Section 01 Part 04 – Registers

“Life is like a cash register, in that every account, every thought, every deed, like every sale, is registered and recorded.” ~Fulton J. Sheen.

Introduction

Processors have memory spaces called registers, and different processors have different types of registers for different reasons. They’re used for moving, adding, subtracting, numbers quickly and effectively to and from places. We’re going to look at the common 16 registers of the 68k and what they can do.

Data Registers

The 68k has 8 “data” registers, and are named:

d0, d1, d2, d3, d4, d5, d6 and d7

“Data registers” are used for storing and changing numbers mathematically, each register has a long-word of space:

register name	space (in hexadecimal)
d0	00 00 00 00
d1	00 00 00 00
d2	00 00 00 00
d3	00 00 00 00
d4	00 00 00 00
d5	00 00 00 00
d6	00 00 00 00
d7	00 00 00 00

Using the previous instruction as an example:

move.w #$04F0,d0

This will move the word 04F0 into data register d0, after the instruction is processed by the 68k, d0 will contain 00 00 04 F0.

You can perform byte, word and long-word instructions on any of them at any time, and you can use most instructions on them, but we’ll get into them in more detail later.

Address Registers

The 68k has another set of 8 registers called “address” registers, and are named:

a0, a1, a2, a3, a4, a5, a6 and a7

“Address registers” also have long-word spaces each:

register name	space (in hexadecimal)
a0	00 00 00 00
a1	00 00 00 00
a2	00 00 00 00
a3	00 00 00 00
a4	00 00 00 00
a5	00 00 00 00
a6	00 00 00 00
a7	00 00 00 00

They can be used in much the same way as data registers, except, you cannot perform byte instructions on them (only word or long-word), and some instructions that work on data registers, will not work on address registers, but we’ll get into those details later on.

These registers are used for storing addresses (i.e. offsets), and can write to or read from those offsets, for example:

movea.l #$00000039,a0

This will put the long-word 00000039 into a0, a0 will now contain 00000039, now that is pretty much the same as a data register, but now that we have the long-word inside a0, we can use it to our advantage:

move.b #$44,(a0)

You’ll notice that with this instruction, a0 is inside brackets, this will tell the 68k that instead of moving 44 into a0, it should move it to memory at the offset inside a0, a0 contains 00000039, so this instruction will put 44 into memory at the offset 00000039:

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	00	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	00	00	00
00000030	00	00	00	00	00	00	00	00	00	44	00	00	00	00	00	00
etc

As you can see, a0 acts as a gateway to memory.

The “address registers” have other features too, such as “indexing”, for example (and we’ll pretend that a0 still has 00000039 inside of it):

move.b #$9B,$04(a0)

The $04 you see right before the bracket, is the index value, this will add to the offset inside a0 to give you a new offset. So a0 contains 00000039, you add the 04 which gives you 0000003D, so, 9B is moved to offset 0000003D:

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	00	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	00	00	00
00000030	00	00	00	00	00	00	00	00	00	00	00	00	00	9B	00	00
etc

Here’s another example to clarify:

movea.l #$00000010,a0

move.b #$20,$01(a0)

move.b #$40,$02(a0)

move.b #$F0,$0F(a0)

move.b #$0E,(a0)

So, 00000010 is moved into a0, 20 is moved to 00000010 plus 01, 40 is moved to 00000010 plus 02, F0 is moved to 00000010 plus 0F, and 0E is moved to 00000010 directly, which gives us:

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	0E	20	40	00	00	00	00	00	00	00	00	00	00	00	00	F0
00000020	00	00	00	00	00	00	00	00	00	00	00	00	00	00	00	00
00000030	00	00	00	00	00	00	00	00	00	00	00	00	00	00	00	00
etc

Auto Increment/Auto Decrement

Another feature is auto increment and auto decrement, observe:

movea.l #$00000020,a0

move.b #$B5,(a0)+

move.b #$11,(a0)+

You’ll notice the + symbols on the right end of the brackets, this is auto increment.

• So first off, 00000020 is moved into a0.
• Then B5 is moved to the address at a0 (B5 goes to offset 00000020).
• The + symbol will then “increment” a0; this basically means it’ll increase the offset inside a0 by 1 (Now a0 contains 00000021).
• The next instruction moves 11 to the address at a0 (11 goes to offset 00000021).
• The + symbol will then “increment” a0; adding 1 to it (Now a0 contains 00000022).

As you can see, once the byte has been written to the address, a0 is increased by 1.

Here’s another example, using word:

movea.l #$00000020,a0

move.w #$A90E,(a0)+

• So first off, 00000020 is moved into a0.
• Then word A90E is moved to the address at a0 (A9 goes to offset 00000020, 0E goes to offset 00000021).
• The + symbol will then “increment” a0; adding 2 to it (Now a0 contains 00000022).

The reason why it increases by 2 this time, is because we moved a “word” of data, a word is 2 bytes. If we moved a long-word, it would have incremented by 4, because a long-word is 4 bytes.

Auto decrement is similar, but in reverse:

movea.l #$00000020,a0

move.b #$2E,-(a0)

You’ll notice in this example, there’s a – symbol, and instead of being on the right side, it is on the left side

• So first off, 00000020 is moved into a0.
• The - symbol will “decrement” a0; subtracting 1 from it (Now a0 contains 0000001F).
• THEN the number 2E is moved to the address 0000001F.

For decrement, the address register is always decreased before the instruction moves the number.

And there you have it. There are other indexing methods used with address registers, but that’s better left for another later part.

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