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Section
01 Part 03 – Instruction Set Out |
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“Computer Science is no more about computers than astronomy is about telescopes.” ~Edsger W. Dijkstra. |
Introduction
The
68k has a set of “instructions” that you can use (i.e. commands). You can use these instructions to get the 68k
to do what you want it to do. They can
be instructions that change numbers, control where the processor reads from or
writes to, and many other things.
In
this part, we’re going to look at the basic set out of an instruction.
Mnemonics
In
computing, instructions are read by the processor in binary (bits), here’s an
example of an instruction:
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0011 0000 0011 1100 0000 0100 1111 0000 |
Of course,
we don’t want to work exclusively with binary, it would simply take too long to
memorise and work with. This is where
assembly comes in.
We
use what are known as “mnemonics”, and they are simply words written in our
language to make it easier for us to program.
Here’s the same instruction, but in mnemonic form:
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move.w #$04F0,d0 |
Now,
you may notice that I’ve broken the instruction up into different colour coded
sections, we have; the command, size, source operand, and destination operand.
So,
let’s take a look at an instruction you’ll find easy to understand:
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move.b #$2C,$0000001E |
This
instruction will move the byte 2C into memory at offset 0000001E. As you can see, it does exactly what it says
on the tin.
After
the 68k processes that instruction, the byte in memory at offset 0000001E will be 2C:
|
Offset |
0 |
1 |
2 |
3 |
4 |
5 |
6 |
7 |
8 |
9 |
A |
B |
C |
D |
E |
F |
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00000000 |
00 |
00 |
00 |
00 |
00 |
00 |
00 |
00 |
00 |
00 |
00 |
00 |
00 |
00 |
00 |
00 |
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00000010 |
00 |
00 |
00 |
00 |
00 |
00 |
00 |
00 |
00 |
00 |
00 |
00 |
00 |
00 |
2C |
00 |
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00000020 |
00 |
00 |
00 |
00 |
00 |
00 |
00 |
00 |
00 |
00 |
00 |
00 |
00 |
00 |
00 |
00 |
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00000030 |
00 |
00 |
00 |
00 |
00 |
00 |
00 |
00 |
00 |
00 |
00 |
00 |
00 |
00 |
00 |
00 |
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etc |
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It
really is that simple.
When
you assemble your code, the assembler will convert all of your mnemonics:
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move.w #$04F0,d0 |
Into
binary:
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0011 0000 0011 1100 0000 0100 1111 0000 |
So
the 68k can read it.
The # and $
symbols
You
may have also noticed the # and $ symbols that appear before the 2C and 0000001E, the $ symbol is to tell the
assembler that the number is a “hex” number, not “decimal” number, if you had
no $ symbol for example:
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move.b #32,$0000001E |
The
assembler will convert 32
(decimal) to 0010
0000 (binary)
when it assembles it. 0010 0000 is 20 in hex, so writing 32 is the same as writing $20. If you wanted to write binary numbers, you can
use the % symbol.
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move.b #%00100000,$0000001E move.b #$20,$0000001E move.b #32,$0000001E |
All
of these above are exactly the same, the top is the binary version (%), the
middle is hexadecimal ($) and the bottom is decimal. We’ll be using hex and binary in this
tutorial more than decimal to help you get a good feel and understanding of
them.
The #
symbol on the other hand, is to tell the assembler, that the number is an
“immediate” value, and not an offset. So
what’s an “immediate” value? Hold that
thought for just one moment, and let us look at an example without the #
symbol:
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move.b $00000010,$0000002D |
This
will read the byte held at offset 00000010, and copy it over to offset
0000002D, if the byte at offset 00000010 was 49, then 0000002D will now also be
49:
|
Offset |
0 |
1 |
2 |
3 |
4 |
5 |
6 |
7 |
8 |
9 |
A |
B |
C |
D |
E |
F |
|
00000000 |
00 |
00 |
00 |
00 |
00 |
00 |
00 |
00 |
00 |
00 |
00 |
00 |
00 |
00 |
00 |
00 |
|
00000010 |
49 |
00 |
00 |
00 |
00 |
00 |
00 |
00 |
00 |
00 |
00 |
00 |
00 |
00 |
00 |
00 |
|
00000020 |
00 |
00 |
00 |
00 |
00 |
00 |
00 |
00 |
00 |
00 |
00 |
00 |
00 |
49 |
00 |
00 |
|
00000030 |
00 |
00 |
00 |
00 |
00 |
00 |
00 |
00 |
00 |
00 |
00 |
00 |
00 |
00 |
00 |
00 |
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etc |
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And
now back to “immediate”, as far as I’m concerned, it’s just a fancy name for
“raw number”, the # symbol tells the 68k that the number is not an
offset/address.