.PRG File Format

Introduction

A .prg file is what the Commodore 64’s SAVE command writes and LOAD reads: two bytes giving the address to load at, and then the bytes that go there. It holds BASIC programs, machine-code programs, and anything else a program is made of. Most C64 games and demos distributed as single files are PRG files. Emulators such as VICE run them directly.

The file has no signature, no length and no start address. A machine-code program is normally started from a one-line BASIC program, a stub such as 10 SYS 2064, that sits at the start of BASIC memory and calls the machine code that follows it. The explorer finds that SYS and takes its number as the program’s entry point.

A PRG file dropped on any page opens in the explorer. It shows the load address, lists the BASIC lines with their keywords written out, finds the SYS target, and disassembles the machine code with an instruction decoded at every byte, laid out in columns so that a jump into the middle of another instruction can be followed. Calls to the C64’s KERNAL and accesses to the VIC-II, SID and CIA chips and to well-known zero-page locations are shown with their names, and undocumented and unstable opcodes are highlighted (see the instruction set). The processor can be changed in the options bar above the viewer.

Because the file has no signature, the explorer recognises a PRG by its extension alone: a file is shown this way only when its name ends in .prg. Any two or more bytes could be a PRG file, so nothing in the content can confirm it.

Structure

The data’s length is the file’s length minus two. A file whose data would run past $FFFF is damaged or is not a C64 program; the explorer says so.

The processor

The C64 has a 6510, a 6502 with a built-in I/O port. A PRG does not say which processor it was written for, so the processor is a choice in the explorer’s options bar: the 6510 (the default), the NMOS 6502, which decodes the same, and the 65C02. See the 6502 instruction set for every instruction.

Not described

Known, not yet covered

• Reference and viewer: PRG files of other Commodore machines. The VIC-20, the PET, the Plus/4 and the C128 use the same file shape with different load addresses ($1001 or $1201, $0401, $1001, $1C01), and the C128’s BASIC 7 has more tokens. The viewer shows them as C64 files: the BASIC is not listed, and the code is labelled with the C64’s addresses.
• Viewer: a SYS whose argument is an expression (SYS 4096+3) or a variable. Only a plain number is read.
• Viewer: which bytes are code and which are data. The disassembly decodes every byte; code that unpacks or moves itself is not followed.
• Reference: the PC64 .p00 wrapper, which puts a header with the file name in front of a PRG; it is a different file format.

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