6502 Assembly programming for the Commodore PET

The PET was the predecessor to the VIC 20, Released in 1976 it was the earliest of the Commodore home computers

Later revisions added more memory and a higher screen resolution, though it was still only 'character based'

Pet 2001 / 3000 Pet 4000 SuperPet 9000
Cpu 1mhz 6502 1mhz 6502 1mhz 6502 + 1mhz 6809
Ram 4/8k + 8 /16k+ 96k
Colors Green / Black Green / Black Green / Black
Sound chip None / Beeper None / Beeper None / Beeper

Thanks to "KnightFire66" for the PET photos!

Useful Documents

Quick Reference
The Pet Revealed

ChibiAkumas Tutorials

Lesson H10 - Hello World on the Commodore PET
Lesson S32 - Char Block clipping on the Commodore PET!
Lesson S19 - Keypad Reading on the Commodore PET!
Lesson P38 - Character Block Graphics on the Pet
Lesson P39 - Key reading on the PET
Lesson P40 - Sound on the PET
Lesson P49 - Sound on the Commodore PET (ChibiSound Pro)
Lesson P63 - Multiplatform Software tilemap on the PET

Memory Map

From To Function
$0000 $3FF OS and Basic storage
$0400 $1FFF Basic Program / 8k main ram
$2000 $7FFF 32k extension
$8000 $83FF VRAM
$C000 $E7FF Basic ROM
$E800 $EFFF I/O

Interrupt vectors

The standard 6502 interrupt vectors from $FFFA+ are ROM, however these jump to vectors in low memory addresses. IRQ and BRK interrupts push A,X and Y onto the stack in that order.

Immediately after the pushes, the interrupt handler at the addresses below are executed.

VBLank interrupts cause a CB1 IRQ interrupt. This can be disabled with "lda #%11111100   sta $e813"

Note: The vector addresses were different on the first generation 2001-8 machine.

2001-8 Others

From To From To Function Registers Pushed
$021A $0090 $0091 IRQ A X Y
$021B $021C $0092 $0093 BRK A X Y
None None $0094 #0095 NMI

Character Map

Write #12 to $E84C

Write #14 to $E84C


Rom Functions

Address Purpose
$FFD2 PrintChar A
$FFE4 ReadChar A
$FFCF Input a character

I/O Map

Port Purpose Bits Notes
$E810 PIA1:Key Row select (0-9) DICcKKKK D=Diagnostic sense / I=IEEE EOI in / C=Cassette sense #2 / c==Cassette sense #1 / K=Keyboard Row select
$E811 PIA1:Screen / Tape T-SSSDCC T=Tape #1 input flag / S = Screen blank (old 8k only) IEE EOI Out CA2 / D=DDRA Access (Data Direction Reg A) / C=Casette #1 read control CA1
$E812 PIA1:Key Data KKKKKKKK K=Keyboard Row Input
$E813 PIA1:Screen / Tape R-CCCDrr R=Retrace I flag / C=Cassette #1 Motor output CB2 / D=DDRB Access (Data Direction Reg B)  / r= retrace interrupt control CB1 (rr=1 Vblank interrupt)
$E820 PIA2:IEEE488 Input IIIIIIII I=IEEE Input
$E821 PIA2:IEEE488 A-INCDia A=ATN I Flag / I=IEEE/ N=Ndac Out/ C=CA2 / D=DDRA Access / i=IEEE control / a=ATN in CA1
$E822 PIA2:IEEE488 Output OOOOOOOO O=IEEE Output
$E823 PIA2:IEEE488 S-IDCdis S=SRQ I Flag / I=IEEE / D=DAV out / C=CB2 / d=DDRB access / i=IEEE control / s=SRQ in CB1
$E840 VIA:IO Port B output DNRCOAnd D=Dav in / N=NFRD In / R=Retrace In / C=Cassette #2 motor / O=Cassette output / O=Atn Out / n=NFRD out / d=NDAC in PB
$E841 VIA:IO Port A output

$E842 VIA:IO Port B direction (1=out 0=in) BBBBBBBB Data Direction register B (For E840)
$E843 VIA:IO Port A direction AAAAAAAA Data Direction register A (For E84F)
$E844 VIA:Timer 1 L LLLLLLLL
$E845 VIA:Timer 1 H HHHHHHHH
$E846 VIA:Timer 1 Latch L LLLLLLLL
$E847 VIA:Timer 1 Latch H HHHHHHHH
$E848 VIA:Timer 2 L LLLLLLLL 64-255 = Sound Frequency
$E849 VIA:Timer 2 H HHHHHHHH
$E84A VIA:Shift Register SSSSSSSS 15/51/85 = Sound Octive
$E84B VIA:Aux Control Register 1O2SSSPP 1=T1 Control PB7 Out / O=One shot free run / 2=T2 control PB6 sense / S=Shift register control / P=PB PA Latch control ... (16= Sound On)
$E84C VIA:Peripheral control BBBbAAAa B=CB2 (Pup) control in/out / b=CB1 in casette #2 / A= CA2 graphics lowecase in out / a=CA1 in polarity .... 12=Graphics
$E84D VIA:Interrupt Flag Register (IFR) ITtBBSAa I=IRQ status / T=T1 interrupt / t=T2 interrupt / B=CB1 Casette / S=SR Interrupt / A=CA1 Interrupt / a=CA2 Interrupt
$E84E VIA:Interrupt enable register ETtBbSAa E=Enable or disable following ints / T=T1 interrupt / t=T2 interrupt / B=CB1 interrupt / b=CB2 Interrupt / S=SR interrupt / A=CA1 Interrupt / a=CA2 interrupt
$E84F VIA:IO Port A Output without handshaking AAAAAAAA Paralell user port I/O - Port A
$E856 CRTC:Reg select RRRRRRRR
$FFF0 64k Expansion Control Register EIS-BbWw E=Enable I=IO Ram $E800-EFFF S=Scrn Ram $8000-$8FFF B=bank 1/3 $C000-$FFFF b=bank 0/2 $8000-$BFFF W=Write protect 1/3 w=Write protect 0/2


The PET did not have any built in sound, but a speaker can be connected to the IO port.

The PET is capable of up to 4 octaves... one of 3 possible pairs can be selected with $E84A

Address $E84B will turn the sound on or off... we write #16 to turn it on, #0 to turn it off.
Address $E84A can be used to select the Octave with value 15/51/85
Address $E848 can be used to select the note, a value of 64-255 should be passed.

It is not possible to set the volume, or play multiple tones at the same time.

Note Freq octave=15 octave=51 octave=85
Octave 0 Octave 1 Octave 1 Octave 2 Octave 2 Octave 3
B 251 125 251 125 251 125
C 238 118 238 118 238 118
C# 224 110 224 110 224 110
D 210 104 210 104 210 104
D# 199 99 199 99 199 99
E 188 93 188 93 188 93
F 177 88 177 88 177 88
F# 168 83 168 83 168 83
G 158 78 158 78 158 78
G# 149 74 149 74 149 74
A 140 69 140 69 140 69
A# 133 65 133 65 133 65

(Sound info taken from the PetFaq)


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