.PRG File Format – BASIC program and stub

Introduction

A BASIC program is stored in memory, and so in the file, in tokenised form: every keyword is replaced by one byte of $80 or more, so that PRINT takes one byte and not five. The program starts at $0801, so a PRG file with a BASIC program loads at $0801. This page describes BASIC V2, the BASIC of the C64.

A line

The explorer checks that each link is the address of the line after it, and reports one that is not.

The end of the program

The program ends with a link of $0000: the two bytes after the last line’s zero byte. Anything after them in the file is not part of the BASIC program. In a program that carries machine code, the code usually follows straight after.

The SYS stub

A machine-code program is started from BASIC by SYS address, which calls the code at that decimal address (and returns to BASIC if the code ends in RTS). A PRG file of this kind starts with a short BASIC program that does only that:

10 SYS 2061

The stub takes up some bytes ($0801 to $080C here, with its end marker) and the code starts straight after, at $080D, which is 2061. Some stubs put a comment after the number, or other statements; some leave a gap before the code. The explorer takes the first SYS that is followed by a plain decimal number, spaces allowed, and uses it as the entry point if it lies in the file. It does not evaluate expressions, so SYS 4096+3 and SYS A are not followed. After RUN, BASIC runs the stub, and the stub starts the machine code.

The tokens

BASIC V2 has 76 tokens, $80 to $CB, and $FF for the constant π. A byte of $80 or more outside quotation marks and REM is a token; the explorer writes a byte in that range that is not a token as {$cc}.

Links