80386 Barrel Shifter
nand2mario.github.io52 points by jamesbowman 2 days ago
52 points by jamesbowman 2 days ago
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?
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).
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.
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.