Data and address registers

The M68K processor has sixteen registers for everyday work. It divides them into two named groups:

• eight data registers, named d0 through d7;
• eight address registers, named a0 through a7.

The letter tells you the register's usual job. The number picks one register from that group. For example, d3 is data register 3 and a3 is address register 3. They are two separate registers.

Data registers

Data registers hold values that a program is working with. A value might be a score, a counter, a price or the result of a calculation.

A program could use some data registers like this:

register	value being kept there
d0	the current total
d1	the number of items
d2	the latest result

The meanings in the right-hand column come from the program. The register names themselves are fixed: the programmer chooses what each data register will hold and keeps track of that choice.

Address registers

Memory is made of many locations. Each location has an address: a number that identifies that location. An address works like a numbered place where the processor can find some stored data.

Suppose memory location 1200 contains the value 25. These are two different numbers with different jobs:

place	value
address register a0	1200
memory location numbered 1200	25

Here, a0 holds the address 1200. That address tells the processor which memory location is of interest. The data at that location is 25.

Address registers are used for location numbers like 1200. A program might use a0 for the start of some text and a1 for the next memory location it plans to visit. This gives the processor one group of registers for finding places in memory and another group for values used in calculations.

a7 and sp

The last address register has two names: a7 and sp. Both names refer to the same visible register. If its value changes under one name, the value shown under the other name changes too.

sp means stack pointer. The processor uses this register for stack bookkeeping, so reserve a7/sp for that job. For ordinary addresses, choose from a0 through a6.

Check your understanding

For each job, choose data register, address register, or a7/sp.

1. Keep a running total.
2. Keep the number of the memory location where a message begins.
3. Keep count of how many items have been processed.
4. Handle stack bookkeeping.

Then answer this question: if a2 contains 1200 and memory location 1200 contains 25, which value is the address?

Show answers

1. A data register such as d0.
2. An address register such as a0.
3. A data register such as d1.
4. a7/sp.

The address is 1200. The value 25 is the data stored at that memory location.