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17_10_87/T\OSB01
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: | 17_10_87/T\OSB01 |
| Read OK: | ✔ |
| File size: | 3372 bytes |
| Load address: | 0000 |
| Exec address: | 0000 |
File contents
OSBITS- An Exploration of the BBC Micro at Machine Level
by Programmer
...........................................................
Part 1: The BBC Assembler.
BBC BASIC is not the only language you get with your BBC
Micro. Built into the BASIC language ROM is an assembler.
This is the program that helps you to program using machine
code in the same way that the BASIC chip itself in your
machine helps you to program in BASIC.
Programming a computer in machine code means giving the
machine commands in the number code it directly understands.
Generally when people talk about programming in machine code
they really mean using an assembler. The assembler takes
your program, written in 'assembly language', and translates
it into machine code. Your original assembly language
program is called your 'source code'. Once the assembler
has translated your source into machine code and 'assembled'
it into memory you no longer need the source. You could
throw it away and the chunk of machine code would still do
its job all by itself.
This can make for faster and more efficient programs than in
BASIC for two reasons. Firstly you are writing the program
in a form very close to the form in which the machine actually
computes. Secondly BASIC stays in the memory and is
translated into machine code line by line, command by
command, as the program runs. Each command is first turned
into machine instructions and then executed, which is time
consuming. BBC BASIC is called an 'interpreted' language
because it works in this way and the assembler, which does
the translation before, rather than during, executing the
code is said to be 'compiled'. Generally compiled languages
are faster that interpreted ones.
Besides wanting a faster program than BASIC there are some
things that can only be done in machine code. These are
explorations of the very innards of the computer, and we
will try some of these later in the course. (Personally I
find them more fun that just being able to multiply faster!)
It is possible to write the numbers of machine code directly
into memory, but actually entering the numbers that tell the
computer to add the contents of two memory locations
together (for example) is a little error-prone. The
assembler, which allows mnemonics and address labels to be
used, clarifies this process.
Here's our example, which adds 5 to 32 and puts the result
into memory location &2021:
In machine code In BBC assembler
&A9 &20 LDA #&20 Put &20 in the accumulator
&18 CLC Clear the carry flag
&69 &05 ADC #&05 Add 5 (with carry)
&85 &21 &20 STA result Store in memory labelled
'result' (&2021)
Now at this stage the LDA/CLC/ADC bits in the middle may be
just as obscure as those numbers at the left, however I hope
you will agree that LDA is easier to remember than &A9,
especially since the code to put a number in the accumulator
changes depending on whether the number is put in directly
(as here) or loaded from memory, and where in memory, and
how you get there! In fact LDA can represent 8 different
machine code instructions depending on exactly what you want
to do. The assembler sorts out which one is appropriate.
(Incidentally, putting a & in front of a number is the way
you tell BBC BASIC that a number is in base 16, hexadecimal,
instead of base 10, decimal. The accumulator is a number
store in the microprocessor with which we can do
arithmetic.)
LDA is a mnemonic, standing for LoaD the Accumulator. When
you run an assembly language program it takes the mnemonics
and labels and numbers and converts them into the
appropriate machine code and assembles the code into the
section of memory you require.
The BBC assembler is actually part of BBC BASIC. You use it
from BASIC which gives you certain advantages. As we will
see later you can use the structure of BASIC to help your
assembler programming and you can also use BASIC commands
within the assembler. For example if you want to load the
accumulator with the ASCII code for a space you can use any
of these methods:
LDA #&20
LDA #32
LDA #ASC(" ")
char%=32
LDA char%
They all mean exactly the same thing.
The names with which you label memory locations in your
assembler program are BASIC variables and follow the same
rules. This means you can use long and clear label names if
you want to, and since the source code is not needed in the
machine when you run the machine code the length of your
variable names does not affect the length of your program,
or its speed, unlike BASIC.
