being
printed at the graphics cursor
If Bit 6 is set then the input and output cursors are
separated and cursor editing is in progress
If Bit 7 is set then the VDU is disabled following a VDU21
You can test the value held in X bit by bit using the
logical operators AND or BIT which I'll come onto in a later
module.
OSBYTE 129 operates like BASIC's INKEY. The X and Y
registers hold the time limit for waiting for a key. X
contains the Lower Significant Byte (time MOD 256) and Y
contains the MSB (time DIV 256) up to a limit of &7FFF.
This is &7FFF hundredths of a second giving an upper limit
of about 5 and a half minutes.
On exit from the routine the X register will hold the ASCII
value of the key pressed. Y will hold zero and the Carry
flag will be clear. If no key was pressed during that time
then Y will contain &FF and the Carry flag will be set. If
escape was pressed then Y will contain &1B (27) and the
Carry flag will be set.
If the figure held in X and Y is negative then this is the
equivalent to INKEY(-N) where N is a key number. Here the
routine will tell you if the key with that number was
pressed at the moment the routine scanned the keyboard.
On exit X and Y will both contain &FF if the key is pressed.
The file B/osb05 with this transmission contains a small
program which uses a similar OSBYTE call, number 121, which
scans the keyboard and returns a number corresponding to the
key pressed down at the time. This is then used to play a
note using an OSWORD call.
OSWORD operates in a similar way to OSBYTE except that here
the parameters you need to pass to and from the routines are
too big to fit into a couple of bytes. In this case,
instead of using X and Y to hold parameters, you use X and Y
to point to a block of memory in which you put your
parameters. The accumulator holds the OSWORD number in the
same way it did with OSBYTE. OSWORD is called at &FFF1.
There are only 14 OSWORD routines in the BBC Model B
operating system, and 16 in the Master. They are as
follows:
OSWORD 0 - Read a line from input
OSWORD 1/2 - Read/write system clock
OSWORD 3/4 - Read/write interval timer
OSWORD 5/6 - Read/write I/O processor memory
OSWORD 7 - Sound command
OSWORD 8 - Envelope definition
OSWORD 9 - Read value of a pixel on the screen
OSWORD 10 - Read definition of a character
OSWORD 11/12 - Read/write palette
OSWORD 13 - Read the last two graphics cursor positions
OSWORD 14/15 - Read/write CMOS clock in Master
OSWORD calls with other values in the accumulator are passed
to sideways ROMS and you find them applying to filing
systems and the like. For example:
OSWORD 127 enables you to read/write a sector of a disc in
the disc filing system
OSWORD 122 enables you to work with the Acorn or BBC
telesoftware ROMS
OSWORD 64 accesses the status of the AMX mouse if you have
this software in your machine.
In all cases the OSWORD block is set up like this. You set
aside the relevant number of bytes for your parameter block
by using, perhaps, the FNEQU or EQU functions in your
assembler program. Let's say that this block starts at the
address labelled 'osword_block'
LDA #number_of_OSWORD_call
LDX #(osword_block MOD 256)
LDY #(osword_block DIV 256)
so that the X register contains the LSB of the address of
the parameter block and the Y register contains the MSB of
this address.
The parameter block varies in size depending on the OSWORD
call and in many cases, like with the teletext OSWORD call,
the byte held at the start of the block selects different
operations within the one OSWORD call.
Let's look at the SOUND call, OSWORD 7. The parameter block
for this is as follows:
osword_block + 0 LSB of the channel number
osword_block + 1 MSB of the channel number
osword_block + 2 LSB of the amplitude/envelope number
osword_block + 3 MSB of the amplitude/envelope number
osword_block + 4 LSB of the pitch
osword_block + 5 MSB of the pitch
osword_block + 6 LSB of the duration
osword_block + 7 MSB of the duration
In fact the structure exactly follows the BASIC SOUND
command. The program B/osb05 in this week's telesoftware
transmission is a program that takes the internal value of a
key pushed (the basis for its value for -INKEY) and uses it
to select the pitch of a note that is then played. It's not
particularly amazing but it will show you how the OSBYTE and
OSWORD calls are set out in assembler.
I have used OSBYTE 121 which scans the keyboard upwards from
the internal key number held in X. The internal key number
is the INKEY number of the key with all its bits reversed.
