Author: James Robertson
Date: 14:20:48 01/27/99
Go up one level in this thread
On January 27, 1999 at 16:45:03, Eugene Nalimov wrote: >On January 27, 1999 at 15:40:06, James Robertson wrote: > >>On January 27, 1999 at 15:00:37, Eugene Nalimov wrote: >> >>>On January 27, 1999 at 14:50:59, James Robertson wrote: >>> >>>>On January 27, 1999 at 01:32:28, Eugene Nalimov wrote: >>>> >>>>>On January 26, 1999 at 22:38:37, James Robertson wrote: >>>>> >>>>>>Once again I show my absolute ignorance of assembly with these two questions: >>>>>>I am trying to acces the 3rd [+ 4th] byte of a register. How? E.g., what is next >>>>>>after al, ah, ? >>>>>>How do I pop something off the stack without moving the stack pointer? >>>>>> >>>>>>Thanks, >>>>>>James >>>>> >>>>>1. Use shift instruction. E.g. >>>>> shr eax, 16 >>>>> mov byte ptr [esi], al >>>>> You can also use rotate instruction (it'll not destruct >>>>> other bytes): >>>>> ror eax, 16 >>>>> mov byte ptr [esi], al >>>>> ror eax, 16 >>>>> But if I remember it correctly, rotate is worse than shift >>>>> (cannot be executed in parallel) on both Pentium and P6 family. >>>>>2. mov reg32, dword ptr [esp] >>>>> >>>>>Eugene >>>> >>>>Ok.... that would work. Because I already have stuff stored in ax, and I know >>>>the exact values I want to put into the 'e' part, I wrote: >>>> >>>>or eax,0xeeee0000 >>>>and eax,0x0000ffff >>>>or eax,0xeeee0000 >>>> >>>>where eeee=what I want to put in. This seems to work. Are there any problems I >>>>am missing? >>>> >>>>James >>> >>>First instruction is unnecessary - one "and" and one "or" will >>>be enough. >>> >>>Also, try to use 32-bit values instead of 16-bit values where >>>possible. 16-bit operations are more expensive than either 8-bit >>>or 32-bit. >> >>I'm confused. Aren't these 32 bit values? >> >>James >> >>> >>>Eugene > >Yes, those values are all 32-bit. But you wrote "stuff already >stored in ax", so I guessed that you made so using instruction >that operates with 16-bit values. That instruction is usually >worse than instruction that operates with all 32 bits. Actually, I have the results of another 32-bit operation in eax. The maximum value for the result is 63, which is < 16 bits, so I can be sure that the higher bits of eax are free for other stuff. > >One more hint (as you still refusing to read documentation): That's not true. :) I downloaded several files, but I read slowly. >if >you used 16-bit move to load ax, or if you done that using 2 >8-bit moves, and then immediately (less than 20 instructions >later) try to use eax as a whole, than P6/PII will stall for up >to 10 CPU clocks before executing those instructions (there are >some exceptions to that rule - you can read about it Intel >manuals). I wonder why Intel decided to do that..... James > >Eugene
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