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+// sbex.h
+// The SplitBit loadable program format, version one.
+//
+// A program that is not the one the machine booted from has to say where it wants to
+// live, because nothing relocates it. This is a header saying that, in front of the
+// bytes themselves. It is the same idea as the load address on the front of a C64 .PRG,
+// with room for the machine to ask a few more questions later.
+//
+// Two things read this: whatever builds one on the host, and the loader running on
+// SplitBit. As with the filesystem, nothing is shared between them but the specification.
+//
+// All multi byte numbers are most significant byte first.
+//
+//   0   4  "SBEX"
+//   4   1  Version
+//   5   1  How many vectors follow the data, zero in a version 1 file
+//   6   2  Where the code goes in Program Memory
+//   8   2  Where to start running, an address in Program Memory
+//   10  2  How many bytes of code there are
+//   12  2  Where the data goes in Data Memory
+//   14  2  How many bytes of data there are
+//   16     The code, then the data, then the vectors
+//
+// ---- Vectors, added in version two ----
+//
+// Four bytes each, the same shape a boot image uses: the address of the vector table slot,
+// then the address to put in it. Both most significant byte first. Saying the slot outright
+// rather than the vector number means the loader does no arithmetic and does not need to
+// know where either vector table begins, and one entry can name a software or a hardware
+// vector without saying which.
+//
+// A PROGRAM CARRYING VECTORS SAYS VERSION TWO, and one carrying none stays version one and
+// loads anywhere. The version is not decided by the format's age but by whether the file
+// needs something of its loader: an older loader meeting a version two file refuses it and
+// says so, which is the right answer, because a program whose handlers were quietly dropped
+// is not the program somebody asked for. It would run, and then fail later at a place with
+// nothing to connect it back to loading.
+//
+// INSTALLING THEM IS THE LOADER'S JOB, AND SO IS TAKING THEM BACK. A vector points into the
+// program that supplied it, so leaving one installed after that program is gone aims an
+// interrupt at whatever occupies those addresses next. A loader restores what it found.
+#define SBEX_VECTOR_ENTRY_BYTES 4
+//
+// Sixteen bytes, so the code begins at a round offset and finding it is one step rather
+// than an arithmetic. Nothing here relocates anything: the addresses are where the
+// program was built to live, and putting it anywhere else would leave every branch and
+// every SETD inside it pointing at the wrong place.
+//
+// Written by Anachronaut
+
+#ifndef SBEX_H
+#define SBEX_H
+
+#define SBEX_MAGIC "SBEX"
+#define SBEX_MAGIC_BYTES 4
+// What a program says when it asks nothing of its loader beyond code and data, and what
+// it says when it also brings vectors that have to be installed.
+#define SBEX_VERSION 1
+#define SBEX_VERSION_VECTORS 2
+#define SBEX_HEADER_BYTES 16
+
+#define SBEX_VERSION_AT   4
+#define SBEX_VECTORS_AT   5
+#define SBEX_CODE_AT      6
+#define SBEX_ENTRY_AT     8
+#define SBEX_CODE_LEN_AT  10
+#define SBEX_DATA_AT      12
+#define SBEX_DATA_LEN_AT  14
+
+#endif // SBEX_H