The posh thing to say is that is has been EORd with &FF.
More on EOR in another module. I have started my scan from
16 upwards which means I avoid any problems caused by having
keyboard links set. In fact OSBYTE 122 does exactly this
but using it would not illustrate the OSBYTE setting up as
much. In fact I have not used the Y register in these
OSBYTEs but you would LDY with whatever value you wanted
before the JSR. Y register use in OSBYTES is not as common
as X register use.
For more detailed information on OSWORD and OSBYTE calls I
recommend you consult the advanced user or reference guide
for your particular machine.
Points to note in B/osb05 are that the routine loops
continuously, albeit via BASIC to make it easy to ESCAPE out
of it. If no key is pressed, i.e. &FF is returned in X,
then the SOUND section of the code is skipped. Also the
sound buffer is flushed using OSBYTE 21 when no key is
pressed, which cuts off the note when you lift your finger
from the key. In line 440 note that UNTIL0 means exactly
the same as UNTIL FALSE since the value of FALSE is 0 in BBC
BASIC.
To recap on addressing modes, you will see in lines like 140
and 150 that the number is preceded by a hash (#) which
tells the assembler that the actual number following is to
be used. Without the hash the assembler treats the
following number as an address and reads from that. It's a
common mistake to forget the hash when you intend immediate
addressing (as this is called).
If you have BASIC2 or later you can replace each occurrence
of OPT FNEQUW(number) with EQUW number and you can, of
course, also drop the function definition from the end of
the program.
In the next module we'll look at how you shift the bits of a
byte about and how to use the stack for temporary storage.
The program will take a byte entered as a character from the
keyboard and print its value in binary, hexadecimal and
decimal.
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 2e 0d 0d 0d 50 61 72 74 20 35 3a 20 |........Part 5: | 00000090 4f 70 65 72 61 74 69 6e 67 20 53 79 73 74 65 6d |Operating System| 000000a0 20 43 61 6c 6c 73 20 28 49 29 20 2d 20 4f 53 42 | Calls (I) - OSB| 000000b0 59 54 45 20 61 6e 64 20 4f 53 57 4f 52 44 0d 0d |YTE and OSWORD..| 000000c0 0d 49 74 27 73 20 77 6f 72 74 68 20 74 61 6b 69 |.It's worth taki| 000000d0 6e 67 20 61 20 62 72 65 61 6b 20 66 72 6f 6d 20 |ng a break from | 000000e0 61 6c 6c 20 74 68 65 20 6d 61 74 68 65 6d 61 74 |all the mathemat| 000000f0 69 63 61 6c 20 73 74 75 66 66 20 74 6f 0d 6c 6f |ical stuff to.lo| 00000100 6f 6b 20 61 74 20 73 6f 6d 65 20 6f 66 20 74 68 |ok at some of th| 00000110 65 20 6f 70 65 72 61 74 69 6e 67 20 73 79 73 74 |e operating syst| 00000120 65 6d 20 63 61 6c 6c 73 2e 20 20 54 68 65 20 4f |em calls. The O| 00000130 53 20 63 61 6c 6c 73 0d 70 72 6f 76 69 64 65 20 |S calls.provide | 00000140 61 6e 20 