amiga-bootcamp/03_loader_and_exec_format/hunk_debug_info.md

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HUNK Debug Information

Overview

Two optional hunk types carry debug information in AmigaOS executables:

• HUNK_SYMBOL ($3F0) — a simple name→offset symbol table
• HUNK_DEBUG ($3F1) — arbitrary debug data (most commonly stabs records)

Both are ignored by the loader and only used by debuggers.

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HUNK_SYMBOL

The simplest debug hunk. Contains a list of (name, offset) pairs for the current hunk:

HUNK_SYMBOL ($000003F0)

[Repeat:]
  <name_len>      Length of name in longwords (0 terminates)
  <name...>       Symbol name padded to longword boundary
  <value>         Symbol value (offset within current hunk)

<0>               Terminator: name_len = 0
HUNK_END

Example

Two symbols in a code hunk:

$000003F0           HUNK_SYMBOL
$00000001           name = 1 long (4 chars)
"_foo"              symbol name
$00000000           at offset 0
$00000002           name = 2 longs (8 chars)
"_bar\0\0\0\0"
$00000040           at offset $40
$00000000           terminator

Use in Debuggers

MonAm, wack, and IDA Pro all parse HUNK_SYMBOL to provide named labels in the disassembly. IDA's Amiga loader maps these directly to function/data names.

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HUNK_DEBUG

HUNK_DEBUG carries arbitrary debug data. The most common format used by AmigaOS compilers is stabs records (as produced by SAS/C 6.x and GCC).

HUNK_DEBUG ($000003F1)

<size_in_longs>     Total size of the debug data in longwords
<data...>           Compiler-specific debug data
HUNK_END

SAS/C Stabs Format

SAS/C 6.x emits stabs-format debug info. The first longword in the debug data is a tag identifying the format:

$3D415053   Tag = "=APS" — SAS/C stabs

Following the tag: standard BSD/UNIX stabs records:

struct stab_entry {
    ULONG  n_strx;   /* offset into string table */
    UBYTE  n_type;   /* stab type code */
    UBYTE  n_other;
    UWORD  n_desc;   /* line number or misc */
    ULONG  n_value;  /* symbol value */
};

Common stab type codes:

Code	Name	Meaning
$24	N_FUN	Function start
$44	N_SLINE	Source line number
$64	N_SO	Source file name
$84	N_LSYM	Local symbol / type
$A0	N_GSYM	Global symbol
$C0	N_RSYM	Register variable

GCC Stabs Format

GCC (m68k-amigaos-gcc) emits similar stabs, usually with tag $3D474343 ("=GCC") or no tag at all.

Line Number Information

Stabs records with N_SLINE provide source-to-address mapping, enabling source-level debugging in tools like wack:

N_SLINE: n_desc = source_line_number
         n_value = offset in current code hunk

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Reading Debug Info in IDA Pro

IDA Pro's Amiga HUNK loader (standard IDA plugin) parses:

• HUNK_SYMBOL → applies as function/data names automatically
• HUNK_DEBUG → partially parsed; stab N_FUN entries become function names

To see IDA's parsed symbols after loading:

• View → Open Subviews → Names — all named locations including HUNK_SYMBOL entries
• View → Open Subviews → Segments — hunk-to-segment mapping

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Stripping Debug Info

To produce a smaller executable without debug info:

SAS/C:

slink lib/c.o + myobj.o TO myexe NODBG

GCC:

m68k-amigaos-strip --strip-debug myexe

This removes HUNK_SYMBOL and HUNK_DEBUG records, reducing file size.

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Worked Hex Example (HUNK_SYMBOL)

Binary fragment from a real executable:

Offset  Hex Bytes           Decoded
$1A00:  00 00 03 F0         HUNK_SYMBOL
$1A04:  00 00 00 01         name_len = 1 (4 chars)
$1A08:  5F 6D 61 69         "_mai"
$1A0C:  6E 00 00 00         "n\0\0\0"
$1A10:  00 00 00 00         value = 0 (entry at start of code hunk)
$1A14:  00 00 00 02         name_len = 2 (8 chars)
$1A18:  5F 70 72 6F         "_pro"
$1A1C:  63 65 73 73         "cess"
$1A20:  00 00 00 78         value = $78
$1A24:  00 00 00 00         terminator
$1A28:  00 00 03 F2         HUNK_END

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References

• NDK39: dos/doshunks.h — HUNK_SYMBOL, HUNK_DEBUG constants
• SAS/C 6.x Programmer's Guide — debug output format
• GCC internals — stabs format documentation
• IDA Pro Amiga loader source (community) — stabs parsing