Re: [hatari-devel] 68000: Wrong order of bus accesses for move.l xx,-(Ay)

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Le 27/01/2018 à 18:00, Thorsten Otto a écrit :
On Freitag, 26. Januar 2018 20:50:26 CET Toni Wilen wrote:
different instruction variants
modify address register differently:

Oh god. That will be an endless story then. How is that possible? It does not
make much sense to me.


It doesn't make sense because when it was designed by Motorola the goal was not to have a behaviour that looks "logical" to us from the outside, but to divide each opcode in some sub-instructions to try to maximize bus access with logical/arithmetic operations, all that with a very limited number of transistors in the end.

So sometimes, some opcode will share the same sub-instructions, even if it would not seems to be the most "logical" way, but it allows to limit the number of sub-instruction.

In the end, it's true that even some braod rules appear, it sometimes looks like if many instruction/address mode have their own hardcoded behaviour.

Nicolas



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