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Section
01 Part 06 – Addressing & Program Flow |
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“If builders built buildings the way programmers wrote programs, then the first woodpecker that came along would destroy civilization.” ~One of Murphy's Laws of Technology |
Introduction
There
are a few important things about addressing, which need to be covered before
learning other instructions. We’ll also
take a look at the standard flow and direction of programming assembly code.
Odd
addressing
By
now, you will of course have seen examples of moving data in and out of memory,
for example:
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move.w $00000046,d0 |
What
you don’t know however, is that you cannot move “word” or “long-word” sized
data in and out of memory, if the address is an odd number:
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move.w $00000047,d0 |
Because
00000047 is an odd number (an odd address/offset), the 68k has trouble working
with it, and will crash with an “address error”.
You
can only access data on odd addresses using “byte”:
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move.b $00000047,d0 |
The
above example is OK, because only a byte is loaded from that odd address.
The
same applies to “address registers”, if you have an odd address inside an
address register, and then attempt to load a “word” or “long-word” in or out of
memory from it, the 68k will crash:
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movea.l #$00000047,a0 move.w (a0),d0 |
This
applies to ANY instruction, not just
the MOVE instruction. As long as you
make sure that only bytes are move to and from odd addresses, then you’ll do
fine.
Program
Flow
You’ve
probably already worked this one out by now, but I digress. When programming one instruction after
another, the direction is always downwards:
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move.b #$20,d0 move.b d0,d1 move.w d2,d0 |
So,
always starting at the top:
Always
heading down, each instruction is dealt with, one by one.
There
are instructions available that will change the flow for a brief moment, but
we’ll be coming to those at a later part.
Homework
This is
a chance for you to test what you’ve learnt so far. Below is a list of instructions which will
move numbers to and from places:
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move.w #$0123,d0 move.b d0,d1 move.w d1,d2 move.l #$FFFF0000,d3 move.w d0,d3 move.l d3,d0 move.w d2,d0 |
All
of the data registers will start with 00000000 to begin with. After all of this is processed, what will d0
contain?
The
answer and working out are on the next part, be sure to give it a good go
before moving on.