Help with simple assembly mmx exercise

Question

Given a vector of bytes with length multiple of 8, how can I, using mmx instructions, convert all 2's to 5's, for example?

.data
v1 BYTE 1, 2, 3, 4, 1, 2, 3, 4

Thanks.

edit: 2's and 5's are just an example. They are actually parameters of a procedure.

Answer 1

I'm sure there are several ways to do this. For instance, the following should work:

1) make (or load) a mask of 5's and one of 2's in two mmx registers (mm0-mm7)

2) load data into another mmx register, e.g using MOVQ

3) compare the register holding data to be tested with the mask of 2's, e.g. using PCMPEQB, this will result in a mask of FFh and 00h according to whether the element in the register was 2 or not

4) use MASKMOVQ, the register with 5's and the mask generated by the compare to selectively write out 5's to those positions that previously held 2's. MASKMOVQ will store data for the mask positions that held FFh values.

5) Repeat this until finished.

6) at the end, issue EMMS to exit MMX state. Also issue an SFENCE or MFENCE instruction at the end of the routine (because MASKMOVQ generates a non-temporal hint).

If you use MMX rather than XMM, you won't have to worry about alignment.

Edit: If you are having trouble with the details of the instructions, the Intel® 64 and IA-32 Architectures Software Developer's Manual, Instruction Set Reference (Volumes 2A and 2B), should contain everything you'll ever want to know. You can find them here.