docs(asm): open the assembly language reference

Assisted-by: GLM 5.3 Flash

CHANGELOG.md

@@ -9,6 +9,15 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Added
 
+- **The Plan 9 assembly language reference.**  [docs/asm/](docs/asm/README.md)
+  opens the complete language reference with its common core: the lexicon,
+  statement structure and constant expressions, the operand grammar with
+  the pseudo-registers and symbol naming, the directives and the function
+  flag vocabulary, preprocessing with `#define` and `#include`, and the
+  Go-embedded layer (ABI0, prototypes, `go_asm.h`, `funcdata.h` and the
+  runtime contract).  Every claim is verified against `go tool asm` of
+  Go 1.27.1 and gasm's differential tests; the per-architecture pages and
+  generated instruction appendices follow.
 - **GOOBJ format specification.**  [docs/GOOBJ.md](docs/GOOBJ.md)
   documents the Go object file format in full: both containers, the 96
   byte header and all 19 blocks, every structure with its byte

README.md

@@ -186,8 +186,9 @@ verified the way the code is verified: an encoding documented here is
 one that differential tests against `go tool asm` confirm
 byte-for-byte, and a format field documented here is one the linker
 demonstrably reads. The work has begun: [docs/GOOBJ.md](docs/GOOBJ.md)
-specifies the object file format completely. The assembler language
-reference follows.
+specifies the object file format completely, and
+[docs/asm/README.md](docs/asm/README.md) opens the language reference
+with its common core. The per-architecture pages follow.
 
 ## Direction
 
@@ -315,6 +316,7 @@ recipe.
   them
 - [docs/ARCHITECTURE.md](docs/ARCHITECTURE.md): components and data flow
 - [docs/GOOBJ.md](docs/GOOBJ.md): the GOOBJ object file format specification
+- [docs/asm/](docs/asm/README.md): the Plan 9 assembly language reference
 - [docs/DEVELOPMENT.md](docs/DEVELOPMENT.md): development setup and recipes
 - [CHANGELOG.md](CHANGELOG.md): release history
 

