Article by Ayman Alheraki on January 11 2026 10:37 AM
The instruction table is the core reference structure that an assembler uses to map mnemonic representations of machine instructions to their corresponding binary encodings, operand patterns, and encoding rules. It acts as the bridge between human-readable assembly language and raw machine code generation.
In a modern x86-64 assembler, the instruction table is extensive, optimized, and structured to efficiently handle the complexity of the ISA, including multiple encoding schemes, operand size variants, register classes, instruction extensions (e.g., AVX, EVEX), and conditional forms.
The instruction table performs several critical functions:
Mnemonic Matching: Resolves assembly mnemonics like MOV, ADD, JMP, etc.
Operand Validation: Checks operand count, types, and compatibility with the instruction.
Encoding Rule Mapping: Determines the correct opcode byte(s), ModR/M, SIB, prefixes, and immediate values.
Instruction Variant Resolution: Selects the correct instruction variant based on operand types and sizes.
Extension Management: Handles instructions from optional CPU extensions (e.g., AVX-512, BMI2, FMA).
Each instruction entry in the table includes metadata required for matching and encoding. A typical instruction descriptor includes:
| Field | Description |
|---|---|
mnemonic | The textual name of the instruction (e.g., MOV). |
operand_count | Number of operands expected. |
operand_types[] | Encoded description of allowed operand types (register, memory, immediate). |
opcode | Primary opcode byte(s), possibly including opcode maps or escape bytes. |
modrm_required | Boolean indicating whether a ModR/M byte is needed. |
sib_required | Indicates whether a SIB byte may be generated. |
prefixes | Optional prefixes like REX, VEX, EVEX, operand-size override, segment override. |
immediate_size | Size of any immediate operand (in bytes). |
encoding_flags | Flags for encoding rules (e.g., direction bit, operand size override). |
isa_level | The instruction set level required (e.g., base x86-64, AVX2, SSE4.2). |
To improve lookup speed, many assemblers organize this structure into hash tables or decision trees, indexed first by mnemonic and second by operand pattern.
The instruction table must distinguish between variants of the same mnemonic. For example:
MOV r/m64, r64
MOV r64, r/m64
MOV r64, imm32
Each form is stored as a separate entry with a unique operand type signature. These signatures are encoded as combinations of enums or bit fields, such as:
OP_REG_R64 – 64-bit general-purpose register
OP_MEM – memory reference (may include displacement or scale)
OP_IMM32 – 32-bit immediate
OP_XMM – 128-bit vector register
OP_YMM – 256-bit vector register
Some modern assemblers compress these patterns using tables that collapse similar forms and apply encoding rules dynamically.
The instruction table supports multi-phase encoding:
Mnemonic Resolution: Find the mnemonic (MOV) in the instruction table.
Operand Matching: Search all forms of MOV for a signature matching the provided operands.
Prefix Determination: Decide if a REX, VEX, or EVEX prefix is required based on operand size and register class.
Opcode Construction: Emit base opcode and any escape bytes (e.g., 0F, 0F38, 0F3A).
ModR/M Encoding: If applicable, encode register and addressing mode using ModR/M and SIB bytes.
Immediate Handling: Emit immediate bytes, ensuring correct size and little-endian representation.
All of this relies on the instruction table to drive correct binary output.
Post-2020 assembler designs often implement dynamic support for multiple ISA levels:
Baseline ISA (x86-64 core)
AVX, AVX2, AVX-512 (with VEX/EVEX prefixes)
BMI, FMA, SHA, AESNI
TSX, CET, and other Intel/AMD-specific instructions
The instruction table entries include an isa_level field or capability flags so that the assembler can warn or error if the current target CPU does not support a given instruction.
Some assemblers allow conditional enabling or disabling of instruction subsets during parsing, using directives like .cpu, .arch, or specific feature toggles.
Modern assemblers may use one of several strategies to organize the instruction table efficiently:
Flat table with hashing: Quick lookup by mnemonic, then filter by operand types.
Trie-based mnemonic indexing: Especially useful when supporting instruction aliases or pseudo-instructions.
Opcode-First Lookup: Optimized for binary disassembly, where opcode is known and decoding proceeds from there.
Decision DAGs: Reduce ambiguity and efficiently resolve instruction forms based on operand types.
Some designs preload these tables from a compact intermediate format generated during the assembler’s own build process, allowing more flexible or automated updates when ISA extensions are added.
Recent enhancements to assembler design have influenced how instruction tables are built and maintained:
Auto-generated tables from Intel XML/JSON opcode data sources.
Instruction compression tables for embedded environments, using minimal representation of operand forms.
Encoding templates and macros reduce repetition in instruction definitions.
Instruction form overloading with embedded encoder logic to reduce table size and support emerging ISA patterns.
Runtime instruction patching for JIT use cases, enabling live code mutation based on instruction templates.
These modern techniques increase the maintainability, extensibility, and performance of instruction table management in both static and dynamic assemblers.
The instruction table is the assembler’s formal map of all supported instruction forms, encoding requirements, and operand patterns. It underpins parsing, validation, and code generation. A robust and extensible instruction table is key to supporting the full breadth of x86-64, including legacy and modern instruction set extensions. As the architecture evolves, maintaining this table dynamically and efficiently becomes central to assembler design excellence.