65 61 73 79 20 61 63 63 65 73 73 20 74 |an easy access t| 00000150 6f 20 6f 70 65 72 61 74 69 6f 6e 73 20 74 68 61 |o operations tha| 00000160 74 20 77 6f 75 6c 64 20 6f 74 68 65 72 77 69 73 |t would otherwis| 00000170 65 20 62 65 0d 71 75 69 74 65 20 64 69 66 66 69 |e be.quite diffi| 00000180 63 75 6c 74 2e 20 20 54 68 65 79 20 61 6c 73 6f |cult. They also| 00000190 20 68 65 6c 70 20 70 72 65 73 65 72 76 65 20 74 | help preserve t| 000001a0 68 65 20 63 6f 6d 70 61 74 69 62 69 6c 69 74 79 |he compatibility| 000001b0 0d 6f 66 20 73 6f 66 74 77 61 72 65 20 61 73 20 |.of software as | 000001c0 74 68 65 20 6f 70 65 72 61 74 69 6e 67 20 73 79 |the operating sy| 000001d0 73 74 65 6d 20 63 68 61 6e 67 65 73 2e 0d 0d 4f |stem changes...O| 000001e0 53 42 59 54 45 20 66 69 72 73 74 2e 0d 0d 4f 53 |SBYTE first...OS| 000001f0 42 59 54 45 20 69 73 20 61 20 6d 61 63 68 69 6e |BYTE is a machin| 00000200 65 20 63 6f 64 65 20 73 75 62 72 6f 75 74 69 6e |e code subroutin| 00000210 65 20 74 68 61 74 20 6c 69 76 65 73 20 69 6e 20 |e that lives in | 00000220 74 68 65 20 6d 69 63 72 6f 0d 61 74 20 61 64 64 |the micro.at add| 00000230 72 65 73 73 20 26 46 46 46 34 2e 20 20 54 68 65 |ress &FFF4. The| 00000240 72 65 20 61 72 65 20 32 35 36 20 70 6f 73 73 69 |re are 256 possi| 00000250 62 6c 65 20 4f 53 42 59 54 45 20 63 61 6c 6c 73 |ble OSBYTE calls| 00000260 20 28 6e 6f 74 0d 61 6c 6c 20 6f 66 20 74 68 65 | (not.all of the| 00000270 6d 20 75 73 65 64 29 20 61 6e 64 20 74 68 65 20 |m used) and the | 00000280 6f 6e 65 20 79 6f 75 20 77 69 73 68 20 74 6f 20 |one you wish to | 00000290 75 73 65 20 69 73 20 73 65 6c 65 63 74 65 64 20 |use is selected | 000002a0 62 79 0d 74 68 65 20 76 61 6c 75 65 20 69 6e 20 |by.the value in | 000002b0 74 68 65 20 61 63 63 75 6d 75 6c 61 74 6f 72 20 |the accumulator | 000002c0 77 68 65 6e 20 79 6f 75 20 65 6e 74 65 72 20 74 |when you enter t| 000002d0 68 65 20 73 75 62 72 6f 75 74 69 6e 65 2e 20 0d |he subroutine. .| 000002e0 54 68 65 20 58 20 61 6e 64 20 59 20 72 65 67 69 |The X and Y regi| 000002f0 73 74 65 72 73 20 63 61 72 72 79 20 70 61 72 61 |sters carry para| 00000300 6d 65 74 65 72 73 20 66 6f 72 20 74 68 65 20 70 |meters for the p| 00000310 61 72 74 69 63 75 6c 61 72 0d 63 61 6c 6c 20 79 |articular.call y| 00000320 6f 75 20 61 72 65 20 75 73 69 6e 67 2e 0d 0d 49 |ou are using...I| 00000330 6e 20 42 41 53 49 43 20 79 6f 75 20 63 61 6c 6c |n BASIC you call| 00000340 20 4f 53 42 59 54 45 20 65 69 74 68 65 72 20 62 | OSBYTE either b| 00000350 79 20 75 73 69 6e 67 20 2a 46 58 41 2c 58 2c 59 |y using *FXA,X,Y| 00000360 20 28 66 6f 72 0d 4f 53 42 59 54 45 20 61 6e 64 | (for.OSBYTE and| 00000370 20 2a 46 58 20 63 61 6c 6c 73 20 61 72 65 20 74 | *FX calls are t| 00000380 68 65 20 73 61 6d 65 20 74 68 69 6e 67 29 20 6f |he same thing) o| 00000390 72 20 62 79 20 75 73 69 6e 67 20 42 41 53 49 43 |r by using