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name: disk-io-1541 description: > Use this skill for all 1541 disk drive programming on the Commodore 64. Covers DOS commands, file types (sequential, relative, program), direct access, error handling, the BAM, directory structure, and disk utilities. Sources: The Anatomy of the 1541 Disk Drive (Revised Edition), COMPUTE!‘s Mapping the C64.
1541 Disk Drive Programming
Overview
The VIC-1541 disk drive is a single-sided, single-density (SS/SD) 5.25” floppy drive connected to the C64 via the serial bus (IEC bus). It contains its own 6502 CPU and 2KB of RAM.
- Device number: Default 8 (configurable 8-11)
- Disk capacity: 170KB (664 blocks free on a formatted disk)
- Tracks: 35 (numbered 1-35)
- Sectors per track: 17-21 (varies by zone)
- Block size: 254 usable bytes per sector (2 bytes used as track/sector link)
- Communication: Via Kernal's serial bus routines
Track/Sector Layout
| Tracks | Sectors/Track | Cumulative Sectors |
|---|---|---|
| 1-17 | 21 | 0-356 |
| 18-24 | 19 | 357-490 |
| 25-30 | 18 | 491-598 |
| 31-35 | 17 | 599-682 |
- Directory track: Track 18 (always)
- BAM: Track 18, Sector 0
Logical File Model
Every disk operation goes through logical files. The steps are:
SETLFS— assign logical file number, device, secondary addressSETNAM— assign filenameOPEN— open the fileCHKIN/CHKOUT— redirect input/output to the fileBASIN/BSOUT— read/write dataCLOSE— close the file
From BASIC, use: OPEN LFN, DEV, SA, "FILENAME"
File Types
| Type | SA Range | Description |
|---|---|---|
| Program | 0-1 | Machine language or BASIC programs |
| Sequential | 2 | Sequential data files (read/write, not both at once) |
| Relative | any+SA | Fixed-length record random-access files |
| Command | 15 | Command channel (send DOS commands, read error) |
BASIC Disk Operations
Load and Save Programs
LOAD "FILENAME",8 ' Load BASIC program
LOAD "FILENAME",8,1 ' Load at original address (ML programs)
SAVE "FILENAME",8 ' Save BASIC program
SAVE "@:FILENAME",8 ' Save and overwrite existing file
VERIFY "FILENAME",8 ' Verify saved file matches memory
Directory
LOAD "$",8 ' Load directory as BASIC program
LIST ' Display directory
Sending DOS Commands
OPEN 15,8,15,"COMMAND" ' Open command channel with command
CLOSE 15 ' Close command channel
Reading the Error Channel
10 OPEN 15,8,15 : REM Open command channel
20 INPUT#15,EN,EM$,ET,ES : REM Read error: number,message,track,sector
30 PRINT EN;EM$;ET;ES
40 CLOSE 15
The error channel should be read after every disk operation.
DOS Commands
Formatting
OPEN 15,8,15,"N0:DISKNAME,ID" : CLOSE 15
' N0: = NEW on drive 0; ID = 2-char disk ID (e.g., "A1")
' WARNING: Destroys all data on disk
Scratching (Deleting) Files
OPEN 15,8,15,"S0:FILENAME" : CLOSE 15
' Wildcards supported: S0:TEST* deletes all files starting with TEST
Renaming Files
OPEN 15,8,15,"R0:NEWNAME=OLDNAME" : CLOSE 15
Copying Files
OPEN 15,8,15,"C0:DEST=0:SOURCE" : CLOSE 15
Validating Disk (clean up)
OPEN 15,8,15,"V0:" : CLOSE 15
' Reclaims space from improperly closed files
Initializing Disk
OPEN 15,8,15,"I0:" : CLOSE 15
' Re-reads BAM; use after swapping disks
Wildcards
*— matches any sequence of characters?— matches any single character
OPEN 15,8,15,"S0:TEST*" : CLOSE 15 ' scratch all TEST files
Sequential File Access
Sequential files are read or written in order only (not random access).
Writing a Sequential File
10 OPEN 1,8,2,"MYDATA,S,W" ' LFN=1, DEV=8, SA=2, write sequential
20 PRINT#1,"LINE 1 DATA"
30 PRINT#1,"LINE 2 DATA"
40 PRINT#1,NUMBER ' Numbers also work
50 CLOSE 1
Reading a Sequential File
10 OPEN 1,8,2,"MYDATA,S,R" ' SA=2, read sequential
20 IF ST<>0 THEN 60 ' ST = status byte; non-zero = EOF/error
30 INPUT#1,LINE$
40 PRINT LINE$
50 GOTO 20
60 CLOSE 1
Appending to a Sequential File
OPEN 1,8,2,"MYDATA,S,A" ' SA=2 with ,A flag appends
Relative File Access
Relative files provide random access to fixed-length records.
