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Teknisk => Generelt teknisk => Emne startet av: ATC på 27. ſeptember 2008, 18:24 pm

Tittel: C64: BASIC program storage in RAM
Skrevet av: ATC på 27. ſeptember 2008, 18:24 pm

How is a BASIC program stored in the Commodore 64?

Tittel: [Solved] C64: BASIC program storage in RAM
Skrevet av: ATC på 27. ſeptember 2008, 18:24 pm

The BASIC program code always* starts with a byte #$00 at $0800, followed by one or more records in the following format:

NEXT LOW (byte)
NEXT HIGH (byte)
BASIC line number LOW (byte)
BASIC line number HIGH (byte)
Bytecode ASCIIZ string, bytes > #$80 are instruction codes

Example code:
0800: 00 0c 08 01 00 9e 20 34
0808: 30 39 36 00 00 00

Code breakdown:
0800: 00 # Always* zero
0801: 0c 08 # Next record begins at $080c
0803: 01 00 # Line number 1
0805: 9e 20 34 30 39 36 # SYS 4096
080b: 00 # ASCIIZ string terminator
080c: 00 00 # Next record begins at $0000 (End of program)

*) The first byte is always zero in the code I have examined. If this byte is changed, the BASIC interpreter will yield syntax errors but still run the code that follows.