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Commodore PET

The Commodore PET (Personal Electronic Transactor, 1977) was Commodore's first computer and one of the first successful personal computers ever sold. It predates the Apple II and TRS-80 by only months and established Commodore as a serious computing company.

The PET uses a MOS 6502 CPU and runs Commodore BASIC 2.0 (or 4.0 on later models) — the same BASIC family that descended into the VIC-20 and C64. The machines are characterised by their all-in-one design (keyboard, screen, and cassette tape in one unit) and their distinctive green phosphor display.

Languages in the IDE

Language Notes
Commodore BASIC V2 Built-in ROM BASIC — line-numbered, interpreted
CC65 C C cross-compiler with PET target
6502 assembly ca65 assembler

Quick start

  1. Select Commodore PET as the platform.
  2. Choose your language preset.
  3. The IDE loads a PET emulator — you'll see the familiar READY. prompt.
  4. Build and run.

Which PET?

The IDE emulates a specific PET model (check the preset). Common targets are the PET 2001 (original, 40-column) and PET 8032 (80-column business machine). Screen width and RAM differ between models.

Hardware overview

CPU — MOS 6502 at 1 MHz

Standard 6502 — see 6502 assembly for the full instruction set. The PET runs the 6502 at 1 MHz, the same as the C64.

Video — 6545 CRTC

The PET uses a Motorola 6845/6545 CRTC (the same chip as the Amstrad CPC) to drive its built-in monitor. The display is character-based only — no bitmap graphics mode on most PETs.

Model Screen Notes
PET 2001/3000 series 40×25 characters 1 KB video RAM
PET 4000 series 40×25 characters BASIC 4.0, disk commands
PET 8000 series 80×25 characters Business machine

Memory map (PET 2001 / 32K)

Range Contents
$0000–$00FF Zero page
$0100–$01FF Stack
$0200–$03FF OS work area
$0400–$0FFF Video RAM (1 KB at $8000 on early models)
$1000–$7FFF User RAM (up to 32 KB)
$8000–$8FFF Video RAM (later models)
$9000–$9FFF I/O (VIA chips, CRTC)
$A000–$BFFF BASIC ROM
$C000–$DFFF BASIC ROM (continued)
$E000–$FFFF Kernal ROM + editor ROM

Video RAM location varies

Early PET 2001 models have video RAM at $8000. Later models moved it. Always check which model your preset targets before hard-coding screen addresses.

VIA chips — I/O

Two MOS 6522 VIA chips handle keyboard, IEEE-488 bus (for disk drives and printers), cassette, and the speaker.

The keyboard is read as a matrix via VIA 1 (port B = column select, port A = row readback).

Commodore BASIC on the PET

The PET runs essentially the same BASIC V2 as the C64 — see Commodore BASIC V2 for the full guide. Key differences:

No colour — the PET display is monochrome (green or white phosphor). POKE addresses for colour don't apply.

No sprites — the PET has no sprite hardware.

PETSCII characters — the same character set as the C64. Block graphics characters (codes 160–223) provide crude graphics.

Screen RAM location — not at $0400 like the C64. Check your model:

10 REM PET 2001 early: screen at $8000 = 32768
20 POKE 32768, 1    : REM Block character top-left
30 REM PET 4000/8000: check model documentation

No SID chip — the PET has a simple speaker driven by a VIA timer. You can produce beeps but not the rich sound of the C64.

A simple PET BASIC program:

10 PRINT CHR$(147)        : REM Clear screen
20 FOR Y=0 TO 24
30   FOR X=0 TO 39
40     PRINT CHR$(INT(RND(1)*5)+160);  : REM Random block char
50   NEXT X
60 NEXT Y
70 GOTO 20

CC65 C on the PET

CC65 has a pet target. Basic usage:

#include <peekpoke.h>
#include <conio.h>
#include <stdio.h>

int main(void) {
    clrscr();
    gotoxy(10, 5);
    printf("HELLO FROM CC65!\n");

    /* Write directly to screen RAM */
    POKE(0x8000, 0x01);   /* Block character, top-left (PET 2001) */

    return 0;
}

The cc65 pet.h and conio.h headers handle screen addressing differences between PET models for the standard I/O functions.

PET graphics — block characters

Without a graphics mode, PET programmers use the PETSCII block graphic characters to draw shapes. The block set provides characters that fill different portions of a character cell:

Code range Description
160–175 Solid and partial blocks
176–191 Diagonal and special shapes
224–254 More block graphics

By combining these characters you can produce surprisingly detailed graphics — many PET games are pure character graphics.

IEEE-488 — disk and printer I/O

The PET uses the IEEE-488 (GPIB) bus for storage and printing rather than the serial bus used by the C64. This is faster but the devices (disk drives, printers) were expensive. The Kernal provides OPEN/CLOSE/GET/PUT routines for IEEE-488 access just like the C64 does for its devices.

10 OPEN 1,8,0,"MYFILE,S,R"   : REM Open file on disk drive 8
20 INPUT#1, A$                : REM Read a line
30 CLOSE 1

Kernal routines (call via SYS / JSR)

Address Name Action
$FFD2 CHROUT Print character in A
$FFE4 GETIN Get character from keyboard
$FF9C CLALL Close all files
$FFB4 OPEN Open a file
$FFC0 CLOSE Close a file
$FFA5 CHKIN Set input device
$FFA8 CHKOUT Set output device

These are the same Kernal entry points as the C64 — deliberate design continuity across the Commodore line.

Tips

Use CHR$(147) to clear — PRINT CHR$(147) clears the screen via the PETSCII control code (same as on C64). CLR only clears variables.

PETSCII vs ASCII — the PET uses PETSCII throughout, not ASCII. Many special characters are in different positions. ASC("A") = 65 in PETSCII and ASCII, but lowercase letters differ.

64K PET — some PET models have 64 KB RAM and use a different memory map. The upper 32 KB is accessed by banking out the BASIC ROM.

The PET is the ancestor — the C64's Kernal, BASIC structure, and many hardware conventions trace directly back to the PET. If you understand the PET, you understand the whole Commodore 8-bit family.

See also