amiga-bootcamp/03_loader_and_exec_format/hunk_relocation.md

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HUNK Relocation Mechanics

Overview

Relocation is the process of patching absolute addresses in a loaded executable to reflect its actual memory location. Since AmigaOS allocates memory dynamically, a program cannot know its load address at compile time — all inter-hunk references must be fixed up at runtime.

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Why Relocation Is Necessary

An Amiga executable contains references like:

LEA  DataTable(PC), A0   ; PC-relative — no relocation needed
MOVE.L #DataTable, A0    ; Absolute — MUST be relocated

The linker places DataTable at some hunk-relative offset (e.g., offset 0 in the data hunk). The absolute address is only known at load time. The relocation table tells the loader which longwords in the code contain these absolute values.

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HUNK_RELOC32 Format

HUNK_RELOC32 ($000003EC)

[Repeat until terminator:]
  <num_offsets>       Number of longword addresses to patch for this target hunk
  <target_hunk>       Index of the hunk whose base address is added
  <offset_0>          Byte offset within the current hunk to patch
  <offset_1>
  ...

<0>                   num_offsets = 0 terminates the reloc list
HUNK_END ($000003F2)

Patching Algorithm

For each entry in HUNK_RELOC32 of hunk H:

foreach (target_hunk, offsets[]):
    base = segment_base_address[target_hunk]
    foreach offset in offsets:
        *(ULONG *)(H_base + offset) += base

The value at H_base + offset already contains the hunk-relative address written by the linker. Adding the actual base produces the final absolute address.

Example

Code hunk references data hunk at two sites:

Before load (raw file values):
  code[0x18] = $00000000   ; linker placed "data offset 0" here
  code[0x2C] = $00000010   ; linker placed "data offset 0x10" here

HUNK_RELOC32:
  num_offsets = 2
  target_hunk = 1           ; data hunk
  offsets = [0x18, 0x2C]

After load (data hunk loaded at $20000):
  code[0x18] = $00000000 + $20000 = $00020000
  code[0x2C] = $00000010 + $20000 = $00020010

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HUNK_RELOC16 and HUNK_RELOC8

Same format as HUNK_RELOC32 but patch 16-bit or 8-bit values:

• HUNK_RELOC16 ($3ED): patches UWORD at offset
• HUNK_RELOC8 ($3EE): patches UBYTE at offset

These are rare in practice — the 68000 requires even-aligned word accesses and only supports 16-bit displacement in most addressing modes.

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HUNK_DREL32 — Short Relocation (32-bit)

HUNK_DREL32 ($3F7) is an alternative relocation format used by some linkers (e.g., BLink) for smaller reloc tables:

HUNK_DREL32

[Repeat:]
  <num_offsets>  (WORD, not LONGWORD)
  <target_hunk>  (WORD)
  <offset_0>     (WORD)
  ...

<0>              terminator

By using 16-bit values, this format is more compact for programs with many relocations and small hunk sizes (<64 KB). AmigaOS InternalLoadSeg supports both formats.

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PC-Relative References (No Relocation Needed)

The 68020+ supports PC-relative addressing with 32-bit displacements:

LEA  symbol(PC), A0    ; PC-relative load effective address
MOVE.L  data(PC), D0   ; PC-relative data read

PC-relative references do not require relocation — the offset is relative to the instruction, so it is valid regardless of where the code is loaded. GCC for 68k generates PC-relative code by default (-fpic), significantly reducing the size of relocation tables.

SAS/C generates absolute references by default and relies heavily on HUNK_RELOC32.

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Relocation at Runtime — Segment Chain

The loader tracks loaded segments as a BPTR chain (singly-linked list). The segment list head is returned by LoadSeg():

Segment 0 (code):
  BPTR → Segment 1
  [code data]

Segment 1 (data):
  BPTR → 0 (NULL)
  [data]

Each segment begins with a 4-byte BPTR to the next segment. Hunk index n corresponds to segment n in this chain.

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Viewing Relocations with Tools

IDA Pro

After loading a HUNK file with the Amiga plugin, IDA resolves relocations automatically. The fixup table is visible in View → Open Subviews → Fixups.

hexdump + manual

Locate HUNK_RELOC32 ($3EC) in raw hex:

xxd mybinary | grep "0003 ec"

Then read num_offsets and target_hunk longwords that follow.

hunkinfo (custom tool)

hunkinfo mybinary    # shows all hunks, sizes, reloc counts

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References

• NDK39: dos/doshunks.h
• Amiga ROM Kernel Reference Manual: Libraries — AmigaDOS chapter, InternalLoadSeg
• vlink linker documentation — relocation section
• http://amigadev.elowar.com/read/ADCD_2.1/Libraries_Manual_guide/node01E0.html