BASIC| 000003a0 27 73 0d 77 61 79 20 6f 66 20 73 65 74 74 69 6e |'s.way of settin| 000003b0 67 20 41 2c 58 20 61 6e 64 20 59 20 77 68 69 63 |g A,X and Y whic| 000003c0 68 20 69 73 3a 0d 0d 20 20 20 20 20 20 20 20 20 |h is:.. | 000003d0 20 20 20 20 20 20 20 20 20 20 20 20 20 41 25 3d | A%=| 000003e0 6e 75 6d 62 65 72 5f 6f 66 5f 4f 53 42 59 54 45 |number_of_OSBYTE| 000003f0 5f 63 61 6c 6c 0d 20 20 20 20 20 20 20 20 20 20 |_call. | 00000400 20 20 20 20 20 20 20 20 20 20 20 20 58 25 3d 58 | X%=X| 00000410 0d 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 |. | 00000420 20 20 20 20 20 20 20 59 25 3d 59 0d 20 20 20 20 | Y%=Y. | 00000430 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 | | 00000440 20 20 43 41 4c 4c 20 26 46 46 46 34 0d 0d 49 74 | CALL &FFF4..It| 00000450 27 73 20 77 6f 72 74 68 20 6e 6f 74 69 6e 67 20 |'s worth noting | 00000460 74 68 61 74 20 62 65 63 61 75 73 65 20 2a 46 58 |that because *FX| 00000470 20 63 61 6c 6c 73 20 61 72 65 20 6f 70 65 72 61 | calls are opera| 00000480 74 69 6e 67 0d 73 79 73 74 65 6d 20 63 61 6c 6c |ting.system call| 00000490 73 20 61 6e 64 20 6e 6f 74 20 61 63 74 75 61 6c |s and not actual| 000004a0 6c 79 20 70 65 72 74 20 6f 66 20 74 68 65 20 42 |ly pert of the B| 000004b0 41 53 49 43 20 6c 61 6e 67 75 61 67 65 20 79 6f |ASIC language yo| 000004c0 75 0d 63 61 6e 20 75 73 65 20 74 68 65 6d 20 69 |u.can use them i| 000004d0 6e 20 63 6f 6d 6d 61 6e 64 20 6d 6f 64 65 20 77 |n command mode w| 000004e0 69 74 68 20 6f 74 68 65 72 20 6c 61 6e 67 75 61 |ith other langua| 000004f0 67 65 20 52 4f 4d 53 20 6c 69 6b 65 0d 56 49 45 |ge ROMS like.VIE| 00000500 57 20 61 6e 64 20 42 43 50 4c 2e 0d 0d 49 6e 20 |W and BCPL...In | 00000510 42 41 53 49 43 32 20 61 6e 64 20 61 62 6f 76 65 |BASIC2 and above| 00000520 20 79 6f 75 20 63 61 6e 20 61 6c 73 6f 20 75 73 | you can also us| 00000530 65 20 4f 53 43 4c 49 20 69 66 20 74 68 65 72 65 |e OSCLI if there| 00000540 20 69 73 20 61 0d 76 61 72 69 61 62 6c 65 20 79 | is a.variable y| 00000550 6f 75 20 77 69 73 68 20 74 6f 20 69 6e 63 6c 75 |ou wish to inclu| 00000560 64 65 2e 20 20 49 20 77 6f 6e 27 74 20 67 6f 20 |de. I won't go | 00000570 69 6e 74 6f 20 64 65 74 61 69 6c 20 77 69 74 68 |into detail with| 00000580 0d 61 6e 79 20 6f 66 20 74 68 65 73 65 20 42 41 |.any of these BA| 00000590 53 49 43 20 6d 65 74 68 6f 64 73 20 68 65 72 65 |SIC methods here| 000005a0 2c 20 77 65 27 6c 6c 20 73 74 69 63 6b 20 74 6f |, we'll stick to| 000005b0 20 61 73 73 65 6d 62 6c 65 72 2e 0d 0d 54 6f 20 | assembler...To | 000005c0 65 6e 74 65 72 20 74 68 65 20 4f 53 42 59 54 45 |enter the OSBYTE| 000005d0 20 73 75 62 72 6f 75 74 69 6e 65 20 79 6f 75 20 | subroutine you | 000005e0 68 61 76 65 20 74 6f 20 73 65 74 20 74 68 65 20 |have to set the | 000005f0 41 2c 58 20 61 6e 64 20 59 0d 72 65 67 69 73 74 |A,X and Y.regist| 00000600 65 72 73 2c 20 70 6f 73 73 69 