Amiga HUNK File Format

Introduction

A HUNK file is how AmigaOS stores a program. The loader reads it block by block, puts each hunk (a piece of code, data or zeroed memory) at an address of its own, and adds that address to the places the file lists, so the program can be loaded anywhere. The same block format serves three kinds of file, told apart by the first block: an executable (a load file) starts with HUNK_HEADER, an object file from an assembler or compiler starts with HUNK_UNIT, and a link library starts with HUNK_LIB. Every number is big-endian, as on the 68000. The code is for the Motorola 68000 family.

An Amiga program has no extension (Dir, diskfont.library, trackdisk.device), so the explorer recognises a HUNK file by its first long, $000003F3, $000003E7 or $000003FA, in a file of at least 24 bytes. A file inside an Amiga disk image (.adf) opens the same way.

Structure

The header page has the first block, and the hunks page the blocks that hold the program. Relocations says how an address is fixed up, and symbols and debug what a debugger gets. Objects and libraries is the linker side, and system addresses lists the fixed addresses the viewer names in the disassembly: address 4, the custom chips, the CIAs, and the library call offsets.

The three kinds of file

Not described

Known, not yet covered

• Reference: the layouts here come from third-party descriptions (Wikipedia, the amiga-dev wiki, the amitools source and the vasm assembler’s source); the fetch tool that read the web pages returned summaries, so the layouts rest on those summaries and on the real files below. Not covered: HUNK_RELOC32SHORT’s exact use by each linker, the PowerPC extended hunks other than HUNK_PPC_CODE and HUNK_RELRELOC26, the line-debug formats other than LINE (HCLN, HEAD, ODEF, OPTS, ZMAGIC), the overlay manager protocol, and how HUNK_ABSRELOC16 is laid out (the two sources disagree).
• Viewer: the explorer opens executables, object files and libraries: every block as a node, the hunk table with sizes and memory types, the relocations, symbols, external symbols, LINE debug tables, the library index, text in data hunks, and the code of every HUNK_CODE hunk disassembled with the 68000-family decoder (68000 to 68060 as an option), with relocated longs marked, hardware addresses named, and library calls named for the library you choose. It was checked on 2,547 real HUNK files (the Workbench disks 1.0 to 3.1 and loose Amiga files, read in memory) and on files made by the vasm assembler, but the real files are all executables: no real object file or link library was found on this machine, so those two variants are checked on assembler output and built files only. It decodes every even address as an instruction (code and data are not told apart), does not follow jumps or the overlay manager, shows HUNK_PPC_CODE hunks as bytes, and has not been run in a browser by its author.

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