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Section
05 Part 01 – The PC (Program Counter) |
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“The opposite for courage is not cowardice, it is conformity.
Even a dead fish can go with the flow” ~Jim Hightower |
Introduction
There
are instructions that change the location of where the 68k is reading its
instructions. To allow the 68k to do
this, they use the PC and SP, so let’s take a look.
The PC
(Program Counter)
The
PC (Program Counter) is another 68k register; it is designed to hold the
current location of where the 68k is currently reading. Now, all of the instructions you have learnt
so far, have to exist somewhere in memory, for example:
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move.w #$0F20,d0 add.l d1,d0 lsr.w #$02,d0 move.w d0,$00200000 |
I’ve
colour coded each instruction differently for this example. These instructions have to exist in memory
somewhere for the 68k to be able to read them, so we’ll pretend they’re in
memory at offset 00001000:
|
Offset |
0 |
1 |
2 |
3 |
4 |
5 |
6 |
7 |
8 |
9 |
A |
B |
C |
D |
E |
F |
|
00001000 |
30 |
3C |
0F |
20 |
D0 |
81 |
E4 |
48 |
33 |
C0 |
00 |
20 |
00 |
00 |
00 |
00 |
|
00001010 |
00 |
00 |
00 |
00 |
00 |
00 |
00 |
00 |
00 |
00 |
00 |
00 |
00 |
00 |
00 |
00 |
|
00001020 |
00 |
00 |
00 |
00 |
00 |
00 |
00 |
00 |
00 |
00 |
00 |
00 |
00 |
00 |
00 |
00 |
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00001030 |
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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As I
said before, the 68k reads instructions in binary, but you can probably see the
similarities, like the move instruction, where the 303C is the move.w and d0, and you can see after it the
0F20.
The
PC is 24-bit, meaning it’ll only hold the end three bytes of the offset.
In
our example above, the PC will start with 001000.
As
you can see, the PC is the main control over where the 68k reads. In this section, we’ll be looking at jump and
branch instructions that can change the PC, and allow you to change where the
68k reads its instructions from.