Category:68000 Assembly: Difference between revisions

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====The Stack====

The 68000's stack is commonly referred to as <code>SP</code> but it is also address register <code>A7</code>. This register is handled differently than the other address registers when pushing bytes onto the stack. A byte value pushed onto the stack will be padded to the <b>right</b> with zeroes. The stack needs to pad byte-length data so that it can stay word-aligned at all times. Otherwise the CPU would crash as soon as you tried to use the stack for anything other than a byte! ~~If a byte is popped, the zeroes are placed on the left side of the lower word of the data register, and the actual byte goes in the right side.~~

The 68000's stack is commonly referred to as <code>SP</code> but it is also address register <code>A7</code>. This register is handled differently than the other address registers when pushing bytes onto the stack. A byte value pushed onto the stack will be padded to the <b>right</b> with zeroes. The stack needs to pad byte-length data so that it can stay word-aligned at all times. Otherwise the CPU would crash as soon as you tried to use the stack for anything other than a byte!

<lang 68000devpac>MOVE.B #$FF,-(SP) ;push #$FF then #$00 onto the stack, in that order.

MOVE.B (SP)+,D0 ;The values are popped in the order #$00 #$FF.</lang>

You can abuse this property of the stack to quickly swap bytes around. Suppose you had a number like <code>#$11223344</code> stored in <code>D0</code> and you wanted to change it to <code>#$11224433</code>:

<lang 68000devpac>MOVE.W D0,-(SP) ;push #$3344 onto the stack

ROL.W #8,D0

MOVE.B (SP)+,D0 ;pop them in the order #$44 #$33.</lang>