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06_11_87/T\OSB03
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: | 06_11_87/T\OSB03 |
| Read OK: | ✔ |
| File size: | 2885 bytes |
| Load address: | 0000 |
| Exec address: | 0000 |
File contents
OSBITS - An Exploration of the BBC Micro at Machine Level
By Programmer
............................................................
Part 3: The Registers and Flags
By now you should know how to set up the assembler in BBC
BASIC and have some ideas about using binary arithmetic.
What we'll cover this week is basic information about how
the microprocessor works with bytes and what you can
therefore do with it.
Let's start by looking at the useful parts of the
microprocessor. The stores you work with that are actually
part of the microprocessor are its 6 registers.
The registers you directly affect are the Accumulator and
the X and Y register. All your calculating is done with the
accumulator while X and Y can be used to show an offset from
a base address (as in the program in the first module) or as
counters. You can use them as temporary stores too.
Firstly, let's deal with the accumulator.
The name comes from the way it holds the current result of
your calculation, it is accumulating the result in the same
way an accumulator bet does. The two simplest things you
can do with it are to load it and save it.
The assembler mnemonic for loading the accumulator is LDA.
You can only hold a number between 0 and 255 in the
accumulator, and the assembler will generate a 'byte' error
if you try to load anything bigger. A hash (#) is used to
tell the assembler that you want to load a number directly,
e.g. LDA #32 means 'load the number 32 into the
accumulator'. Without the hash the accumulator would be
loaded from the memory location so that LDA 32 loads with
the byte stored in memory location 32 (&20). It's very easy
to forget the hash when loading a number directly and you
will get strange results if you do.
If you have committed this file to paper it is possible that
your printer has printed a pound sign instead of a hash,
this is a common problem in the UK. You should try to make
sure that in answer to ASCII code 35 you get a hash, it
looks like a sharp sign in music, as printed on the number 3
key on your micro keyboard. This will save you much bother
in dealing with assembly listings.
To store the contents of the accumulator in an address you
use STA. STA &123 will store the accumulator contents in
address &123. For both loading and storing you can, and
most often will, use labels. Here are some examples:
LDA #space Load variable 'space'
LDA pigeon_hole Load contents of address 'pigeon_hole'
STA hideaway Store in address 'hideaway'
The X and Y registers operate like the accumulator using the
mnemonics LDX STX LDY STY.
You can use the X and Y registers to offset your loading and
saving either directly or indirectly. For the time being
let's just deal with direct indexing, as it is called.
LDA pigeon_hole, X Load contents of 'pigeon_hole' + X
STA pigeon_hole, Y Save contents in 'pigeon_hole' + Y
In the first example work out ('pigeon_hole' + X) first,
this produces a new address X away from 'pigeon_hole', and
then load from there. Similarly with the second example.
Both the X and Y registers can be used as indices with both
LDA and STA. This gives you a way of working through a list
in memory, like a string to be printed out, or to move a
block of memory from one place to another if the block or
the list are no larger than 256 bytes.
Similarly to the accumulator X can be used as an index to
LDY and STY and Y as an index to LDX and STX.
You can increase or decrease the X and Y registers by one,
for counting, by using INX and INY to increase and DEX and DEY
to decrease. There is no equivalent for the accumulator in
basic 6502 assembler although INC and DEC will work on the
contents of memory locations.
The accumulator and the X and Y registers can be affected
directly by the programmer. Indirectly you can affect the
program counter, which holds the address of the next
instruction, and the status register.
The program counter is affected by any instruction that can
move execution to a different part of the program. There
are branches and jumps that do this. A branch can go back
or forwards by 127 and a jump can be to anywhere in the
address space. Rather than think of these instructions as
changing the program counter we should concentrate on what
they mean in terms of programming.
The status register has seven flags. Each flag is a bit of
the byte held in the register. This is where decisions can
be made since the microprocessor can be instructed to branch
to a different part of the program depending on the status
of some of the flags.
Bit 0 is the carry flag which is set on a carry during
addition and cleared by a borrow on subtraction. In some
ways you can think of it as a ninth bit of the byte you are
working on.
