AnachronautandClaude Opus 5
ca34e077ad
The loadable program format's header has been in the working tree since the loader was written and has never been in the repository: the gitignore rules matched Source/Assembler as a directory, so it was silently untracked. Anything cloning this repository could not build it, because secondPass.c includes this file. This is the failure the previous commit describes, having already happened once without being noticed. Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
71 lines
3.1 KiB
C
71 lines
3.1 KiB
C
// sbex.h
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// The SplitBit loadable program format, version one.
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//
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// A program that is not the one the machine booted from has to say where it wants to
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// live, because nothing relocates it. This is a header saying that, in front of the
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// bytes themselves. It is the same idea as the load address on the front of a C64 .PRG,
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// with room for the machine to ask a few more questions later.
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//
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// Two things read this: whatever builds one on the host, and the loader running on
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// SplitBit. As with the filesystem, nothing is shared between them but the specification.
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//
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// All multi byte numbers are most significant byte first.
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//
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// 0 4 "SBEX"
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// 4 1 Version
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// 5 1 How many vectors follow the data, zero in a version 1 file
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// 6 2 Where the code goes in Program Memory
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// 8 2 Where to start running, an address in Program Memory
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// 10 2 How many bytes of code there are
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// 12 2 Where the data goes in Data Memory
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// 14 2 How many bytes of data there are
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// 16 The code, then the data, then the vectors
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//
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// ---- Vectors, added in version two ----
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//
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// Four bytes each, the same shape a boot image uses: the address of the vector table slot,
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// then the address to put in it. Both most significant byte first. Saying the slot outright
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// rather than the vector number means the loader does no arithmetic and does not need to
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// know where either vector table begins, and one entry can name a software or a hardware
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// vector without saying which.
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//
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// A PROGRAM CARRYING VECTORS SAYS VERSION TWO, and one carrying none stays version one and
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// loads anywhere. The version is not decided by the format's age but by whether the file
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// needs something of its loader: an older loader meeting a version two file refuses it and
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// says so, which is the right answer, because a program whose handlers were quietly dropped
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// is not the program somebody asked for. It would run, and then fail later at a place with
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// nothing to connect it back to loading.
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//
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// INSTALLING THEM IS THE LOADER'S JOB, AND SO IS TAKING THEM BACK. A vector points into the
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// program that supplied it, so leaving one installed after that program is gone aims an
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// interrupt at whatever occupies those addresses next. A loader restores what it found.
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#define SBEX_VECTOR_ENTRY_BYTES 4
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//
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// Sixteen bytes, so the code begins at a round offset and finding it is one step rather
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// than an arithmetic. Nothing here relocates anything: the addresses are where the
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// program was built to live, and putting it anywhere else would leave every branch and
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// every SETD inside it pointing at the wrong place.
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//
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// Written by Anachronaut
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#ifndef SBEX_H
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#define SBEX_H
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#define SBEX_MAGIC "SBEX"
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#define SBEX_MAGIC_BYTES 4
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// What a program says when it asks nothing of its loader beyond code and data, and what
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// it says when it also brings vectors that have to be installed.
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#define SBEX_VERSION 1
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#define SBEX_VERSION_VECTORS 2
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#define SBEX_HEADER_BYTES 16
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#define SBEX_VERSION_AT 4
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#define SBEX_VECTORS_AT 5
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#define SBEX_CODE_AT 6
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#define SBEX_ENTRY_AT 8
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#define SBEX_CODE_LEN_AT 10
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#define SBEX_DATA_AT 12
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#define SBEX_DATA_LEN_AT 14
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#endif // SBEX_H
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