docs/asm/DIRECTIVES.md

@@ -0,0 +1,148 @@
+# Directives: TEXT, DATA, GLOBL and the annotations
+
+Layer 1, the common language, with the flag vocabulary both layers share.
+Verified against `go tool asm` of Go 1.27.1, against the shipped headers
+`textflag.h` and `funcdata.h` in `$GOROOT/pkg/include`, and against gasm's
+parser. Where gasm extends a directive, the extension says so and is marked.
+
+Six directives exist. Three define things: TEXT, DATA, GLOBL. Three
+annotate: FUNCDATA, PCDATA, PCALIGN.
+
+## TEXT
+
+```text
+// func Add(a, b int64) int64
+TEXT ·Add(SB), NOSPLIT, $0-24
+	...instructions...
+	RET
+```
+
+```text
+TEXT symbol(SB), [flags,] $framesize[-argsize]
+```
+
+- The symbol is an `·Name(SB)` reference into the current package, or a
+  fully qualified name.
+- The optional flag argument is a constant expression, normally an OR of the
+  names from `textflag.h`, the table below. Without `#include "textflag.h"`
+  the names are not macros and the assembler reports the misleading error
+  `illegal or missing addressing mode for symbol NOSPLIT`: include the
+  header first.
+- `$framesize-argsize` is two constants, not a subtraction: the local frame
+  size in bytes, and the caller's argument area in bytes. The argument size
+  may be omitted entirely, `$16`, which marks the argument size unknown
+  (0x80000000 in the object, the value of `ArgsSizeUnknown` from
+  `funcdata.h`); a frame size may be negative only in the generated ABI
+  wrappers.
+- A function whose last instruction is not a branch cannot fall through into
+  the next TEXT: the toolchain appends a jump to itself, so end functions
+  with `RET` deliberately.
+- One TEXT per symbol; redeclaring is an error. The TEXT line also fixes the
+  function's source line for traceback: it is the line number that pcln
+  reports for the function's start.
+
+The framesize and argsize fields do real work: the framesize drives the
+stack-split preamble (RUNTIME.md carries the contract), and both travel into
+the FuncInfo record of the object (GOOBJ.md carries its layout).
+
+### The flag table
+
+Values from `textflag.h`, in agreement with `cmd/internal/obj/textflag.go`:
+
+| Name | Value | Applies to | Meaning |
+|---|---|---|---|
+| NOPROF | 1 | both | do not profile; deprecated |
+| DUPOK | 2 | both | the linker may keep one of several duplicates |
+| NOSPLIT | 4 | TEXT | no stack-split preamble |
+| RODATA | 8 | data | put the data in a read-only section |
+| NOPTR | 16 | data | the data contains no pointers |
+| WRAPPER | 32 | TEXT | a wrapper; must not disable `recover` |
+| NEEDCTXT | 64 | TEXT | a closure consuming the context register |
+| TLSBSS | 256 | data | a thread local word in BSS |
+| NOFRAME | 512 | TEXT | no frame setup; only valid with a frame size of 0 |
+| REFLECTMETHOD | 1024 | TEXT | the function calls `reflect.Type.Method` or `MethodByName` |
+| TOPFRAME | 2048 | TEXT | the outermost frame; unwinders stop here |
+| ABIWRAPPER | 4096 | TEXT | an ABI transition wrapper |
+
+Rules with teeth:
+
+- `NOSPLIT` removes the split check, so the frame plus everything the
+  function calls must fit in the stack segment that remains. It exists to
+  protect the splitting code itself; reaching for it to save two instructions
+  is how stack overflows corrupt memory. On amd64 the assembler additionally
+  marks small leaf functions NoSplit itself and omits the check, so the
+  absence of the preamble is not proof the flag was written.
+- A TEXT whose symbol is declared `ABIInternal` must carry NOSPLIT: the
+  assembler rejects it otherwise, because it cannot generate
+  the split path for a register-ABI function.
+- `RODATA` implies NOPTR for the garbage collector.
+
+## DATA
+
+```text
+DATA ·table+0(SB)/8, $0x0102030405060708
+DATA ·msg+0(SB)/14, $"hello, world\n"
+GLOBL ·msg(SB), RODATA, $14
+```
+
+```text
+DATA symbol+offset(SB)/width, value
+```
+
+- `width` is exactly 1, 2, 4 or 8: the initialiser is written into the data
+  image at `symbol+offset` in that many bytes.
+- The value is an integer or character constant of the width, or a string
+  literal whose byte length equals the width exactly; escapes count. Long
+  data is written as successive DATA lines at increasing offsets; bytes the
+  directives never name are zero.
+- Every symbol initialised with DATA ends with a GLOBL line declaring its
+  total size, after all of its DATA lines.
+
+A symbol containing pointers cannot be defined in assembly, because the
+collector cannot see into it: define it in Go and refer to it by name. As a
+rule, data that is not read-only belongs in Go.
+
+Extension, gasm only: a DATA initialiser may name a symbol,
+`DATA ·fn+0(SB)/8, $·handler(SB)`, which gasm lays down as an absolute
+relocation on that field. The toolchain offers no ground truth for this
+form; gasm's behaviour is verified by linking and execution.
+
+## GLOBL
+
+```text
+GLOBL symbol(SB), [flags,] $size
+```
+
+Declares the symbol global with its total size in bytes. The useful flags
+are RODATA, NOPTR, DUPOK and TLSBSS from the table above. Uninitialised
+bytes are zero, which makes GLOBL with no DATA the language's BSS.
+
+## FUNCDATA and PCDATA
+
+```text
+FUNCDATA $functypeid, symbol(SB)
+PCDATA $pctypeid, $value
+```
+
+The compiler's annotations for the garbage collector and traceback, named by
+the ids in `funcdata.h`: FUNCDATA 0 to 7 (args pointer maps, locals pointer
+maps, stack objects, inline tree, open-coded defer info, argument info,
+argument liveness, wrap info), PCDATA 0 to 4 (unsafe point, stack map index,
+inline tree index, argument liveness index, panic bounds). Assembly code
+normally reaches them only through the macro forms in `funcdata.h`, which
+RUNTIME.md explains. Outside the macros, hand-written PCDATA is meaningless:
+the values are pc-value tables the compiler builds from its own view of the
+program.
+
+## PCALIGN
+
+```text
+PCALIGN $32
+```
+
+Pads the code so that the next instruction lands on the given boundary,
+which must be a power of two and at least the target's instruction
+alignment. Supported on amd64, arm64, ppc64, loong64 and riscv64. The
+padding instructions are the target's NOP encoding, so the bytes between
+functions differ from what the instruction stream alone would produce, which
+matters to anyone comparing encodings byte for byte.