To set up the assembler you should have a piece of BASIC
like this:
10 REM Assembler set-up example
20
30 DIM code% 256
40
50 FOR pass% = 0 TO 3 STEP 3
60 P% = code%
70 [OPT pass%
80 ...... the first line of assembler
490 ...... the last line of assembler
500 ]
510 NEXT
In line 30 we are allocating some memory in which to
assemble the machine code with the alternative use for the
DIM statement. The soft loving protection of BASIC is not
provided in machine code so the assembler will not allocate
memory for you nor will it stop you allocating yourself a
silly piece of memory. DIM code% 256 claws back a little
help from BASIC by taking 256 bytes of memory starting at
the address now held in the variable code%. BASIC will not
overwrite this block of memory but it will also not stop you
trying to use more than you asked for, although usually the
assembly will fail.
By silly memory I mean things like accidentally assembling
the code into the space in which BASIC is storing your
source code, or putting it into a crucial part of the
computer's work space. Tricks like this will certainly
crash the machine or lose you your source code, but at least
the computer won't explode! (Honest!)
The code is assembled in two passes, hence the FOR/NEXT
loop. This loop, by setting a variable pass% is instructing
the assembler to work in a certain way by setting an OPT
function (not to be confused with *OPT) at the start of the
code. The general idea is that during the first pass the
assembler will learn where all the labelled addresses are
and in the second pass will place those addresses into the
code. During the first pass (with OPT=0) error trapping is
turned off; otherwise a label pointing to a piece of memory
later in the program would cause a 'no such variable' error.
Also the assembler is instructed not to list the assembly to
screen in the first pass. In the second pass (with OPT=3)
error trapping is turned on and a listing is produced. This
listing of the assembly is optional but useful.
P%, set equal to code% in line 60, is a BASIC variable which
is tied to the value held in one of the microprocessor's
registers or number stores, called the program counter. As
your code is assembled the assembler sets P% to the address
of the next piece of code to be assembled. At the beginning
of each pass of the assembly P% is reset to the start of
your chunk of memory. That is why P% is set inside the
FOR/NEXT loop. When your machine code is executed, fancy
machine code jargon for 'run', the real program counter in
the microprocessor keeps track of the next command to be
carried out. In other words P% keeps track of the addresses
of instructions during assembly in the same way the program
counter proper does during execution.
The square brackets in lines 70 and 500 mark the start and
finish of the assembler in your BASIC program. In mode 7
they come out as arrows but mean the same thing.
Here's a little more code:
100 .first
110 LDA #32 \ this is a comment
120 CLC:ADC#20:STA &70
At line 100 is a label. It is simply a variable name that
will hold the address at this point. The assembler knows
it's a label because you have put a full stop in front of
it. It is important to remember that in assembler you store
a number at an address and the address is often held as a
variable. Don't think of the number as being stored in the
variable itself.
The second part of line 110 is a comment. Anything
following a back-slash is ignored by the assembler, like REM
in BASIC. Comment your programs liberally!
Layout of the assembly program is up to you. It is legal to
join statements with colons just like BASIC, but there is no
real benefit and it reduces the readability of your source
code. You can put labels on the same line as statements but
my personal preference is to put the labels on separate
lines.
The program B/osb01 in this telesoftware transmission is a
simple assembler program to print out a message to the
screen, a sort of confidence test. This is how it works:
10REM OSbits Module B/osb01
20REM Introduction to the BBC Assembler
30REM Version 1.0 - 17.8.86
40
It's always a good idea to identify your programs and put a
version number and date on them.
50 *KEY1MO.3|M|NL.|M
60
I always work in Mode 3 for assembler. This gives plenty of
room for comments and also things like the backslash and
square brackets come out right, which they don't in Mode 7.
So I set up function key 1 to give me a listing in Mode 3.
70 osasci=&FFE3
OSASCI is one of a set of operating system routines built
into the BBC micro. It starts at address &FFE3. I'll be
dealing with operating system routines in detail later.
OSASCI takes the number in the Accumulator (which I'll call
A) and sends it to the screen, exactly like VDU ?A would in
BASIC if you could read the contents of the accumulator with
a ?. (The Accumulator and the X and Y registers are the
three places to store and manipulate numbers that are
actually within the microprocessor, again more later.)
The variable is in lower case because I give all my
assembler variables lower case names. I find it aids
readability and keeps you completely clear of any problems
of clashes with BASIC reserved words.
