| name | c64-io |
| description | The Commodore 64 input/output system and its device/file model: logical device numbers, OPEN/CLOSE/PRINT#/INPUT#/GET#/CMD, secondary addresses, and the ports behind them — output to TV/printer/modem, the RS-232 interface, the serial IEC bus, the user port, the expansion (cartridge) port, and the Z-80 cartridge. Use this skill WHEN a user asks "how do I OPEN a channel", "what device number is the printer/screen/RS-232", "what does the secondary address mean", "how does the serial bus work", "what's on the user port or expansion port", "how do I set up RS-232 / a modem", or works with EXROM/GAME, ATN/CLK/ DATA, $DD00, $DE00. Pairs with c64-disk, c64-tape, c64-game-ports, c64-cia, c64-kernal, and c64-memory-map. |
C64 Input/Output System
Every peripheral on the C64 is reached through one model: you OPEN a logical
file onto a device number (optionally with a secondary address and a name),
then use PRINT#/CMD to send and INPUT#/GET# to receive, and CLOSE when done. The
screen, keyboard, tape, RS-232, printer, and disk are all the same machinery
from BASIC's point of view — only the device number and the meaning of the
secondary address change.
Logical device numbers
| Dev | Device | Notes |
|---|
| 0 | Keyboard | default input |
| 1 | Cassette (Datassette) | default for LOAD/SAVE with no device — see c64-tape |
| 2 | RS-232 | built-in serial interface, runs on CIA2 |
| 3 | Screen | default output |
| 4, 5 | Printer | on the serial bus |
| 6, 7 | (plotter / extra serial devices) | |
| 8–11 | Disk drives | 8 is the usual 1541 — see c64-disk |
Devices 4–11 live on the serial IEC bus; 1 (tape), 2 (RS-232), 3 (screen),
0 (keyboard) do not.
The OPEN model
OPEN logical#, device#, secondary#, "name"
- logical# (1–255): the handle you pass to PRINT#/INPUT#/GET#/CMD/CLOSE. Use
1–127; numbers >127 add a line feed after each carriage return.
- device#: from the table above.
- secondary# (a.k.a. sa / channel): a per-device setup code (see below).
- "name": filename, or device-specific setup bytes (printer mode, RS-232
control/command bytes, disk "drive:name,type,mode").
100 OPEN 4,4: PRINT#4,"WRITING ON PRINTER" : REM printer, dev 4
110 OPEN 3,8,3,"0:DISK-FILE,S,W": PRINT#3,"HI" : REM disk seq write
120 OPEN 1,1,1,"TAPE-FILE": PRINT#1,"WRITE ON TAPE": REM tape write
130 OPEN 2,2,0,CHR$(10): PRINT#2,"SEND TO MODEM" : REM RS-232
Companion statements: PRINT#lf,... writes; INPUT#lf,var / GET#lf,var$ read;
CMD lf redirects normal PRINT output to that file (e.g. CMD to a printer then
LIST); CLOSE lf finishes. Read the reserved variable ST after I/O for
status (end-of-file, errors). Pitfall: after CMD, you must PRINT#lf then
CLOSE lf to flush and unhook output, or the channel stays redirected.
Secondary address meaning (per device)
| Device | sa | Meaning |
|---|
| Cassette | 0 / 1 / 2 | read / write / write-with-EOT marker |
| Printer | 0 / 7 | upper-case+graphics / upper+lower case |
| RS-232 | 0 | always 0; the name field carries control/command bytes |
| Disk | 2–14 | data channel; 15 = command channel (DOS commands) |
The secondary address is how the C64 hands setup to a device — e.g.
OPEN 1,4,7 tells printer 4 to switch to upper/lower case.
Output to screen, printer, modem
- Screen (dev 3): plain
PRINT. ; joins with no space, , tabs to the
next 10-column field, both suppress the trailing RETURN when last on the line.
TAB/SPC position the cursor. For colors, sprites, raster: see c64-vic-ii.
- Printer (dev 4/5): like the screen, but
TAB is wrong (it uses the screen
cursor, not the paper) — use SPC. Control via PETSCII codes you simply PRINT:
14/15 double-width on/off, 18/146 reverse on/off, 17/145 switch case set, 8 dot
graphics. Full table in references/io-guide.md.
- Modem (dev 2): OPEN with control/command bytes selects baud/parity; you
must translate PETSCII↔ASCII yourself. The modem is the RS-232 interface —
see below.
RS-232 interface
The C64 has a built-in RS-232 at TTL levels (0–5V), not true ±12V — you need
a level-converting cable/cartridge. It runs as a software UART driven by CIA2
NMIs in the background. OPEN allocates two 256-byte FIFO buffers at the top of
memory (512 bytes total).
OPEN 2,2,0, CHR$(<control>)+CHR$(<command>)
- control byte sets baud/word-length/stop-bits (e.g.
CHR$(6) = 300 baud).
- command byte sets parity/duplex/handshake (3-line vs x-line).
- Read ST for status: parity/framing/overrun/break errors, buffer-empty,
CTS/DSR missing. Prefer
GET# over INPUT# (INPUT# can hang the system).
Critical gotcha: OPEN an RS-232 channel before creating variables/arrays —
the OPEN does an automatic CLR to allocate buffers, and if 512 free bytes aren't
available your program gets corrupted with no error. Only one RS-232 channel at a
time. Full register maps, status bits, buffer pointers ($F7/$F9), and zero-page
usage live in references/rs232.md.