docs/asm/LANGUAGE.md

@@ -0,0 +1,140 @@
+# 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.

docs/asm/OPERANDS.md

@@ -0,0 +1,114 @@
+# Operands: grammar, pseudo-registers, addressing and symbols
+
+Layer 1, the common language. Verified against `go tool asm` of Go 1.27.1 and
+against gasm's parser. The operand grammar is the part of the language that
+varies most between targets, so this page fixes the common grammar and the
+pseudo-registers; the per architecture pages carry the register names and the
+addressing quirks each target adds.
+
+## The four operand kinds
+
+Every operand is one of four kinds:
+
+```text
+R1                  register
+$4                  immediate
+label               branch target or symbol
+-8(BX)(DI*4)        memory
+```
+
+**Operands go source first, destination last**: `MOVQ x+0(FP), AX` loads the
+argument into AX. This is the opposite of Intel order and the same order as
+AT&T, with the sigils removed: registers are bare names, immediates take
+`$`, memory is `offset(base)`.
+
+## Registers
+
+A register operand is its bare name, with no prefix: `AX`, `X15`, `R14` on
+amd64; `R0` to `R30`, `ZR`, `V0` to `V31` on arm64; `X0` to `X31`, `F0` to
+`F31`, `V0` on riscv64; `R0` to `R31`, `F0` to `F31`, `V0` on loong64.
+Sub-register and width selection rides the mnemonic, not the operand: the
+amd64 family spells `MOVB`, `MOVW`, `MOVL`, `MOVQ`, and the arm64 family
+suffices `B`, `H`, `S`, `D`, `Q` on the shared forms. Each architecture page
+lists its registers and the reserved ones.
+
+## Immediates
+
+`$` introduces a constant: `$42`, `$-1`, `$0x2a`, `$'A'`, `$bufSize`. The
+`$` applies to the whole constant expression that follows, so
+`$(4*8+reader__size)` is one immediate. Without the `$`, a number in operand
+position is an address, not a value; the classic error `ADDQ 1, AX` asks the
+assembler for the byte at address 1.
+
+The one place a `$` number is not an immediate is the frame and argument
+size field of TEXT, `$16-24`, which is two separate constants and not a
+subtraction; DIRECTIVES.md specifies it.
+
+## Memory
+
+```text
+offset(base)
+offset(base)(index*scale)
+```
+
+Both parts are optional where the target allows them: `(BX)` is the memory
+at BX, `foo+16(SB)` is a global, and on amd64 `foo+32(SP)(R9*8)` adds a
+scaled index. `offset` is a constant expression, optionally carrying a
+symbol name. The extensions beyond `offset(base)` are where the targets
+diverge, and each belongs to its architecture page: amd64 carries the
+`index*scale` form with scale 1, 2, 4 or 8 and its own rules on which
+registers may index; loong64 writes base plus index as `(R4)(R5)`; the ARM
+family attaches shift amounts to the index register in its own spelling.
+
+The address arithmetic is on **byte addresses**: the offset is added to the
+base as it stands, whatever the operand width of the instruction. Loading
+the third 8-byte word of an array at BX is `16(BX)`, not `2(BX)`.
+
+## The four pseudo-registers
+
+Four names denote locations no target register holds, and they mean the same
+on every architecture:
+
+- **FP**, the frame pointer: the arguments and results of the current
+  function, at positive offsets, in the order the Go prototype declares
+  them. Every FP reference must carry a name: `x+0(FP)`, and an unnamed
+  `0(FP)` is rejected. Results follow arguments; an unnamed result is called
+  `ret`.
+- **SP**, the virtual stack pointer: the high end of the function's local
+  frame, so locals live at negative offsets, `x-8(SP)`. A reference without
+  a name and without a plus, `-8(SP)`, addresses the **hardware** stack
+  pointer instead: the two spellings are one character apart and mean
+  different registers. That is the sharpest edge in the language and the
+  source of the deepest bugs.
+- **SB**, the static base: the origin of memory, used for globals and
+  cross-package symbols, always with a name: `foo(SB)`, `foo+4(SB)`.
+- **PC**, the program counter: branch targets, and the explicit relative
+  form `1(PC)`.
+
+## Symbol names
+
+A symbol's full name is the package path, a period, and the base name. In
+source, the period is written U+00B7 (`·`) and a slash in the path U+2215
+(`∕`), because the parser treats the ASCII forms as punctuation. Inside the
+package's own file, `·Name` is enough and is the preferred spelling, since
+it survives a rename of the import path.
+
+| Spelling | Meaning |
+|---|---|
+| `·Name(SB)` | this package's Name |
+| `runtime·morestack(SB)` | another package's morestack |
+| `sourcedock.dev∕petrbalvin∕pkg·Name(SB)` | fully qualified |
+| `msg<>(SB)` | file-local, the static of this language; `<>` also makes the ABI field static in the object |
+| `Name<ABIInternal>(SB)` | ABI-qualified reference, the ABI in angle brackets after the name |
+
+The object file these symbols produce, with the index rules that decide what
+is referenced by name and what by index, is specified in
+[GOOBJ.md](../GOOBJ.md).
+
+## What vet adds in Go
+
+Inside a Go package, `go vet`'s asmdecl analyzer checks every FP offset and
+name against the Go prototype, and checks the declared argument area against
+the frame. That layer, the prototype requirement and `go_asm.h`, belongs to
+RUNTIME.md; the grammar above is the whole of what the assembler itself
+requires.

