| 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'
|
 Thanks to "KnightFire66" for the PET photos! |
Useful Documents
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 | |||
| 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)
