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Commodore DOS Quick Reference

commodorec641541reference

This is the Commodore DOS reference I actually use. Not exhaustive — that’s what the official 1541 manuals are for — just the commands that come up over and over again when you’re working with a 1541 from a C64 BASIC prompt.

All the examples below assume the drive is device 8 on the IEC bus. If your drive is set to device 9, substitute 9 wherever you see 8.

Loading and saving

LOAD "TEST",8           : REM load the file TEST from disk
LOAD "$",8              : REM load the directory
SAVE "TEST",8           : REM save the current program as TEST
SAVE "@0:TEST",8        : REM save and overwrite an existing TEST

The @0: prefix on SAVE is the overwrite-if-exists modifier. Without it, the drive refuses to write over an existing file. The 0 is the drive number — always 0 on a 1541, which is single-drive.

Gotcha: LOAD "$",8 reads the directory into BASIC’s program memory, which means it overwrites whatever program you have in there. If you typed something you wanted to keep, save it first. After the directory loads, LIST to see it, then NEW (or just power-cycle) before you keep coding — the directory listing isn’t a runnable program and you don’t want it underfoot.

The command channel

Anything that isn’t a load or save goes through the command channel, file 15. Open it once, send commands, close it when you’re done.

OPEN 15,8,15
PRINT#15, "command goes here"
CLOSE 15

The three arguments to OPEN are:

  • 15 — the file number you’ll use locally (any number, but 15 is the convention)
  • 8 — the device number on the IEC bus
  • 15 — the secondary address, which is the drive’s command channel

You can also pass a one-shot command as an optional fourth argument: OPEN 15,8,15,"command".

Format a new disk

OPEN 15,8,15
PRINT#15, "NEW0:MYDISK,ID"
CLOSE 15
  • NEW — the format command
  • 0 — the drive number (always 0 on a 1541)
  • MYDISK — the disk name (16 characters max)
  • ID — a two-character ID written to every sector

Delete a file

OPEN 15,8,15
PRINT#15, "SCRATCH0:FILENAME"
CLOSE 15
  • SCRATCH — the delete command
  • 0 — the drive number
  • FILENAME — the file to delete

Reset the drive after a command failure

OPEN 15,8,15
PRINT#15, "INITIALIZE"
CLOSE 15

INITIALIZE resets the drive after a failed command. If the drive is sitting on a flashing red error LED, this is the first thing to try — the drive holds the error state until you read the error channel or initialize it.

Read the error channel

Same command-channel file (15), but read it instead of write to it. The drive replies with four fields: error code, error message, track, sector.

OPEN 15,8,15
INPUT#15, A, B$, C, D
PRINT A; B$; C; D
CLOSE 15

After a clean operation you’ll get 00 OK 00 00. After a failure you’ll get the specific code and message, plus the track and sector where it happened (zeros if not applicable). Reading the error channel also clears the error LED — same effect as INITIALIZE for that purpose, so on a flashing 1541 you can either reset it or just read the error and learn what went wrong at the same time.

Common error codes

The handful you’ll actually see day-to-day on a 1541:

  • 00 OK — no error.
  • 21 READ ERROR — no sync mark on the track. Usually a bad disk, sometimes a head-alignment problem.
  • 22 READ ERROR — data block not present where the directory says it should be.
  • 23 READ ERROR — checksum error in a data block. The block is there but corrupted.
  • 26 WRITE PROTECT ON — the write-protect notch is covered (or there’s no notch). Cover it with tape and try again.
  • 29 DISK ID MISMATCH — you swapped disks without an INITIALIZE. Send INITIALIZE and it clears.
  • 62 FILE NOT FOUND — the filename you asked for isn’t on the disk.
  • 63 FILE EXISTS — you tried to SAVE without the @0: overwrite prefix and there’s already a file by that name.
  • 73 CBM DOS V2.6 1541 — the power-up banner, not an error. If you see this on any other operation, you’ve got a 1571 / 1581 / second drive answering with its own banner.
  • 74 DRIVE NOT READY — no disk in the drive, or the disk isn’t seated.

A complete list lives in the 1541 User’s Guide appendix and in the service manuals.

Change the device number temporarily

The 1541’s device number is set in hardware by solder pads on the controller board, but you can override it in software. Send a memory-write to the drive’s zero page:

OPEN 15,8,15
PRINT#15, "M-W"+CHR$(119)+CHR$(0)+CHR$(2)+CHR$(N+32)+CHR$(N+64)
CLOSE 15

Where N is the desired device number (8, 9, 10, or 11). For example, N=9 sends bytes 41 and 73.

What this does: the two bytes at zero-page addresses 119 ($77) and 120 ($78) hold the LISTEN and TALK addresses the drive responds to on the IEC bus, which default to device+32 and device+64. Overwriting them with N+32 and N+64 makes the drive answer to a different IEC address, so to the rest of the bus it looks like a different device number.

The change lives in the drive’s RAM only. Power the drive off and the hardware-set number takes over again — meaning you have to re-run the one-liner on every cold boot. That’s why nobody really used this as the actual answer for running two drives on the same chain; the trace-cut hardware mod was the practical standard.