docs/asm/PREPROCESSOR.md

@@ -0,0 +1,79 @@
+# Preprocessing: include, define and selection
+
+Layer 1, the common language. Verified against the preprocessor inside
+`go tool asm` of Go 1.27.1 (`cmd/asm/internal/lex`), whose directives are
+`#define`, `#undef`, `#include`, `#ifdef`, `#ifndef`, `#else`, `#endif` and
+`#line`, and against gasm's implementation, which is differentially tested
+against the toolchain's.
+
+Input runs through a simplified C preprocessor before the parser sees it.
+The set is deliberately small: there is no `#if` with constant expressions
+and no token pasting with `##`. `#line` is honoured, so it changes the
+positions the assembler reports and records.
+
+## #include
+
+```text
+#include "textflag.h"
+#include "go_asm.h"
+#include "defs_linux_amd64.h"
+```
+
+The search path, in order: the directory of the including file, then the
+directories given by repeatable `-I` flags. The assembler seeds no default
+of its own: a bare `go tool asm` invocation finds none of the standard
+headers, and it is the `go` build system that passes `$GOROOT/pkg/include`
+among the `-I` directories when it drives the build. That directory ships
+`textflag.h`, `funcdata.h` and the per architecture register headers.
+Includes nest; a file included twice through different paths is processed
+twice, which is why headers guard their defines.
+
+## #define and #undef
+
+```text
+#define bufSize 1024
+#define MOVD(d, s) MOVQ s, d
+#undef bufSize
+```
+
+- An object macro replaces its name with its token sequence at the point of
+  use.
+- A parameterised macro takes its arguments in parentheses and substitutes
+  them into the body. Macro parameters compose with the rest of the
+  language: an argument used with an element suffix, as in `A.S4` on the
+  vector forms, substitutes correctly.
+- Redefinition is an error; `#undef` first, or pick a new name.
+- The `-D name[=value]` flag predefines an object macro from the command
+  line, repeatable, exactly as `#define` would; a `-D` without a value
+  defines the name as `1`.
+- Expansion happens when the name is used, so a macro may expand to
+  instructions, operands or fragments of either, and a macro body may use
+  macros defined before it.
+
+`textflag.h` and `funcdata.h` are themselves ordinary `#define` files: the
+flag names and the runtime macros are preprocessor definitions, not language
+keywords. That is why a missing include produces a parser error at the first
+use of `NOSPLIT` rather than a complaint about the name.
+
+## #ifdef, #ifndef, #else, #endif
+
+```text
+#ifdef GOOS_windows
+	#define SYSCALL_INT 0x2b
+#endif
+```
+
+Selection is by defined-name only: `#ifdef`, `#ifndef`, `#else`, `#endif`,
+nesting freely. There is no `#if defined(x) && y`, because the preprocessor
+evaluates no expressions; reach that with a build-tag Go file generating a
+header, which is exactly how the runtime's own `go_asm.h` and defs headers
+are produced.
+
+## What preprocessing does not cover
+
+The preprocessor is textual and runs first, so it knows nothing of assembly
+semantics: it does not check that a macro expansion is a legal instruction,
+and it does not participate in the constant expression evaluator, which runs
+later, in the parser. A constant folded with `#define` and a constant folded
+in an operand expression end at the same value through different doors;
+GOOBJ.md records both in the object identically.

