# Language: lexicon, statements and expressions

Layer 1, the common language, the same on every target. Verified against
`go tool asm` of Go 1.27.1 and against gasm's parser, which is differentially
tested against the toolchain. The authoritative sources behind this page are
the assembler's lexer (`cmd/asm/internal/lex`), its parser
(`cmd/asm/internal/asm/parse.go`) and the toolchain's own test data.

## Source files and targets

An assembly source is a `.s` file. The Go build convention names a
target-specific file with the architecture suffix, `_amd64.s`, `_arm64.s`,
`_riscv64.s` or `_loong64.s`; files without a suffix are portable across
targets. The same assembler program assembles every target: `go tool asm`
picks the target from the `GOOS` and `GOARCH` environment variables, and gasm
from the file name suffix or the `--arch` flag.

## Character set and identifiers

Sources are ASCII text. An identifier is a sequence of ASCII letters, digits
and underscores, digits never first, with exactly two additions:

- U+00B7, the middle dot `·`, stands for the period in a symbol's
  package-qualified name;
- U+2215, the division slash `∕`, stands for the slash in a package path.

The two substitutions exist because the parser treats a real period and a
real slash as punctuation. The syntax is otherwise uppercase throughout:
instructions, registers and directives are written in upper case. The one
inherited exception is the `g` register name on 32-bit ARM.

## Comments

Two comment forms, both Go's:

```text
// a line comment
/* a block comment */
```

A comment of the form `//go:build` or the legacy `+build` comment is not a
plain comment: the lexer reports it to the build system as a build
constraint.

## Statements

The grammar of one line, from the parser:

```text
{label:} WORD[.qualifier] [ arg {, arg} ] (';' | '\n')
```

- A **label** is an identifier followed by a colon. Labels are
  function-local: two functions in one file may reuse the same name, and a
  reference resolves within the function that contains it. A branch
  instruction names its target with a bare label operand, and the assembler
  resolves it PC-relative. The explicit forms `offset(PC)`, a constant
  counting instructions from the branch, and `name(SB)`, a cross-function
  static reference, appear as branch targets as well.
- **WORD** is the instruction or directive name, upper case. On the ARM
  family the word may carry a dot qualifier selecting a condition or shift
  mode, such as the condition suffixes on 32-bit ARM; the amd64, arm64,
  riscv64 and loong64 assemblies carry no instruction qualifiers apart from
  their own width suffixes, which are part of the mnemonic.
- **Arguments** are separated by commas, with no trailing comma.
- A statement ends at a newline or at a semicolon, so several statements fit
  on one line separated by `;`. Blank lines are free.

The first word of a line is a directive if it is one of the directive names
(TEXT, DATA, GLOBL, FUNCDATA, PCDATA, PCALIGN) and an instruction otherwise.
Unknown instruction names are errors; the instruction set is the set the
toolchain itself defines per target, plus the common pseudo-instructions.

## Literals

| Form | Examples | Notes |
|---|---|---|
| Integer | `0`, `42`, `0x2a`, `0o52`, `0b101010`, `1_000` | decimal, hexadecimal, octal and binary forms with Go's digit separators |
| Character | `'a'`, `'\n'`, `'\x41'` | single quoted, Go escape rules |
| String | `"this program can only run\n"` | double quoted, Go escape rules; accepted where an operand takes raw bytes, in practice a DATA initialiser |
| Float | `1.5`, `1e9` | accepted by the lexer; only meaningful where the target's encoding takes a float operand |

## Expressions

Constant expressions may appear wherever a constant is expected: in
immediates after `$`, in memory offsets, in frame and data sizes. The
evaluator works on unsigned 64-bit values with Go's operator precedence, and
the parser states its grammar in exactly those terms:

```text
expr   = term   { '+' term | '-' term | '|' term | '^' term }
term   = factor { '*' factor | '/' factor | '%' factor | '<<' factor | '>>' factor | '&' factor }
factor = const  | '+' factor | '-' factor | '~' factor | '(' expr ')'
```

Two consequences are worth naming, because the arithmetic surprises people
who read it as C:

- Shifts bind at the multiplicative level, next to `*` and `&`, while `|`
  and `^` bind at the additive level. `$x<<1|3` computes `(x<<1)|3`, which
  differs from `x*2+3` whenever `x` is odd. Plan 9 arithmetic is Go
  precedence applied to a byte-oriented language, not the C expression it
  resembles.
- The evaluator is unsigned and guarded: division or modulo by zero is an
  error, and so is dividing a value with the high bit set; shift counts must
  be non-negative; and a right shift of a value with the high bit set is
  rejected rather than sign-extended.

An address expression such as `(index*4)(base)` is evaluated at assembly
time only if every name in it is a constant; a name that resolves to a
symbol turns the expression into a relocation request, never into a folded
constant.

Named constants enter expressions through the preprocessor (`#define`,
`-D`) and, in Go-embedded packages, through the generated `go_asm.h`; see
PREPROCESSOR.md and RUNTIME.md.

## The common pseudo-instructions

A handful of instructions exist on every target, assembled by the assembler
itself rather than the encoder: `NOP`, which emits the target's no-operation
encoding, and the frame-management pseudo-instructions the compiler emits
(`FUNCDATA`, `PCDATA`) which DIRECTIVES.md specifies. Everything else is the
target's own instruction set, and the assembler knows only the instructions
the toolchain's compiler emits; a hand-written kernel wanting more lays the
encoding down with `BYTE` on amd64 or waits for the extended layer.

## Case study: three lines, decomposed

```text
B.EQ	1(PC)                  // arm64: condition qualifier on the mnemonic,
                               // target one instruction past the branch
JMP	done                   // every target: bare label, function-local,
                               // resolved PC-relative
MOVQ	$reader__size>>3, CX   // amd64: expression over a go_asm.h constant
```

The first shows a qualifier and the explicit relative target form; the second
the ordinary label reference; the third an expression over a generated
constant. Labels are reusable between functions without conflict.