USF format specification v0.0 INITIAL PRE-SPEC WHICH IS IN NO WAY FINAL:

See PSF spec, version code 0x21.

Code area:

Contains sparse version of N64 ROM. ROM is 32-bit swapped. Sparse format is:
4 byte identifier: "SR64"
each chunk:
4 byte little endian length (if 0 this ends the sparse file)
4 byte little endian offset
n bytes data
Any unspecified data is assumed to be previously loaded (for USFlib/miniUSF purposes, if never specified it is assumed zero).

Reserved area contains sparse version of Project 64 1.4 save state.
Save state format:
(DWORD is 4-byte little endian, _int64 is 8-byte little endian)
0x000: DWORD: 0x23D8A6C8, identifier
0x004: DWORD: RDRAM size in bytes
0x008: ROM header, little endian first 0x40 bytes of ROM
0x048: DWORD: VI timer (PJ64 internal)
0x04c: DWORD: PC
0x050: 32 _int64: CPU General Purposes Registers
0x150: 32 _int64: Floating Point Registers
0x250: 32 DWORD: COP0 Registers
0x2d0: 32 DWORD: Floating Point Control Registers
0x350: _int64: HI
0x358: _int64: LO
0x360: 10 DWORD: RDRAM Registers
0x388: 10 DWORD: SP Registers
0x3b0: 10 DWORD: DP Registers
0x3d8: 4 DWORD: MI Registers
0x3e8: 14 DWORD: VI Registers
0x420: 6 DWORD: AI Registers
0x438: 13 DWORD: PI Registers
0x46c: 8 DWORD: RI Registers
0x48c: 4 DWORD: SI Registers
0x49c: 32 TLB entries (20 bytes each), see PJ64 source for format
0x71c: PIF RAM, 0x40 bytes 32-bit swapped
0x75c: RDRAM, 32-bit swapped, whatever size specified by 0x004.
0x75c+RDRAM size: 0x1000 bytes RSP DMEM, 32-bit swapped
0x175c+RDRAM size: 0x1000 bytes RSP IMEM, 32-bit swapped

The _lib tag is used to open up another file which is loaded, both ROM and save state, before the current file. This behaves recursively. Either ROM or save state size can be zero and the USF will still be valid (most often the only contents of a miniUSF is a song specifier in the save state).

I use only the sparse storage format and not and sort of compression because the main point of compression is to remove zero/unused bytes, which sparse accomplishes. This also allows for more efficient external compression of the files.