🚀 go-pugleaf

In article <34228@grapevine.uucp> johnz@grapevine.uucp (John Zolnowsky ext. 33230) writes:
>
>Although virtual memory was desired for the 68000, it was deemed too costly
>to provide an instruct restart model.  This required many more temporary

>
>After the 68000 went to market, the instruction continuation model was
>conceived.  This is best understood as an interrupt at the microcode level.

>
>Presumably, in later processors from the family, the provision of extra
>hardware to reduce instruction cycle counts also leads to a reduction
>in the indeterminancy of the values of registers.  This makes the cost
>of instruction restart more tractable.
>
So what you're saying is that motorola first thought that virtual
memory support could only be supported through instruction restart
later on they conceived the concept of instruction continuation.

The point is still this: why go to instruction restart seeing that

	(1) you _can't_ provide virtual (memory-mapped) i/o with instruction 
	    restart (this is the virtual machine part that can't be supported)

	(2) you can have big problems with memory-mapped i/o based
	    on reads

Instruction restart appears to be _less_ powerful than instruction 
continuation. Just look at the algorithm recently defined here to
prevent instruction restart from doing multiple read accesses. 
-- 
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