62 6c 79 20 62 79 |ers, possibly by| 00000610 20 75 73 69 6e 67 20 4c 44 41 2c 20 4c 44 58 20 | using LDA, LDX | 00000620 61 6e 64 20 4c 44 59 20 6c 69 6b 65 20 74 68 69 |and LDY like thi| 00000630 73 3a 0d 0d 20 20 20 20 20 20 20 20 20 20 20 20 |s:.. | 00000640 20 20 20 20 20 20 6f 73 62 79 74 65 3d 26 46 46 | osbyte=&FF| 00000650 46 34 0d 0d 20 20 20 20 20 20 20 20 20 20 20 20 |F4.. | 00000660 20 20 20 20 20 20 4c 44 41 20 23 6e 75 6d 62 65 | LDA #numbe| 00000670 72 5f 6f 66 5f 4f 53 42 59 54 45 5f 63 61 6c 6c |r_of_OSBYTE_call| 00000680 0d 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 |. | 00000690 20 20 20 4c 44 58 20 23 58 5f 70 61 72 61 6d 65 | LDX #X_parame| 000006a0 74 65 72 0d 20 20 20 20 20 20 20 20 20 20 20 20 |ter. | 000006b0 20 20 20 20 20 20 4c 44 59 20 23 59 5f 70 61 72 | LDY #Y_par| 000006c0 61 6d 65 74 65 72 0d 20 20 20 20 20 20 20 20 20 |ameter. | 000006d0 20 20 20 20 20 20 20 20 20 4a 53 52 20 6f 73 62 | JSR osb| 000006e0 79 74 65 0d 0d 41 66 74 65 72 20 74 68 65 20 4a |yte..After the J| 000006f0 53 52 20 79 6f 75 20 6d 61 79 20 66 69 6e 64 20 |SR you may find | 00000700 74 68 61 74 20 74 68 65 20 58 20 61 6e 64 20 59 |that the X and Y| 00000710 20 72 65 67 69 73 74 65 72 73 0d 63 6f 6e 74 61 | registers.conta| 00000720 69 6e 20 75 73 65 66 75 6c 20 69 6e 66 6f 72 6d |in useful inform| 00000730 61 74 69 6f 6e 2e 20 20 49 6e 20 42 41 53 49 43 |ation. In BASIC| 00000740 20 79 6f 75 20 77 6f 75 6c 64 20 75 73 65 20 74 | you would use t| 00000750 68 65 20 55 53 52 0d 66 75 6e 63 74 69 6f 6e 20 |he USR.function | 00000760 74 6f 20 6f 62 74 61 69 6e 20 74 68 69 73 20 62 |to obtain this b| 00000770 75 74 20 69 6e 20 6d 61 63 68 69 6e 65 20 63 6f |ut in machine co| 00000780 64 65 20 74 68 65 20 62 79 74 65 73 20 61 72 65 |de the bytes are| 00000790 0d 6a 75 73 74 20 73 69 74 74 69 6e 67 20 69 6e |.just sitting in| 000007a0 20 74 68 65 20 72 65 67 69 73 74 65 72 73 20 77 | the registers w| 000007b0 61 69 74 69 6e 67 20 66 6f 72 20 79 6f 75 2e 0d |aiting for you..| 000007c0 0d 54 68 65 72 65 20 69 73 20 61 20 6c 69 73 74 |.There is a list| 000007d0 20 6f 66 20 4f 53 42 59 54 45 20 63 61 6c 6c 73 | of OSBYTE calls| 000007e0 20 69 6e 20 6f 6e 65 20 6f 66 20 79 6f 75 72 20 | in one of your | 000007f0 75 73 65 72 20 67 75 69 64 65 73 0d 62 75 74 20 |user guides.but | 00000800 49 20 77 69 6c 6c 20 6c 69 73 74 20 61 20 66 65 |I will list a fe| 00000810 77 20 75 73 65 66 75 6c 20 65 78 61 6d 70 6c 65 |w useful example| 00000820 73 20 68 65 72 65 20 28 77 69 74 68 0d 61 63 6b |s here (with.ack| 00000830 6e 6f 77 6c 65 64 67 6d 65 6e 74 73 20 74 6f 20 |nowledgments to | 00000840 42 72 61 79 2c 20 44 69 63 6b 65 6e 73 20 61 6e |Bray, Dickens an| 00000850 64 20 48 6f 6c 6d 65 73 20 27 41 64 76 61 6e 63 |d Holmes 'Advanc| 00000860 65 64 20 55 73 65 72 0d 47 75 69 64 65 27 29 3a |ed User.Guide'):| 00000870 0d 