80386 Barrel Shifter

Implementing rotate through carry like that was a really bad decision IMO - it's almost never by more than one bit left or right at a time, and this could be done much more efficiently than with the constant-time code which is only faster when the count is > 6.

Is the full microcode available anywhere?

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ajenner 2 hours ago | parent | next [-]

I haven't published it yet as there are still some rough edges to clear up, but if you email me (andrew@reenigne.org) I'll send you the current work-in-progress (the same one that nand2mario is working from).

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kjs3 2 hours ago | parent | prev [-]

Since the shifter is also used for bit tests, the 'most things are a 1-bit shift' might not be the case. Perhaps they did the analysis and it made sense.

	▲	rep_lodsb an hour ago \| parent [-]
		There are separate opcodes for shift/rotate by 1, by CL, or by an immediate operand. Those are decoded to separate microcode entry points, so they could have at least optimized the "RCL/RCR x,1" case. And the microcode for bit test has to be different anyway.