Error Messages and Error Numbers

Every error reported on a BBC machine has an error number (1-byte value in &FD) and an error message (null-terminated string immediately after the BRK opcode that raised it). The error structure is documented in brk.

ERR in BASIC returns the error number. REPORT in BASIC prints the error message. The MOS error-string lookup is by convention only — a paged-ROM can raise any error number with any message, and many do.

BASIC error numbers (0-44)

These are the canonical Acorn BBC BASIC IV errors as shipped on every BBC machine. Per bbc-user-guide Ch 46:

Num	Hex	Message	Cause
0	&00	No room	Untrappable — all memory used
1	&01	Out of range	Branch out of range in assembler
2	&02	Byte	Assembler immediate operand > 255 (e.g. LDA #345)
3	&03	Index	Bad assembler index mode
4	&04	Mistake	Generic “couldn’t make sense of input”
5	&05	Missing ,	Expected comma
6	&06	Type mismatch	Number expected, string offered (or vice-versa)
7	&07	No FN	=FNname outside function body
8	&08	$ range	Attempt to use $ indirection at zero page
9	&09	Missing "	Unterminated string
10	&0A	Bad DIM	DIM A(-3) etc.
11	&0B	DIM space	Insufficient memory for array
12	&0C	Not LOCAL	LOCAL outside procedure/function
13	&0D	No PROC	ENDPROC without DEF PROC
14	&0E	Array	Reference to undeclared array
15	&0F	Subscript	Array index out of bounds
16	&10	Syntax error	Mis-terminated command
17	&11	Escape	ESCAPE key pressed
18	&12	Division by zero	34/0 etc.
19	&13	String too long	>255 chars
20	&14	Too big	Numeric overflow
21	&15	-ve root	SQR(-x), ASN(>1), ACS(>1)
22	&16	Log range	LOG(<=0)
23	&17	Accuracy lost	Trig of very large angle (e.g. SIN(1E7))
24	&18	Exp range	EXP(>88)
25	&19	Bad MODE	Mode change inside PROC/FN or insufficient memory
26	&1A	No such variable	Variable used before assignment
27	&1B	Missing )	Unbalanced bracket
28	&1C	Bad hex	&y etc. (invalid hex digit)
29	&1D	No such FN/PROC	Undefined FN/PROC call
30	&1E	Bad call	PROC/FN called incorrectly
31	&1F	Arguments	Wrong number of args
32	&20	No FOR	NEXT without FOR
33	&21	Can't match FOR	NEXT doesn’t match FOR variable
34	&22	FOR variable	Non-numeric FOR variable
35	&23	Too many FORs	Nesting >10
36	&24	No TO	FOR X=1 (missing TO)
37	&25	Too many GOSUBs	Nesting >26
38	&26	No GOSUB	RETURN without GOSUB
39	&27	ON syntax	Malformed ON … GOTO/GOSUB
40	&28	ON range	Control var <1 or >list length
41	&29	No such line	GOTO/GOSUB to non-existent line
42	&2A	Out of DATA	More READs than DATA
43	&2B	No REPEAT	UNTIL without REPEAT
44	&2C	Too many REPEATs	Nesting >20

Untrappable errors (No room, Bad program, LINE space, Silly, Failed at N, etc.) have no number — they exit to the BASIC prompt directly and can’t be caught by ON ERROR.

Cassette/CFS filing-system errors (216-223)

Raised by the cassette filing system when reading from tape:

Num	Message	Cause
216	Data?	CRC error in file body — rewind, replay
217	Header?	CRC error in file header — rewind, replay
218	Block?	Unexpected block number — rewind, replay
219	File?	Unexpected filename in header
220	Syntax	Bad *OPT or similar
222	Channel	Use of unopened channel
223	Eof	End of file reached

MOS / sideways-ROM errors (250+)

Defined by the MOS / paged-ROM dispatch layer:

Num	Message	Where raised
251	Bad key	*KEY syntax error
253	Bad string	* command string parse error
254	Bad command	No paged ROM recognised the * command

DFS / ADFS / NFS errors (varies)

Filing-system specific. Each filing-system ROM defines its own. Some common ones:

Num	Message	Typical FS
198	Disc fault	DFS / ADFS
199	Disc full	DFS / ADFS
200	Disc changed	DFS / ADFS
201	File locked	DFS / ADFS
202	Not open	DFS / ADFS / NFS
203	Not enabled	DFS / NFS
204	Bad name	DFS / ADFS / NFS
205	Bad drive	DFS / ADFS
206	Bad directory	DFS / ADFS
207	Bad attribute	DFS / ADFS
208	File exists	DFS / ADFS
209	Drive fault	DFS / ADFS
210	Disc read only	DFS / ADFS
211	Not found	DFS / ADFS / NFS
212	Channel	DFS / ADFS
213	End of file	DFS / ADFS
214	File not found	DFS / ADFS / NFS

These are not authoritative; cross-check against *HELP output from the active filing system on real hardware for the exact range. Each filing system has its own error-number convention; conflicts exist between e.g. DFS 200 and NFS 200.

Raising errors from user code

Use the BRK + structure pattern (per brk):

BRK              ; trigger MOS error handler
EQUB error_num   ; error number
EQUS "Message"   ; null-terminated error string
EQUB 0

The MOS sets up &FD (pointer to message), &FE (return PC after BRK), &F0 (saved SP), then calls BRKV (&202). Default BRKV in BASIC prints the message and exits to prompt.

To intercept an error from your own code, hook BRKV before the operation that might fail:

LDA #<my_handler  : STA &202
LDA #>my_handler  : STA &203
; ... do potentially-failing work ...

Restore &202/&203 to the old values before returning. The standard pattern is to save them on entry and restore on RTS.

Compatibility notes

• Error numbers 1-44 are stable across BBC BASIC I, II, III, IV (Model B → Master Compact).
• Error number 0 (No room) is documented as untrappable on Model B per bbc-user-guide Ch 46 (“untrappable error”); behaviour on newer BASIC versions is undocumented in primary sources I’ve ingested — assume untrappable unless you can verify otherwise on the target machine.
• Cassette errors 216-223 only fire on machines that have CFS available (Master 128 has CFS as a ROM image; Master Compact has no CFS at all).
• DFS / ADFS / NFS errors are per-ROM — what error 200 means depends on which filing system is active.
• The Silly and Failed at N errors (from RENUMBER, AUTO) have no numbers and are never ERR-readable.

See also

• brk — BRK structure, BRKV protocol, ROM-raising-errors pattern.
• calls — OSCLI / OSFILE / OSBYTE error-return conventions.
• bbc-user-guide — Ch 46 (canonical BASIC error list).
• zero-page — &FD/&FE (error pointer), &F0 (saved SP).

---

This wiki is curated by Claude following the LLM-Wiki methodology — a human curates source documents, the LLM compiles structured cross-linked markdown. Content may contain errors, omissions, or stale claims. For authoritative information refer to the original source documents in the bbc-documents GitHub archive.