The serial IEC bus
A daisy-chained 6-pin DIN bus (up to 5 devices). The C64 is always the
controller; it commands devices to TALK (put data on the bus) or
LISTEN (receive). Each device has a bus address (4–31); common ones: 4/5
printer, 8 disk. Only one talker at a time; many listeners. Lines:
| Pin | Signal | |
|---|
| 1 | /SERIAL SRQ IN | device requests attention |
| 2 | GND | |
| 3 | SERIAL ATN OUT | C64 pulls low to start a command (all devices listen for the address) |
| 4 | SERIAL CLK IN/OUT | bit timing |
| 5 | SERIAL DATA IN/OUT | one bit at a time |
| 6 | /RESET | |
If an addressed device doesn't answer in time, you get a device-not-present
error in the status word. Disk and printer details: c64-disk, plus the
KERNAL talk/listen routines in c64-kernal.
The user port
A 12+12-pin edge connector wired straight to CIA2. It exposes CIA2 Port B
(PB0–PB7) as 8 general-purpose I/O lines plus FLAG2/PA2 handshake lines, and the
RS-232 signals (Sin/Sout/RTS/CTS/DSR/DCD) ride here too. From BASIC you POKE/PEEK
the port through CIA2:
| Register | Addr | Use |
|---|
| Port B (data) | 56577 / $DD01 | PEEK to read inputs, POKE to drive outputs |
| Port B DDR | 56579 / $DD03 | 1 bit = output, 0 bit = input |
| Port A | 56576 / $DD00 | PA2 is a handshake line |
| Port A DDR | 56578 / $DD02 | |
POKE 56579,255 : REM all 8 PB lines = outputs
POKE 56577,56 : REM drive bits 5,4,3 high (32+16+8)
LDA #$FF
STA $DD03 ; DDR: PB all outputs
LDA #56
STA $DD01 ; write the lines
For the CIA2 register set, timers, FLAG/PC handshaking, and the SP/CNT serial
lines also brought out here, see c64-cia.
The expansion (cartridge) port
A 44-pin edge connector exposing the full address bus, data bus, R/W, clocks,
/IRQ, /NMI, /DMA, and four decode lines:
- /ROML ($8000) and /ROMH ($E000): cartridge ROM banks.
- /I/O1 ($DE00–$DEFF) and /I/O2 ($DF00–$DFFF): cartridge I/O windows.
- /EXROM and /GAME: the two control inputs that tell the PLA how to bank
the cartridge in. Pulling these (with the cart ROM at $8000 containing the
autostart "CBM80" signature) makes a cartridge autostart on power-up.
- Total user-port + cartridge draw ≤ 450 mA; /DMA should only be pulled when φ2
is low and the device must respect VIC-II bus timing.
The exact RAM/ROM/IO banking that /EXROM and /GAME drive is in
c64-memory-map.
Z-80 cartridge (Commodore CP/M)
A cartridge that drops a Z-80 onto the expansion bus so the C64 can run CP/M
software. The 6510 and Z-80 share the buses but run one at a time. Z-80 addresses
are offset by +$1000 from 6510 addresses. The sample turn-on program first
disables 6510 IRQs (POKE 56333,127, the CIA1 ICR), then writes the cartridge
I/O location (POKE 56832,0 = $DE00, the /I/O1 window) to hand control to the
Z-80; the Z-80 self-disables to hand control back. See references/io-guide.md
for the full address-translation table and turn-on program.
Quick answers
- "What device number is the printer / screen / disk / RS-232?" Printer 4 (or
5), screen 3, disk 8, RS-232 2, tape 1, keyboard 0.
- "How do I open a printer channel and print?"
OPEN 4,4: PRINT#4,"TEXT": CLOSE 4.
- "How do I redirect LIST to the printer?"
OPEN 4,4: CMD 4: LIST: PRINT#4: CLOSE 4.
- "How many devices on the serial bus?" Up to 5, addresses 4–31.
How to read the references
references/io-guide.md — the verbatim Ch6 text: TV/printer/modem output,
the OPEN parameter table, full printer control-code table, the user port pin
list + DDR explanation, the serial bus (talk/listen, addresses, timing table),
the expansion port pinout, and the Z-80 cartridge. Read this for any pin,
control code, or bus-timing detail.references/rs232.md — everything RS-232: control/command register bit
maps (Figures 6-1/6-2), opening/getting/sending/closing a channel, the status
register bits (Figure 6-3), two complete sample terminal programs, the
receiver/transmitter buffer base pointers ($F7/$F9), and the zero-page +
$0293-area working storage. Read this when setting baud/parity or debugging
RS-232 timing/buffers.references/io-pinouts.md — App I connector pinouts (control ports,
cartridge slot, audio/video, serial I/O, cassette, user port) with the
physical connector diagrams. Read this when wiring a cable or identifying a pin.
Cross-links
- Disk drives (LOAD/SAVE, directory, command channel) → c64-disk
- Cassette tape (LOAD/SAVE/VERIFY, data files) → c64-tape
- Joysticks, paddles, light pen on the control ports → c64-game-ports
- The CIA chips behind the user port and serial bus → c64-cia
- KERNAL I/O routines (OPEN/CHKIN/CHKOUT/CHRIN/CHROUT/CLOSE) → c64-kernal
- RAM/ROM/IO banking driven by /EXROM and /GAME → c64-memory-map