docs/asm/README.md

@@ -0,0 +1,53 @@
+# The Plan 9 assembly language
+
+This directory is the reference for the Plan 9 assembly language as the Go
+toolchain and gasm accept it, written to be complete enough to implement
+against. It exists because no such reference exists upstream: Go documents
+the language on a single page, and the rest of the knowledge lives in the
+toolchain's source and in the practice of reading it.
+
+Every page carries the same conformance statement: which layer of the system
+it describes, which toolchain release it was verified against, and how the
+claims were checked. Pages in this directory are verified against Go 1.27.1
+and against gasm's own differential test suite, which compares gasm's
+behaviour with `go tool asm` byte for byte and output for output.
+
+## The three layers
+
+The reference deliberately separates three layers, because their rules have
+different owners and different lifetimes:
+
+1. **The common language** (LANGUAGE, OPERANDS, DIRECTIVES,
+   PREPROCESSOR): the syntax, operands, directives and preprocessing, the
+   same on every target and meaningful without a Go runtime.
+2. **The Go-embedded layer** (RUNTIME): everything that exists only because
+   the code runs inside a Go program: the ABI0 contract, generated wrappers,
+   `go_asm.h`, the garbage collector annotations and `go vet` checks.
+3. **The standalone layer** (STANDALONE, planned with the standalone
+   compilation phase): using the language outside Go, through gasm's ELF
+   output and the extended instruction set, where the toolchain offers no
+   ground truth and execution testing is the only verification.
+
+A rule stated in layer 1 holds on every target. A rule stated in layer 2
+says which part of the Go machinery imposes it. Nothing in layer 3 changes
+layers 1 or 2; it extends them.
+
+## Pages
+
+| Page | Layer | Contents |
+|---|---|---|
+| [LANGUAGE.md](LANGUAGE.md) | 1 | lexicon, statement structure, labels, literals, expressions |
+| [OPERANDS.md](OPERANDS.md) | 1 | operand grammar, pseudo-registers, addressing modes, symbol naming |
+| [DIRECTIVES.md](DIRECTIVES.md) | 1 | TEXT, DATA, GLOBL, FUNCDATA, PCDATA, PCALIGN and the function flags |
+| [PREPROCESSOR.md](PREPROCESSOR.md) | 1 | `#include`, `#define`, `#ifdef` and friends, `-D`, `-I` |
+| [RUNTIME.md](RUNTIME.md) | 2 | ABI0, prototypes, `go_asm.h`, `funcdata.h`, `go vet` |
+| AMD64, ARM64, RISCV64, LOONG64 | 1, 3 | per architecture: registers, conventions, addressing, instruction families and the generated instruction appendices |
+| STANDALONE.md | 3 | the language outside Go |
+
+## Status
+
+The common-language core and the Go-embedded layer are written and verified.
+The four per-architecture pages and their generated instruction appendices
+follow, architecture by architecture; STANDALONE.md lands with the standalone
+compilation phase. The object format these pages feed is specified in
+[GOOBJ.md](../GOOBJ.md).

