Commodore VIC-20 and PET: emulator, assembler and debugger

BASIC: type on the screen (10 PRINT CHR$(205.5+RND(1));:GOTO 10, RUN), or use Type into the computer and its BASIC examples…. LOAD"$",8, LOAD"NAME",8 and SAVE"NAME",8 use the disk in the Disk drive and Datassette panel; LOAD / SAVE"NAME" the tape.

Assembly: pick an example — hello world with CHROUT, border colours, screen and colour memory, the VIC’s three voices and noise, reading keys with GETIN, hooking the 60 Hz interrupt through CINV, a custom character set, multicolour, the joystick and a KERNAL SAVE on the VIC-20; on the PET hello world, screen memory at $8000, CB2 sound, the keyboard and the lower-case set — or write your own, then Assemble & run (Ctrl+Enter). The machine is reset, the page waits for READY., loads the code and runs it with SYS; RTS returns. Download .PRG saves it with its load address, Save to disk puts it on the virtual disk.

Run: choose the machine. The VIC-20 can be PAL (6561) or NTSC (6560) and take 3 to 35 KB of expansion — with 8 KB or more in block 1 the screen moves to $1000 and BASIC to $1201, as on the real machine. The PET is a 2001 (8 KB), 3032, 4032 (32 KB) or the 80-column 8032, with the graphics or business keyboard, green, white or amber. Drop a .PRG, .D64, .TAP or .T64 to load it, or a VIC-20 cartridge (.CRT from VICE, a PRG with load address $A000/$2000/$4000/$6000, or a raw 4/8 KB .A0/.BIN image): it starts at reset, as the KERNAL finds the A0CBM signature at $A004.

The monitor (Monitor only, or SYS 65280 from BASIC; prompt .) understands 1000 (show a byte), 1000.103F (dump), 1000: A9 41 20 D2 FF 60 (store), 1000G (run) and X (back to BASIC). On a phone, ⌨ Keys opens an on-screen keyboard.

Video and sound — VIC-20: where the screen, characters and colour RAM are, the size and origin, the raster line, border, background and auxiliary colours, and each voice’s frequency in Hz. PET: the character set, the CRTC registers and the CB2 tone.

VIAs and PIAs — ports and data directions, both timers and their latches, ACR/PCR (with the shift register’s sound mode), and which interrupt flags are set and enabled; the keys held.

Memory map — what is in each block: RAM, expansion, cartridge, ROM or the stub.

The CPU runs instruction by instruction with the documented cycle counts, not cycle by cycle. The VIC-20 screen is drawn a line at a time, so border and colour changes between lines show, but changes in the middle of a line don’t; the border position is approximate. The PET screen is drawn once a frame.

VIC sound is a simple square-wave model (frequency Φ2 / n / (255 − X), n = 256, 128, 64, 32 for the bass, alto, soprano and noise registers) with a 15-bit noise generator, not the real chip’s filters.

Tape and disk work at the KERNAL / BASIC level (no drive CPU, IEEE-488 or serial bus, fast loaders or protected disks); no printer, light pen or paddles; the PET has no CRTC timing tricks. Cartridges with bank switching aren’t supported.

The open BASIC is BASIC 1.1: no OPEN/PRINT# files. Programs that call undocumented ROM routines need the real ROMs. PET BASIC 1 keeps its keyboard buffer elsewhere, so typing into it doesn’t work.

The colours are approximations (palettes for these machines disagree).