Full real Intel instruction decoding tree (like Ghidra view)

1. Full x86-64 Instruction Decoding Tree (Ghidra Style)

Example instruction:

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mov r10, [r8 + r9*4 + 16]

Machine code (conceptual layout):

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REX  OPCODE  ModR/M  SIB  DISP
4C   8B      54      88   10

2. FULL DECODING TREE

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ROOT: RAW INSTRUCTION BYTES
│
├── PREFIX PARSE STAGE
│   │
│   ├── Legacy Prefixes (optional)
│   │     ├─ F0 (LOCK)
│   │     ├─ F2 (REPNE)
│   │     ├─ F3 (REP)
│   │
│   ├── Segment Override (optional)
│   │     ├─ CS / DS / ES / FS / GS
│   │
│   ├── Operand-size override (66h)
│   │
│   └── Address-size override (67h)
│
├── REX PREFIX (x86-64 ONLY)
│   │
│   ├── Format: 0100WRXB
│   │
│   ├── W → 64-bit operand size
│   ├── R → extends ModR/M REG field
│   ├── X → extends SIB INDEX field
│   └── B → extends ModR/M R/M or SIB BASE
│
│   └── OUTPUT:
│         extended register namespace (R8–R15 enabled)
│
├── OPCODE DECODING
│   │
│   ├── Single-byte opcode (e.g. 8B)
│   ├── Two-byte opcode (0F xx)
│   ├── Three-byte opcode (0F 38 / 0F 3A)
│   │
│   └── CLASSIFICATION:
│         mov, add, sub, lea, etc.
│
├── ModR/M BYTE PARSE
│   │
│   ├── Format:
│   │     7 6 | 5 4 3 | 2 1 0
│   │     MOD | REG   | R/M
│   │
│   ├── MOD FIELD
│   │     00 → memory, no displacement
│   │     01 → memory + 8-bit displacement
│   │     10 → memory + 32-bit displacement
│   │     11 → register-direct
│   │
│   ├── REG FIELD
│   │     → destination register (or opcode extension)
│   │
│   └── R/M FIELD
│         → register OR SIB trigger
│
├── SIB BYTE (IF R/M = 100 AND MOD ≠ 11)
│   │
│   ├── Format:
│   │     SCALE | INDEX | BASE
│   │
│   ├── SCALE:
│   │     00 → *1
│   │     01 → *2
│   │     10 → *4
│   │     11 → *8
│   │
│   ├── INDEX:
│   │     register index (extended by REX.X)
│   │
│   └── BASE:
│         base register (extended by REX.B)
│
├── DISPLACEMENT (optional)
│   │
│   ├── 8-bit signed
│   ├── 32-bit signed
│   └── used for memory offset
│
├── IMMEDIATE (optional)
│   │
│   ├── 8-bit
│   ├── 16-bit
│   ├── 32-bit
│   └── 64-bit
│
└── FINAL SEMANTIC DECODE
    │
    ├── Register resolution:
    │       FINAL = (REX bit << 3) + 3-bit field
    │
    ├── Effective Address Computation:
    │       [BASE + INDEX * SCALE + DISP]
    │
    └── Instruction Execution Mapping:
            CPU micro-ops generated

3. Ghidra-Style EXPANDED VIEW (Concrete Example)

Instruction:

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mov r10, [r8 + r9*4 + 16]

RAW BYTES

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4C 8B 54 88 10

DECODE TREE (Ghidra-like)

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Instruction
│
├── Prefixes
│   └── REX = 4C
│       ├── W = 1 (64-bit)
│       ├── R = 1
│       ├── X = 0
│       └── B = 0
│
├── Opcode
│   └── 8B
│       → MOV r64, r/m64
│
├── ModR/M = 54
│   ├── MOD = 01 → memory + 8-bit displacement
│   ├── REG = 010 → R10 (after REX.R)
│   └── R/M = 100 → SIB required
│
├── SIB = 88
│   ├── SCALE = 10 → *4
│   ├── INDEX = 001 → R9 (no REX.X)
│   └── BASE = 000 → RAX/R8? (depends REX.B)
│
├── Displacement = 10h
│
└── Effective Address
    → [r8 + r9*4 + 16]

4. REGISTER RESOLUTION TREE (Important Part)

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REGISTER FIELD RESOLUTION
│
├── Input: 3-bit register field
│
├── Check REX prefix
│   │
│   ├── REX.R → extends REG field
│   ├── REX.X → extends INDEX field
│   └── REX.B → extends BASE / R/M field
│
└── Final register index:
        (REX_bit << 3) | reg_bits

5. MEMORY ADDRESS TREE

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EFFECTIVE ADDRESS CALCULATION
│
├── BASE REGISTER
│     └── from ModR/M or SIB.BASE
│
├── INDEX REGISTER
│     └── from SIB.INDEX
│
├── SCALE FACTOR
│     └── 1 / 2 / 4 / 8
│
├── DISPLACEMENT
│     └── immediate offset
│
└── FINAL ADDRESS:
      BASE + INDEX * SCALE + DISP

6. CPU DECODER PIPELINE VIEW (Real Microarchitecture View)

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FETCH
  ↓
PRE-DECODE
  ↓
PREFIX ANALYSIS
  ↓
REX EXPANSION UNIT
  ↓
OPCODE DECODER (ID stage)
  ↓
ModR/M PARSER
  ↓
SIB ENGINE
  ↓
REGISTER RENAMING
  ↓
MICRO-OP GENERATION
  ↓
EXECUTION UNITS

7. Key Insight (Why This Design Exists)

This structure exists for:

• backward compatibility (x86 legacy preserved)

• minimal encoding changes

• efficient hardware decoding

• modular extension (REX only adds bits, nothing is broken)

8. Final Mental Model (Very Important)

Think of decoding like layers:

Layer 1 → Prefixes (REX, legacy)
Layer 2 → Opcode (what instruction)
Layer 3 → ModR/M (who is involved)
Layer 4 → SIB (how memory is computed)
Layer 5 → Displacement (offset)
Layer 6 → Immediate (constant value)
Layer 7 → Execution (CPU micro-ops)