CS255 Syllabus

Address Register Operands

The address registers of M68000

Recall:

Facts:

Address registers are used to store the address (= location) of a complex variable to help us access a component in the complex variable
An address is always expressed in 32 bits in M68000 !!!
Consequently:

Updating address registers

Rule:

When you store a value into an address register, then:

Example of using an address register:

Before the operation: +----------+----------+----------+----------+ D0 = | 00000000 | 11111111 | 00000000 | 00000011 | +----------+----------+----------+----------+ +----------+----------+----------+----------+ A0 = | 10101010 | 11110000 | 00001111 | 01010101 | +----------+----------+----------+----------+
We execute the operation: MOVE.W D0, A0 (A0 is the destination) After the operation: +----------+----------+----------+----------+ D0 = | 00000000 | 11111111 | 00000000 | 00000011 | +----------+----------+----------+----------+ +----------+----------+----------+----------+ A0 = | 00000000 | 00000000 | 00000000 | 00000011 | +----------+----------+----------+----------+ Notice that all 32 bits in A0 are updated

In contrast, if we use a data register, we will get this result:

Before the operation: +----------+----------+----------+----------+ D0 = | 00000000 | 11111111 | 00000000 | 00000011 | +----------+----------+----------+----------+ +----------+----------+----------+----------+ D1 = | 10101010 | 11110000 | 00001111 | 01010101 | +----------+----------+----------+----------+
We execute the operation: MOVE.W D0, D1 (D1 is the destination) After the operation: +----------+----------+----------+----------+ D0 = | 00000000 | 11111111 | 00000000 | 00000011 | +----------+----------+----------+----------+ +----------+----------+----------+----------+ D1 = | 10101010 | 11110000 | 00000000 | 00000011 | +----------+----------+----------+----------+ Notice that aonly 16 bits (word operand !!) in D1 are updated

DEMO: click here

Comments on instructions that update an address register

Nomenclature:

Assembler instructions that update an address register has an "A" postfix in their nmenomic

Examples:

MOVE.L D0, D1 * Copy the long word in D0 to D1 MOVEA.L D0, A1 * Copy the long word in D0 to A1 ADD.L D0, D1 * Add the long word value in D0 to that in D1 ADDA.L D0, A1 * Add the long word value in D0 to that in A1

Restriction on assembler instructions that update an address register:

You cannot use a byte form of an M68000 instruction when you update an address register

In other words:

MOVEA.B src, AddrReg is not allowed
You can only use: MOVEA.W src, AddrReg MOVEA.L sec, AddrReg