How to interpret x86 opcode map?

Question

In looking at an x86 opcode map such as this:

http://www.mlsite.net/8086/#tbl_map1

It defines mappings, for example:

00: ADD Eb,Gb
01: ADD Ev         


        

Answer 1

You're looking at an opcode map that translates the first byte of an opcode in the instruction pattern that that byte matches. If you want to know about the rest of the bytes of the instruction, you need to look elsewhere.

If you look at the page for the ADD instruction, it will show you something like:

00 /r        ADD r/m8, r8

this tells you that the 00 byte is followed by a ModR/M byte that contains the register r in the register field and that register is an 8-bit register that is the second operand of the ADD instruction (the r8 in the instruction pattern), while the first operand is in the rest of the ModR/M byte

Now if you go look at the documentation for ModR/M bytes, it will tell you that a ModR/M byte has 3 fields -- a 2-bit 'mod' field, a 3-bit 'register/opcode' field and a 3-bit 'r/m' field. It then give a table of all 256 ModR/M byte values noting what the fields mean in each case. This table is (generally) organized as 32 rows of 8 columns -- the 32 rows are split into 4 groups of 8, with the groups corresponding to the 'mod' field bits and the rows within the groups to the 'r/m' field bits, while the columns correspond to the 'register/opcode' field bits. Its a litte weird as the 'mod' is the top 2 bits and the 'r/m' is the bottom 3 bits with the 'register/opcode' in the middle, but it makes sense as the 'mod' and 'r/m' bits are closely associated and go together to describe one operand, while the 'register/opcode' bits are pretty much completely independent, describing the other operand or being part of the opcode.