80 code% = &3000
90
100 FOR pass%=0 TO 3 STEP 3
110 P%=code%
120
130 [OPT pass%
140 LDX #0
150
This sets up the assembly and puts a zero into the X
register, which we will use as a counter. Here we will
assemble the code at address &3000, which should be safe in
Modes 3 and 7, rather than let BASIC find some space. This
is simply so that the listing will be predictable when we
compare results at the end!
160 .loop
170 LDA text,X
LDA text,X is the start of our loop (labelled 'loop') to
print out characters taken from a string stored starting at
address 'text'. LoaD the Accumulator with the number stored
at the address given by 'text'+X.
180 JSR osasci
190 INX
Do a VDU ?A and then INcrease X by one. OSASCI is a
subroutine and JSR means Jump to SubRoutine and then come
back. A subroutine is a self-contained chunk of code (a bit
like a procedure in BBC BASIC) starting at the address to
which you JSR. It always ends with an RTS command (meaning
ReTurn from Subroutine).
200 CMP #13
210 BNE loop
220
CoMPare the contents of A with 13 and then Branch if Not
Equal back to 'loop'. (This is only a rough description of
what CMP/BNE actually means.)
230 RTS
240
Our little machine code programme behaves as if it were a
subroutine called from BASIC so at the end of the active
part of the program we ReTurn from the Subroutine. Try
deleting this line, you will have to press BREAK and then
type OLD to recover after you call your code if you do!
250 .text
260 ]
270 NEXT
280
290 $P%="Hello World, this is machine code!"
300 CALL code%
I've put a string just after the machine code by using the $
indirection operator. This puts the string in memory at P%
followed by a carriage return (ASCII 13). There are other
ways of doing this as we'll see later. Since the label
'text' was at the end of the assembler part of the program
it will have the value of P% at this point.
Finally we CALL the machine code at code% and, lo, there is
the message printed out. Ignoring the comments on each line
in the assembler code in B/osb01, it should look like this:
>RUN
3000 OPT pass%
3000 A2 00 LDX #0
3002
3002 .loop
3002 BD 0E 30 LDA text,X
3005 20 E3 FF JSR osasci
3008 E8 INX
3009 C9 0D CMP #13
300B D0 F5 BNE loop
300D
300D 60 RTS
300E
300E .text
Hello World, this is machine code!
>
Those numbers down the left, 3000 and so on, are the program
counter settings as the assembly was carried out. The
numbers between the program counter values and the nmemonics
and labels are the actual machine code numbers poked into
memory.
So you see, you can do it in assembler. I hope you will now
know what an assembler is, how it is different from BASIC,
and why you might want to use it to write machine code
programs. You should know how to set up the BBC assembler
as part of a BASIC program and will have assembled a short
piece of code which prints out a message.
Next week I'll explain some basics of binary arithmetic and
we'll try some addition and subtraction.
00000000 0d 4f 53 42 49 54 53 1a 2d 20 41 6e 20 45 78 70 |.OSBITS.- An Exp|
00000010 6c 6f 72 61 74 69 6f 6e 20 6f 66 20 74 68 65 20 |loration of the |
00000020 42 42 43 20 4d 69 63 72 6f 20 61 74 20 4d 61 63 |BBC Micro at Mac|
00000030 68 69 6e 65 20 4c 65 76 65 6c 0d 0d 62 79 20 50 |hine Level..by P|
00000040 72 6f 67 72 61 6d 6d 65 72 0d 0d 2e 2e 2e 2e 2e |rogrammer.......|
00000050 2e 2e 2e 2e 2e 2e 2e 2e 2e 2e 2e 2e 2e 2e 2e 2e |................|
*
00000080 2e 2e 2e 2e 2e 2e 0d 0d 0d 0d 50 61 72 74 20 31 |..........Part 1|
00000090 3a 20 20 20 20 54 68 65 20 42 42 43 20 41 73 73 |: The BBC Ass|
000000a0 65 6d 62 6c 65 72 2e 0d 0d 0d 42 42 43 20 42 41 |embler....BBC BA|
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00000100 6c 61 6e 67 75 61 67 65 20 52 4f 4d 20 69 73 20 |language ROM is |
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00003372 
17_10_87/T\OSB01.m0
17_10_87/T\OSB01.m1
17_10_87/T\OSB01.m2
17_10_87/T\OSB01.m4
17_10_87/T\OSB01.m5
.