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

PetFaq
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
$F000	$FFFF	OS ROM

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
$0219	$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

Keymap

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
$E857	CRTC:Reg Data	DDDDDDDD
$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

Sound

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
Note Freq	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