amiga-bootcamp/04_linking_and_libraries/fd_files.md

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Function Definition (.fd) Files

Overview

Function Definition files (.fd) are the authoritative source of truth for AmigaOS library ABIs. They define:

• The name of every library function
• The LVO (Library Vector Offset) — the negative offset used to call it
• The register parameters — which M68k registers carry each argument
• The bias (starting offset, always negative)

Every AmigaOS library call ultimately traces back to an .fd file.

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File Location

In the NDK39:

NDK39/fd/
    dos_lib.fd
    exec_lib.fd
    graphics_lib.fd
    intuition_lib.fd
    icon_lib.fd
    ...

Also part of the AmigaOS developer CD (adjust to your NDK path):

NDK39/fd/

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.fd File Format

##base _DOSBase         ; library base variable name
##bias 30               ; first LVO = -30 (4 mandatory: -6,-12,-18,-24)

##public
Open(name,accessMode)(D1,D2)      ; LVO -30
Close(file)(D1)                   ; LVO -36
Read(file,buffer,length)(D1,D2,D3); LVO -42
Write(file,buffer,length)(D1,D2,D3); LVO -48
...

##private
_InternalOpen(...)                 ; private function, not in public ABI

Grammar Rules

Directive	Meaning
##base _LibBase	Name of the library base global variable
##bias N	Starting LVO magnitude; first public function is at -N
##public	Following functions are part of the public ABI
##private	Following functions are internal/private
FuncName(args)(regs)	Function with argument names and register assignments

The LVO of function n (0-indexed from first public) is:

LVO = -(bias + n × 6)

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Register Notation

Register assignments are listed in order matching the argument list, using Amiga register names:

Open(name, accessMode)(D1, D2)

Means:

• name → D1
• accessMode → D2
• Caller must put library base in A6 (by convention)

Special register names:

• A6 — always the library base (implicit, not listed)
• A0–A3, D0–D7 — data and address registers
• D0 — return value (by convention)

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LVO Calculation Example

For dos_lib.fd with ##bias 30:

LVO	Function
-6	Open (mandatory)
-12	Close (mandatory)
-18	Expunge (mandatory)
-24	Reserved (mandatory)
-30	first public function
-36	second public function
...
-144	SetComment
-150	SetProtection

The actual LVOs match dos/dos.h defines:

#define DOS_Open       (-30)
#define DOS_Close      (-36)
#define DOS_Read       (-42)

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Proto Includes (Generated from .fd)

The proto/ directory contains compiler-specific call wrappers generated from .fd files:

/* proto/dos.h — generated automatically from dos_lib.fd */
#ifndef PROTO_DOS_H
#include <clib/dos_protos.h>   /* function prototypes */
#include <inline/dos.h>         /* inline register call stubs */
#endif

inline/dos.h contains:

#define Open(name,accessMode) \
    ({ ULONG _r; \
       __asm volatile("movea.l %1,a6; jsr -30(a6)" : "=r"(_r) : "r"(_DOSBase) : "a6"); \
       _r; })

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.fd Files and Reverse Engineering

When reversing an Amiga binary, .fd files give you the complete function name and parameter register map for every library call. If you see:

MOVEA.L $00BEEF04, A6   ; graphics.library base
JSR     -$114(A6)       ; LVO -276

Look up -276 in graphics_lib.fd:

-276 ÷ 6 = 46th function from start of table

Or directly look at graphics/graphics.h or proto/graphics.h for the constant:

#define GFX_BltBitMap   (-276)
/* → JSR -276(A6) = BltBitMap() */

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Full LVO Table Reconstruction

See lvo_table.md for the complete process of reconstructing a library's JMP table from scratch during reverse engineering.

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References

• NDK39: fd/ directory — all .fd files
• NDK39: proto/ directory — generated call wrappers
• NDK39: clib/ directory — raw C prototypes
• ADCD 2.1: library chapters reference LVOs
• Community: https://wiki.amigaos.net/wiki/Fd_Files