docs/asm/RUNTIME.md

@@ -0,0 +1,120 @@
+# The Go-embedded layer: ABI0, prototypes and the runtime contract
+
+Layer 2: everything that exists only because the assembly runs inside a Go
+program. Without a Go runtime this page does not apply; the language of
+OPERANDS.md and DIRECTIVES.md still does. Verified against Go 1.27.1, against
+the shipped `funcdata.h` header, and against the object files the toolchain
+produces, which were parsed and checked field by field while writing
+[GOOBJ.md](../GOOBJ.md).
+
+## Hand-written assembly is ABI0
+
+Go functions compiled from source use ABIInternal, the register-based
+calling convention, which the toolchain documents as unstable and free to
+change between releases. A `.s` function is written against ABI0, the stack
+based convention: arguments and results live in the caller's frame at
+positive FP offsets, byte-addressed, in declaration order, with no registers
+assigned at all. The toolchain generates the wrapper that translates between
+the two; a caller in Go calling an assembly function goes through it, and it
+is marked `ABIWRAPPER` in the object. Hand-writing a bridge is never needed
+and never correct.
+
+## Every assembly function carries a Go prototype
+
+```go
+package add
+
+func Add(x, y int64) int64
+```
+
+The body-less declaration is not optional, and not only for the linker: it
+is what tells the garbage collector which arguments and results hold
+pointers, and what `go vet` checks the assembly against. Even a function
+nothing in Go calls gets one. Consequences:
+
+- The FP operand names and offsets are checked by vet's asmdecl analyzer
+  against the prototype: `x+0(FP)` must name an argument that exists, at the
+  offset the prototype says. A file that assembles and links can still fail
+  vet.
+- The declared argument area in `$framesize-argsize` is checked against the
+  prototype's size. An omitted argsize marks the argument size unknown
+  (0x80000000 in the object, the value of `ArgsSizeUnknown` from
+  `funcdata.h`), which is the normal spelling for functions with no Go
+  callers.
+- `//go:noescape` on the declaration tells the compiler that a pointer
+  argument does not escape, for assembly that keeps the pointer beyond the
+  call.
+
+## The frame, the stack and the collector
+
+The runtime owns the stack and the pointer map, and assembly must hold up
+its end of four rules:
+
+1. **Arguments are initialised on entry; results are not.** A function whose
+   results hold live pointers across a call must zero them and then execute
+   `GO_RESULTS_INITIALIZED`. Designing functions that return no pointers
+   avoids the problem.
+2. **A frame with calls and no local pointers says so** with
+   `NO_LOCAL_POINTERS`. A frame with local pointers that the runtime cannot
+   see is not allowed at all: assembly cannot describe a pointer-containing
+   local, so it must not have one. Data symbols containing pointers are the
+   same: define them in Go.
+3. **The stack may move.** Stack growth copies the frame, so no pointer into
+   the frame may be held across a call, and the raw hardware SP register may
+   not be cached across a call either.
+4. **The split check is not optional by default.** Without NOSPLIT, the
+   assembler inserts the stack-growth preamble, including the morestack
+   block for framed functions; NOSPLIT is a contract that the frame and
+   everything below it fit in the remaining stack segment. On amd64 the
+   assembler also marks small leaf functions NoSplit itself and skips the
+   preamble, so silence is not a promise.
+
+The simplest safe shape is a leaf function with no local frame and no calls:
+it needs no annotation beyond the prototype.
+
+## go_asm.h: Go constants and layout in assembly
+
+A package with `.s` files gets a generated header. Include it and use the
+generated names instead of hard-coding layouts, which lie silently when the
+Go side changes:
+
+| Go declaration | Assembly name |
+|---|---|
+| `const bufSize = 1024` | `const_bufSize` |
+| field `r` of `type reader struct` | `reader_r` |
+| size of `type reader struct` | `reader__size` |
+
+The constants arrive as macros, usable as immediates and offsets, computed
+from the Go declarations. An ambiguous name, such as a struct that really
+has a `_size` field, fails the generation with a redefinition error.
+
+## funcdata.h: the runtime macros
+
+`$GOROOT/pkg/include/funcdata.h` defines the PCDATA and FUNCDATA ids and the
+three macros assembly normally uses instead:
+
+| Macro | Expands to | Meaning |
+|---|---|---|
+| `GO_ARGS` | `FUNCDATA $FUNCDATA_ArgsPointerMaps, go_args_stackmap(SB)` | the Go prototype defines the argument pointer map |
+| `GO_RESULTS_INITIALIZED` | `PCDATA $PCDATA_StackMapIndex, $1` | results are initialised; treat them as live from here |
+| `NO_LOCAL_POINTERS` | `FUNCDATA $FUNCDATA_LocalsPointerMaps, no_pointers_stackmap(SB)` | the frame holds no pointers |
+
+`GO_ARGS` is inserted implicitly by the assembler for any function whose
+package-qualified name belongs to the current package, which is why most
+assembly never writes it. `NOSPLIT` leaf functions that call nothing need
+none of the three.
+
+The underlying ids, for reading toolchain output rather than for writing
+source: FUNCDATA 0 to 7 are args pointer maps, locals pointer maps, stack
+objects, inline tree, open-coded defer info, argument info, argument
+liveness and wrap info; PCDATA 0 to 4 are unsafe point, stack map index,
+inline tree index, argument liveness index and panic bounds.
+
+## What the runtime does with all of this
+
+The object file records the annotations as aux symbols and FuncInfo records;
+GOOBJ.md specifies the encoding. The linker assembles them into the runtime's
+pclntable, which traceback and the collector consume. An assembly function
+that misdeclares its frame is not a compile error and usually not a link
+error: it is a wrong collector decision or a wrong traceback at runtime,
+which is why the annotations are a contract and not documentation.