Re: A89: Starting out assembly, need help
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Re: A89: Starting out assembly, need help
In a message dated 11/29/98 9:41:57 PM Eastern Standard Time,
assets@eden.rutgers.edu writes:
> move.w #10,a0 will move 10 into
> the lower word of a0. On the other hand, indirect addressing moves an
> immediate value or another register into the memory pointed to by one of
the
> address registers (those would be a0-a7): move.w #10,(a0) (the parethesis
> indicate indirectness) will move the word 10 into the memory addressed by
> a0; so, if a0 was $10000, then that instruction would move 10 into
($10000).
> The 68k is particular in indirect addressing in that you can only indirect
> address on even bytes of memory (otherwise you get the address error
> exception)
So this is sort of equivalent to the Indirection # in TI-Basic?
> The pre-decrement and post-increment features of the 68k will increment or
> decrement the address register by a word or a longword. Pre-decrement will
> have this effect: move.w #10,-(a0) will decrement a0 (the pointer) by a
word
> and then move the word 10 into the new (a0).
What does decrementing the pointer by a word mean?
> Post-increment will have the
> effect: move.w #10,(a0)+ will move the word 10 into new (a0) and then
> increment a0 by a word. These two addressing methods can be used to fill a
> large space (post-increment's faster than <move.w #10,(a0) / add.l #2,a0>)
> and are especially useful in pushing / popping.
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