4

I saw that for example opcode FF 15 is for CALL on one example that I was looking online, but when I tried to follow that in instruction set reference manual, it says that CALL is for FF /2, FF /3 etc.

So I was wondering, what this /2and /3 represents?

Maybe it's mentioned somewhere in doc, but document is more than 2k pages, and not sure where to look for that.

ShP
  • 1,143
  • 1
  • 12
  • 41
  • 4
    Yes, it is mentioned in the doc. See section _"3.1 INTERPRETING THE INSTRUCTION REFERENCE PAGES"_, in particular section _"3.1.1.1 Opcode Column in the Instruction Summary Table"_ which says _"/digit -- A digit between 0 and 7 indicates that the ModR/M byte of the instruction uses only the r/m (register or memory) operand. The reg field contains the digit that provides an extension to the instruction's opcode."_ – Jester Apr 02 '18 at 13:36
  • So, basically this `15` represent that extension to this `FF` opcode? – ShP Apr 02 '18 at 13:44
  • The `15` is the modr/m byte. Only the `reg` field is the extension the `r/m` is the operand. `reg` is `2` and `r/m` is `5` meaning `disp32`. See section _"2.1.5 Addressing-Mode Encoding of ModR/M and SIB Bytes"_ – Jester Apr 02 '18 at 13:51
  • Thank you @Jester – ShP Apr 02 '18 at 13:59
  • 1
    Possible duplicate of [What does the /4 mean in FF /4?](https://stackoverflow.com/questions/24295464/what-does-the-4-mean-in-ff-4) – phuclv Apr 02 '18 at 14:27

0 Answers0