cclist at sydex.com
Mon Jul 25 11:19:58 CDT 2016
On 07/25/2016 02:31 AM, Peter Corlett wrote:
> Eliminating condition codes just moves the complexity from the ALU to
> the branch logic (which now needs its own mini-ALU for comparisons),
> and there's not much in it either way. Where it *does* win is that
> the useful instructions are all single-output and so one can use the
> noddy code generators found in undergraduate-level compiler
> construction textbooks such as the Dragon Book.
Sorry, I'm not following this bit. I am speaking about a three-address
ISA here, BTW.
Simply adding a flag to each register reflecting its zero/nonzero
content should do the job. The high-order (sign) bit is the only other
bit necessary. Branch instructions need only inquire if either, neither
or both flags are set.
I suppose that one could also have several flags registers, being set
selectively as dictated by a field in the instruction.
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