bagpiper@oxy.UUCP (10/02/87)
In article <2306@sphinx.uchicago.edu> <4478@amd.AMD.COM> ayac071@ut-ngp.UUCP (William T. Douglass) writes: >In article <4478@amd.AMD.COM> ching@amd.UUCP (Mike Ching) writes: >>In article <2306@sphinx.uchicago.edu> cjdb@sphinx.uchicago.edu (Charles Blair) writes: >> The NOPs are never executed and don't slow down the >>processor. > >Actually, I thought that NOPs did an OR of the AX register on itself (or some >similar operation that causes no change to data.) As such, they DO affect the >execution time involved. > >Could someone knowledgeable confirm or refute this? > >Bill Douglass >ayac071@ngp.UUCP In the "The 80386 Programmer's Reference Manual" NOP is listed as Opcode: 90h Clocks: 3 Description: No operation NOP is an alias mnemonis for the XCHG (E)AX,(E)AX So NOP does take some time. The only chip that I have heard of that takes no time on NOP's is the novix forth chip which merges a nop in with the next instruction (so I guess you can actually say that it takes a fraction of a cycle). All in all it has to influence performance in some way since it has to be fetched in the minimal case. Of Course in the case of jump optimization who cares?? You never get to the NOP anyways!! After every JMP the prefetch que is fetched also so the NOP doesn't effect execution in that way. Michael Hunter UUCP : ....{seismo, rutgers, ames}!cit-vax!oxy!bagpiper Box 241 ARPA : oxy!bagpiper@csvax.caltech.edu Occidental College BITNET: oxy!bagpiper@hamlet.bitnet Los Angeles, CA 90041 CSNET : oxy!bagpiper%csvax.caltech.edu@relay.cs.net