BCS means Branch if the Carry flag is Set, BCC means Branch
if the Carry flag is Clear.
Bit 1 is the zero flag which is set if the result of some
operation is zero, otherwise it is cleared.
BEQ means Branch if EQual to zero, i.e. if the zero flag is
set, BNE means Branch if Not Equal to zero, i.e. if the zero
flag is clear.
The next four bits do not enter into our branching and we'll
come back to them at a later stage.
Bit 2 is the interrupt flag, set when interrupts are
disabled.
Bit 3 is the decimal flag. When set the processor
calculates in Binary Coded Decimal.
Bit 4 is the break flag and is set during a BRK interrupt.
Bit 5 is not used.
Bit 6 is the overflow flag which is set if there is a
discrepancy between the carry between bits 6 and 7 and out
of bit 7 during addition and subtraction.
BVS means Branch if oVerflow set and BVC means Branch if
oVerflow clear.
Bit 7 is the negative flag, set if the top bit of the last
operation is set, otherwise clear.
BMI means Branch on MInus, i.e. the negative flag is set,
and BPL means Branch on PLus, i.e. the negative flag is
clear.
Finally there is the stack pointer. The stack is a special
section of memory used by the 6502 and this register holds
the lowest significant byte (LSB) of the next free stack
memory location. The 6502 always keeps its stack in memory
between &100 and &1FF, called Page 1. You can read and
write to the stack pointer and the stack but we'll leave
this to later.
We shall use branching instructions to carry out the
equivalent of IF THEN ELSE instructions. But we also need
to have the equivalent of GOTO and GOSUB and these are JMP
and JSR.
JMP elsewhere - simply jumps to the instruction stored at
'elsewhere'. In addition you can JMP (elsewhere) which
jumps to the instruction stored at the address stored at
'elsewhere' and 'elsewhere'+1. This is an indirect jump of
which more another time.
JSR elsewhere - jumps to the instruction stored at
'elsewhere' but the address of the instruction after the JSR
is stored on the stack and when the program reaches the next
RTS instruction it returns to that address. The piece of
program between 'elsewhere' and the RTS is a subroutine.
One important thing to remember about subroutines is that
they can change the values held in your registers and the
settings of your flags. It's worth noting what changes, if
any, the subroutine makes before it returns. In a later
module I will tell you how to preserve your registers and
flags.
A final set of instructions that will be needed for
branching are CMP, CPX and CPY. The status register flags
are set automatically under some circumstances. For example
the zero, negative, overflow and carry flags can all be
affected after an ADC. Another way of setting these flags
is by using CMP to compare the accumulator with either a
value in memory or directly with a number.
e.g. CMP somewhere or CMP #13
In both cases the value in the accumulator is not changed
but the flags are set and so branching instructions such as
BEQ or BMI can be used. One particular branch to remember
after a CMP is that the carry flag is set if the value in A
is greater than or equal to the value in 'somewhere' (or 13)
in the example above. This makes BCS equivalent to a branch
if greater than or equal. (Do not use BMI or BPL for this.)
In the program B/osb03 in this telesoftware transmission is
a routine that detects whether a key pressed is upper or
lower case, a number, punctuation or a control code and
prints out a message accordingly. It uses an operating
system routine to read in the key press, this is OSRDCH
which is the equivalent of BASIC's GET command. The
branching is done mainly using the carry flag and the
messages are output using indexed addressing and the OSASCI
routine for printing to the screen.
Things to note:
BCS gives a branch on greater than or equal to, not just on
greater than.
This program operates like a series of sieves, each stage
lets through fewer and fewer values in A until those left
can only be punctuation. In this case we start sieving from
the bottom, with control codes.
Note the double labelling of 'text' and 'control'. It takes
up no room in the assembled code of course but I hope it
makes the source code a little clearer.
The very convoluted way of putting the strings into memory
is not necessary, I will show you a more elegant way in the
next file.
In this particular piece of code you can replace every
occurrence of JSR **** followed by RTS with just a JMP ****.