0d 4f 53 42 59 54 45 20 32 20 73 65 6c 65 63 |..OSBYTE 2 selec| 00000880 74 73 20 74 68 65 20 73 6f 75 72 63 65 20 66 72 |ts the source fr| 00000890 6f 6d 20 77 68 69 63 68 20 69 6e 70 75 74 20 69 |om which input i| 000008a0 73 20 74 61 6b 65 6e 2c 0d 63 61 6c 6c 65 64 20 |s taken,.called | 000008b0 74 68 65 20 69 6e 70 75 74 20 73 74 72 65 61 6d |the input stream| 000008c0 2e 20 20 54 68 65 20 69 6e 70 75 74 20 73 74 72 |. The input str| 000008d0 65 61 6d 20 69 73 20 73 65 6c 65 63 74 65 64 20 |eam is selected | 000008e0 62 79 20 58 0d 61 6e 64 20 69 66 20 58 3d 30 20 |by X.and if X=0 | 000008f0 74 68 65 20 6b 65 79 62 6f 61 72 64 20 69 73 20 |the keyboard is | 00000900 73 65 6c 65 63 74 65 64 20 61 6e 64 20 74 68 65 |selected and the| 00000910 20 52 53 34 32 33 20 69 73 0d 64 69 73 61 62 6c | RS423 is.disabl| 00000920 65 64 20 28 77 68 69 63 68 20 6d 65 61 6e 73 20 |ed (which means | 00000930 74 68 61 74 20 6e 6f 20 63 61 72 72 69 65 72 20 |that no carrier | 00000940 77 6f 75 6c 64 20 62 65 20 73 65 6e 74 20 66 72 |would be sent fr| 00000950 6f 6d 20 61 0d 6d 6f 64 65 6d 20 66 6f 72 20 65 |om a.modem for e| 00000960 78 61 6d 70 6c 65 29 2e 20 49 66 20 58 3d 31 20 |xample). If X=1 | 00000970 74 68 65 6e 20 74 68 65 20 52 53 34 32 33 20 69 |then the RS423 i| 00000980 73 20 73 65 6c 65 63 74 65 64 20 61 6e 64 0d 65 |s selected and.e| 00000990 6e 61 62 6c 65 64 2c 20 73 6f 20 79 6f 75 20 63 |nabled, so you c| 000009a0 61 6e 20 72 65 63 65 69 76 65 20 66 72 6f 6d 20 |an receive from | 000009b0 69 74 2e 20 20 49 66 20 58 3d 32 20 74 68 65 6e |it. If X=2 then| 000009c0 20 74 68 65 0d 6b 65 79 62 6f 61 72 64 20 69 73 | the.keyboard is| 000009d0 20 73 65 6c 65 63 74 65 64 20 66 6f 72 20 69 6e | selected for in| 000009e0 70 75 74 20 62 75 74 20 74 68 65 20 52 53 34 32 |put but the RS42| 000009f0 33 20 69 73 20 73 74 69 6c 6c 0d 65 6e 61 62 6c |3 is still.enabl| 00000a00 65 64 2e 0d 0d 4f 6e 63 65 20 74 68 65 20 63 61 |ed...Once the ca| 00000a10 6c 6c 20 68 61 73 20 62 65 65 6e 20 65 78 65 63 |ll has been exec| 00000a20 75 74 65 64 20 74 68 65 20 76 61 6c 75 65 20 69 |uted the value i| 00000a30 6e 20 74 68 65 20 58 20 72 65 67 69 73 74 65 72 |n the X register| 00000a40 0d 77 69 6c 6c 20 62 65 20 30 20 69 66 20 74 68 |.will be 0 if th| 00000a50 65 20 70 72 65 76 69 6f 75 73 20 69 6e 70 75 74 |e previous input| 00000a60 20 77 61 73 20 66 72 6f 6d 20 74 68 65 20 6b 65 | was from the ke| 00000a70 79 62 6f 61 72 64 20 61 6e 64 20 31 0d 69 66 20 |yboard and 1.if | 00000a80 74 68 65 20 70 72 65 76 69 6f 75 73 20 69 6e 70 |the previous inp| 00000a90 75 74 20 77 61 73 20 66 72 6f 6d 20 74 68 65 20 |ut was from the | 00000aa0 52 53 34 32 33 2e 20 20 54 68 69 73 20 63 61 6e |RS423. 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21_11_87/T\OSB05.m0
21_11_87/T\OSB05.m1
21_11_87/T\OSB05.m2
21_11_87/T\OSB05.m4
21_11_87/T\OSB05.m5
.