Full SIB + ModR/M + REX combined diagram

1. Full Instruction Layout (x86-64)

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+--------+---------+----------+-------+--------+--------+
|  REX   | OPCODE  | ModR/M   | SIB   | DISP   | IMM    |
+--------+---------+----------+-------+--------+--------+

Not all fields are always present, but this is the maximum structure.

2. Core Decoding Pipeline (High-Level)

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            MACHINE INSTRUCTION BYTES
                       |
                       v
        +-----------------------------+
        |        REX PREFIX          |
        | 0100WRXB                   |
        +-----------------------------+
           |      |      |      |
           |      |      |      +--> R/M or BASE extension
           |      |      +---------> INDEX extension (SIB)
           |      +-----------------> REG extension
           +-------------------------> 64-bit operand size
                       |
                       v
        +-----------------------------+
        |         OPCODE             |
        +-----------------------------+
                       |
                       v
        +-----------------------------+
        |         ModR/M             |
        |   MOD | REG | R/M         |
        +-----------------------------+
                       |
                       v
        +-----------------------------+
        |          SIB               |
        |   SCALE | INDEX | BASE    |
        +-----------------------------+
                       |
                       v
        +-----------------------------+
        | FINAL REGISTER SELECTION   |
        | 4-bit = REX + 3-bit field  |
        +-----------------------------+

3. ModR/M + SIB + REX Combined View

This is the most important diagram.

​x
                    REX BYTE
              +------------------+
              | W R X B          |
              +------------------+
                | | | |
                | | | +----------------------+
                | | +--------------------+   |
                | +------------------+   |   |
                |                    |   |   |
                v                    v   v   v

+------------------------------------------------------+
|                  ModR/M BYTE                         |
|  7 6      5 4 3       2 1 0                          |
|  MOD        REG        R/M                           |
+------------------------------------------------------+
     |           |          |
     |           |          |
     |           |          +----------------------+
     |           |                                 |
     |           v                                 v
     |   REX.R extends REG                 REX.B extends R/M
     |                                         (or SIB base)
     |
     v
+------------------------------------------------------+
|                     SIB BYTE                         |
|  7 6        5 4 3        2 1 0                      |
| SCALE        INDEX         BASE                     |
+------------------------------------------------------+
                |             |
                |             |
                v             v
        REX.X extends   REX.B extends BASE
          INDEX

4. Full Register Expansion Logic

3-bit → 4-bit transformation

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Final Register = (REX bit << 3) + 3-bit field

So:

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0xxx → RAX–RDI
1xxx → R8–R15

5. Detailed Example (Full Decode Flow)

Instruction:

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mov r10, [r8 + r9*4]

Machine code (conceptual):

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REX  ModR/M  SIB  DISP

Example breakdown:

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4C   04     8C   ...

STEP 1 — REX decode

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4C = 01001100

Split:

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0100 W R X B
     1 1 0 0

Meaning:

• W = 1 → 64-bit operand

• R = 1 → REG extended

• X = 0 → INDEX not extended

• B = 0 → BASE not extended

STEP 2 — ModR/M decode

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04 = 00000100

Split:

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00 000 100
MOD REG R/M

Meaning:

• MOD = 00 → memory addressing

• REG = 000

• R/M = 100 → indicates SIB follows

STEP 3 — SIB decode

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8C = 10001100

Split:

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10 001 100
SCALE INDEX BASE

Meaning:

• SCALE = 10 → multiply by 4

• INDEX = 001

• BASE = 100

STEP 4 — Apply REX extensions

INDEX field:

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REX.X = 0
INDEX = 001 → RCX

BASE field:

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REX.B = 0
BASE = 100 → RSP

STEP 5 — Effective address

Final expression:

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[rsp + rcx * 4]

6. Full Visual Execution Flow

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MACHINE CODE
4C 04 8C ...

        |
        v

+------------------+
| REX = 4C         |
| W R X B = 1 1 0 0|
+------------------+
        |
        v
+------------------+
| ModR/M = 04      |
| REG=000 R/M=100  |
+------------------+
        |
        v
R/M = SIB present
        |
        v
+------------------+
| SIB = 8C         |
| SCALE=4          |
| INDEX=RCX        |
| BASE=RSP         |
+------------------+
        |
        v
+------------------+
| Final Address    |
| [rsp + rcx*4]    |
+------------------+

7. Key Mental Model (Very Important)

Think of decoding like stacking layers:

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REX → extends registers (adds 4th bit)

ModR/M → chooses operation + base structure

SIB → enables complex memory math

DISP → adds constant offset

IMM → immediate value

8. One-Sentence Summary

REX extends register encoding, ModR/M selects operands, and SIB builds complex memory addressing like:

[base + index * scale + displacement]