8051 Assembly (A51)
Syntax highlighting, formatting, diagnostics, hover reference, and IntelliSense for 8051 (MCS-51) assembly in the Keil A51 dialect, for .asm and .a51 files.
Features
Syntax highlighting
TextMate-grammar highlighting for instructions, directives, registers, SFRs/SFR bits, bit addresses, numeric literals (hex/binary/octal/decimal), string literals, $/# compiler-control lines, and labels/symbol/macro definitions.
Format Document and Format Selection align labels, mnemonics, operands, and trailing comments into configurable columns:
; before
loop1: setb led
lcall delay ; go
; after (Format Document)
loop1: setb led
lcall delay ; go
- Content after a top-level
END is left byte-for-byte untouched.
- Formatting is idempotent: formatting an already-formatted file is a no-op.
- Case normalization (if enabled) only ever touches mnemonics, directives, and known registers — your own label and symbol names are never rewritten.
See Extension Settings for the full list of formatting options.
Diagnostics
Lightweight, real-time checks as you type:
- Unknown instruction or directive
- Wrong number of operands for a known instruction
- A branch/call target that doesn't resolve to any label in the file
- Invalid indirect addressing (
@R2-@R7 — only @R0/@R1 are addressable)
- Unmatched or never-closed
MACRO/IF/REPT/WHILE blocks
- A symbol (label,
EQU, BIT, DATA/IDATA/XDATA/CODE/SEGMENT, or macro) defined more than once
Like formatting, diagnostics stop at a top-level END — nothing after it is real code, so nothing after it is checked.
This is a lightweight checker, not a full assembler: it won't catch every semantic error a real A51 build would, and it does not assemble or simulate your code.
Hover reference
Hovering an instruction shows its opcode(s), byte length, cycle count, affected flags, and an addressing-mode-specific explanation, plus relevant notes (e.g. the ACALL/AJMP same-page restriction, relative-jump range, @Ri indirect RAM range, RETI vs. RET).
Hovering any other reference to a label, EQU/SET/BIT/DATA-style constant, or macro — whether it's the definition itself or a use elsewhere, like a branch target — shows its kind, its value or parameter list, and where it's defined.
A ; comment block written directly above a definition (no blank line in between) is treated as that symbol's documentation, and shown wherever it's hovered:
; Blinks the status LED once per second.
; Call after SETUP.
LOOP: SETB LED
LCALL DELAY
SJMP LOOP
Hovering LOOP here — on its definition or on the SJMP LOOP reference — shows that two-line comment as its documentation.
IntelliSense
- Auto-completion for instructions, directives, registers, and every symbol defined in the current file
- Go to Definition for labels, constants, and macros
- Find All References
- Outline / breadcrumbs (document symbols) for labels, constants, and macros
- Code folding for
MACRO/IF/REPT/WHILE blocks
Snippets
Prefix-triggered snippets for common A51 patterns: org (reset-vector + main-loop skeleton), vectors (full interrupt vector table), isr, sub, macro, delay, equ, bit, and cjne.
Localization
Diagnostics messages, hover reference text, and completion details follow VS Code's display language: English, Simplified Chinese (简体中文), and Traditional Chinese (繁體中文) are all supported.
Requirements
None — this extension only provides editor tooling (highlighting, formatting, diagnostics, IntelliSense). It does not assemble, build, or simulate your code, so no external toolchain is required.
Extension Settings
| Setting |
Default |
Description |
a51.format.mnemonicColumn |
8 |
Column where the mnemonic/directive starts. |
a51.format.operandColumn |
16 |
Column where the operand field starts. |
a51.format.commentColumn |
40 |
Column where trailing line comments start. |
a51.format.indentStyle |
spaces |
Whitespace character used to reach each column (spaces or tabs). |
a51.format.tabSize |
8 |
Tab stop width used when indentStyle is tabs. |
a51.format.longLabelStyle |
newline |
How to handle a label too long to fit before the mnemonic column: put the instruction on its own following line (newline), or keep it on the same line separated by a single space (sameLine). |
a51.format.caseStyle |
preserve |
Case normalization for mnemonics, directives, and known registers (preserve, upper, or lower). User-defined label/symbol names are never changed. |
a51.format.commaSpacing |
preserve |
Whether to normalize operand lists to have exactly one space after each comma (preserve or space). |
a51.format.semicolonSpacing |
preserve |
Whether to normalize trailing comments to have exactly one space after ; (preserve or space). |
a51.format.indentMacroBody |
false |
Indent one extra level inside MACRO/ENDM, IF/ENDIF, REPT/ENDM, and WHILE/ENDW blocks. |
a51.format.maxBlankLines |
1 |
Maximum number of consecutive blank lines to keep. |
Known Issues
- Diagnostics are a heuristic, line-based checker rather than a full A51 assembler, so some real assembler errors may go unreported and some unusual-but-valid code may be flagged.
Found a bug or have a feature request? Please open an issue.
Release Notes
See CHANGELOG.md.
License
MIT