Can you see why this works? Modify the source code to do
this and you will find it a little shorter. It will also be
a little faster but I doubt if you will notice that. Notice
what it leads you to do between lines 740 and 870 where you
have JMPs to the next line, which can then be removed
completely.
All in all this code is written to be readable as sections
but can be shortened to be more efficient. It is worth
going through the RTSs to see just where they return to.
When the code is running you can break out of it with
ESCAPE, which also is a control code. Every character
pressed on the black keys will lead to a printed descriptive
line.
I hope you now have some idea of how to use the assembler
for addition and subtraction (from last week) and for
branching and jumping. In the next module we will discuss
better ways of reserving memory for variables (in BASIC 1 as
well as in later BASICs) and where to put machine code
programs. The first thing to look at next week will be the
way the BBC Micro's memory is organised.
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 2e 0d 0d 0d 50 61 72 74 20 33 3a |.........Part 3:|
00000090 20 54 68 65 20 52 65 67 69 73 74 65 72 73 20 61 | The Registers a|
000000a0 6e 64 20 46 6c 61 67 73 0d 0d 0d 42 79 20 6e 6f |nd Flags...By no|
000000b0 77 20 79 6f 75 20 73 68 6f 75 6c 64 20 6b 6e 6f |w you should kno|
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000000e0 20 42 42 43 0d 42 41 53 49 43 20 61 6e 64 20 68 | BBC.BASIC and h|
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00000100 62 6f 75 74 20 75 73 69 6e 67 20 62 69 6e 61 72 |bout using binar|
00000110 79 20 61 72 69 74 68 6d 65 74 69 63 2e 20 0d 57 |y arithmetic. .W|
00000120 68 61 74 20 77 65 27 6c 6c 20 63 6f 76 65 72 20 |hat we'll cover |
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00000140 69 63 20 69 6e 66 6f 72 6d 61 74 69 6f 6e 20 61 |ic information a|
00000150 62 6f 75 74 20 68 6f 77 0d 74 68 65 20 6d 69 63 |bout how.the mic|
00000160 72 6f 70 72 6f 63 65 73 73 6f 72 20 77 6f 72 6b |roprocessor work|
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00000940 0d 4c 44 41 20 23 73 70 61 63 65 20 20 20 20 20 |.LDA #space |
00000950 20 20 20 20 4c 6f 61 64 20 76 61 72 69 61 62 6c | Load variabl|
00000960 65 20 27 73 70 61 63 65 27 0d 4c 44 41 20 70 69 |e 'space'.LDA pi|
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00000990 64 72 65 73 73 20 27 70 69 67 65 6f 6e 5f 68 6f |dress 'pigeon_ho|
000009a0 6c 65 27 0d 53 54 41 20 68 69 64 65 61 77 61 79 |le'.STA hideaway|
000009b0 20 20 20 20 20 20 20 53 74 6f 72 65 20 69 6e 20 | Store in |
000009c0 61 64 64 72 65 73 73 20 27 68 69 64 65 61 77 61 |address 'hideawa|
000009d0 79 27 0d 0d 54 68 65 20 58 20 61 6e 64 20 59 20 |y'..The X and Y |
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00000a20 54 58 20 4c 44 59 20 53 54 59 2e 0d 0d 59 6f 75 |TX LDY STY...You|
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00000a50 6f 20 6f 66 66 73 65 74 20 79 6f 75 72 20 6c 6f |o offset your lo|
00000a60 61 64 69 6e 67 20 61 6e 64 0d 73 61 76 69 6e 67 |ading and.saving|
00000a70 20 65 69 74 68 65 72 20 64 69 72 65 63 74 6c 79 | either directly|
00000a80 20 6f 72 20 69 6e 64 69 72 65 63 74 6c 79 2e 20 | or indirectly. |
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06_11_87/T\OSB03.m0
06_11_87/T\OSB03.m1
06_11_87/T\OSB03.m2
06_11_87/T\OSB03.m4
06_11_87/T\OSB03.m5
.