Creating a Relative File
10 OPEN 2,8,2,"MYREL,L," + CHR$(RECLEN)
' RECLEN = record length in bytes (1-254)
Positioning the Record Pointer
20 OPEN 15,8,15
30 RN = 5 ' desired record number (1-based)
40 LO = (RN-1) AND 255
50 HI = INT((RN-1)/256)
60 PRINT#15,"P" CHR$(2) CHR$(LO) CHR$(HI) CHR$(0)
' Channel 2 (the data file's SA), then record# low, high, byte offset
Reading/Writing Records
' After positioning:
PRINT#2, FIELD1$, FIELD2$ ' write
INPUT#2, FIELD1$, FIELD2$ ' read (re-position first)
Error Codes
| Code | Message | Meaning |
|---|---|---|
| 00 | OK | No error |
| 01 | FILES SCRATCHED | Normal after SCRATCH |
| 20 | READ ERROR | Block header not found |
| 21 | READ ERROR | No sync character found |
| 22 | READ ERROR | Data block not found |
| 23 | READ ERROR | Checksum error in data |
| 24 | READ ERROR | Byte decoding error |
| 25 | WRITE ERROR | Write verify error |
| 26 | WRITE PROTECT ON | Write-protect tab covered |
| 27 | READ ERROR | Checksum error in header |
| 28 | WRITE ERROR | Long data block |
| 29 | DISK ID MISMATCH | Wrong disk in drive |
| 30 | SYNTAX ERROR | DOS command syntax error |
| 31 | SYNTAX ERROR | Invalid command |
| 32 | SYNTAX ERROR | Long line |
| 33 | SYNTAX ERROR | Invalid filename |
| 34 | SYNTAX ERROR | No file given |
| 39 | SYNTAX ERROR | Invalid command |
| 50 | RECORD NOT PRESENT | Past end of relative file |
| 51 | OVERFLOW IN RECORD | Wrote past end of record |
| 52 | FILE TOO LARGE | Relative file too large |
| 60 | WRITE FILE OPEN | File still open for writing |
| 61 | FILE NOT OPEN | File not open |
| 62 | FILE NOT FOUND | File does not exist |
| 63 | FILE EXISTS | File already exists (overwrite with @) |
| 64 | FILE TYPE MISMATCH | Wrong file type for operation |
| 65 | NO BLOCK | Block already allocated |
| 66 | ILLEGAL TRACK OR SECTOR | Bad T/S reference |
| 67 | ILLEGAL SYSTEM T OR S | Bad system track/sector |
| 70 | NO CHANNEL | No free logical channel |
| 71 | DIR ERROR | Directory error (use VALIDATE) |
| 72 | DISK FULL | No free blocks |
| 73 | CBM DOS Vxx.x | Power-on/INITIALIZE message |
| 74 | DRIVE NOT READY | No disk or drive error |
Direct Block Access
The 1541 allows reading/writing individual sectors directly.
Block-Read
OPEN 15,8,15
OPEN 2,8,2,"#" ' open direct-access channel
PRINT#15,"B-R:2 0 " TRACK " " SECTOR
' Read into drive buffer; then read bytes via GETIN or INPUT#
Block-Write
' Write buffer to disk after filling via BSOUT:
PRINT#15,"B-W:2 0 " TRACK " " SECTOR
Block-Pointer (set position within buffer)
PRINT#15,"B-P:2 " POS ' set byte position in buffer (0-255)
Directory Structure on Disk
Track 18, Sector 0 = BAM (Block Availability Map)
BAM format:
- Bytes 0-1: Track/sector of first directory block (18/1)
- Byte 2: DOS version (‘A’)
- Byte 3: unused
- Bytes 4-143: BAM entries for each track (4 bytes per track × 35 tracks):
- Byte 0: free sector count for this track
- Bytes 1-3: bit map of free sectors (1=free, 0=used)
- Bytes 144-161: Disk name (padded with $A0)
- Bytes 162-163: Disk ID
- Byte 164: $A0
- Bytes 165-166: DOS version bytes (“2A”)
Track 18, Sectors 1-18 = Directory
Each directory sector holds 8 directory entries (32 bytes each):
- Byte 0: Track link to next directory sector (0 if none)
- Byte 1: Sector link to next directory sector
- For each of 8 entries starting at byte 2 (or 0 for entry 0):
- Byte 0: File type ($80=DEL, $81=SEQ, $82=PRG, $83=USR, $84=REL; OR $40=locked, $60=opened)
- Bytes 1-2: Starting track/sector
- Bytes 3-18: Filename (padded with $A0)
- Bytes 19-20: Side-sector block track/sector (relative files only)
- Byte 21: Record length (relative files only)
- Bytes 22-25: Unused
- Bytes 26-27: Track/sector of replacement file (during @-SAVE)
- Bytes 28-29: File size in blocks (low, high)
Assembly Language Disk I/O
; Open logical file 1 to disk drive 8, secondary address 2, filename "DATA"
LDA #1 ; logical file number
LDX #8 ; device (disk)
LDY #2 ; secondary address
JSR $FFBA ; SETLFS
LDA #4 ; filename length
LDX #<FNAME
LDY #>FNAME
JSR $FFBD ; SETNAM
JSR $FFC0 ; OPEN
BCS error ; carry set = error
LDX #1 ; logical file number
JSR $FFC9 ; CHKOUT - redirect output to file
LDA #65 ; character 'A'
JSR $FFD2 ; BSOUT - write to file
JSR $FFCC ; CLRCH - restore default channels
LDA #1
JSR $FFC3 ; CLOSE
FNAME .TEXT "DATA"