I'm studying the Intel's IA-32 software developer manual. In particular, I'm reading the following manual: http://www.intel.com/Assets/PDF/manual/253666.pdf. Let's take for example the ADD instruction. On page 79 it is written that you can add an r8 (8-bit register) to an r/m8 (8-bit register or memory location). A few rows below, it is also written that you can add an r/m8 to an r8. The question is: if I add two 8-bit registers, which instruction I am using? Thanks.
+7
A:
The add instruction has multiple versions... same mnemonic can be encoded to different opcodes, depending on what operands you use. (and to answer your specific question: the "add r8,r8" instruction probably has 2 different possible encodings, that do the same thing)
Virgil
2010-04-19 06:45:01
Similar to overloaded functions?
dboarman
2010-04-20 17:27:55
for some definitions of "similar"...... i guess you could say so :)
Virgil
2010-04-22 10:20:56
+1
A:
The ADD instruction has 9 different encoding types and two opcodes are reserved for:
Opcode: 00/r = ADD r/m8, r8
Opcode: 02/r = ADD r8, r/m8
In case "ADD r8, r8" both have some effect.
GJ
2010-04-19 07:22:08