Instruction set - Decode opcode

Question

I'm trying to understand how the /d affects the opcode.

Example: FF /6 PUSH r/m16 M Valid Valid Push r/m16.

How meaning is expressed?

Can anyone give me an example of the difference?

Thanks!

The information you have given is insufficient. Where does this opcode comes into play? Is it in a real hardware or in a simulator? — shazin, Jul 26 '13 at 10:03

score 2 · Answer 1 · answered May 21 '14 at 21:37

There are actually many instructions using FF as opcode:

INC     rm16                                    FF /0
INC     rm32                                    FF /0
INC     rm64                                    FF /0
DEC     rm16                                    FF /1
DEC     rm32                                    FF /1
DEC     rm64                                    FF /1
CALL    rm16                                    FF /2
CALL    rm32                                    FF /2
CALL    rm64                                    FF /2
CALL FAR        mem16:16                        FF /3
CALL FAR        mem16:32                        FF /3
JMP     rm16                                    FF /4
JMP     rm32                                    FF /4
JMP     rm64                                    FF /4
JMP FAR mem16:16                                FF /5
JMP FAR mem16:32                                FF /5
PUSH    rm16                                    FF /6
PUSH    rm32                                    FF /6
PUSH    rm64                                    FF /6

As you may see, the /d part is a 3 bit sequence held in the byte following the opcode (the so called ModR/M byte), which help discriminate the correct instruction.

From the Intel reference documentation:

Many instructions that refer to an operand in memory have an addressing-form spec- ifier byte (called the ModR/M byte) following the primary opcode. The ModR/M byte contains three fields of information:

• The mod field combines with the r/m field to form 32 possible values: eight registers and 24 addressing modes.

• The reg/opcode field specifies either a register number or three more bits of opcode information. The purpose of the reg/opcode field is specified in the primary opcode.

• The r/m field can specify a register as an operand or it can be combined with the mod field to encode an addressing mode. Sometimes, certain combinations of the mod field and the r/m field is used to express opcode information for some instructions.

So that /d value is actually extracted from the reg/opcode field. When the CPU loads up the first opcode, it knows that it should read an additional byte following it, and read that field in order to complete the instruction.

Instruction set - Decode opcode

1 Answers1