Error messages
BBC computer errors are an 8-bit number and a string.
+---------------------------+ | error number, 1 byte | +---------------------------+ | error string, 1-254 bytes | +---------------------------+ | &00 | +---------------------------+
Error numbers < 128 (ie, b7=0) are language errors, error numbers > 127 (ie b7=1) are system (ie MOS, filing system, utility, etc.) errors. As BASIC's errors are all <64 (ie b7=0, b6=0), any error numbers used within a program for its own purposes should be 64-127 (ie b7=0, b6=1).
While the text of an error message can vary between situations, the error number must be consistant. For example, error 214 (&D6) is File not found, the string can vary between File not found, Not found, XXXX not found and File 'XXXX' not found. However, the error number is consistantly 214.
On 32-bit platforms the error number is a 32-bit word, error numbers < 256 are the same as 8-bit error numbers.
Raising an error
6502
BRK EQUB err_num EQUS err_string$ EQUB 0
Z80
RST &38 DEFB err_num DEFM err_string$ DEFB 0
6809/6811/6812
SWI DEFB err_num DEFM err_string$ DEFB 0
80x86
INT &4F DEFB err_num DEFM err_string$ DEFB 0
PDP11
EMT 15 DEFB err_num DEFM err_string$ DEFB 0
Errors can also be returned to the caller:
ADR PDP11ErrorBlock,R0 ; Point R0 to error block SEV ; Set Overflow flag RTS PC ; Return to caller .PDP11ErrorBlock DEFB err_num DEFM err_string$ DEFB 0
32016
Errors cannot be generated within PanOS, but they can be returned to the caller:
ADDR PanErrorBlock,R0 ; Point R0 to error block MOV [R0],R1 ; R1=error number SEF ; Set error flag RET 0 ; Return to caller .PanErrorBlock EQUD err_num16bit ; error numbers < 256 are the same as 8-bit error numbers EQUB LEN err_string$ EQUS err_string$
ARM
ADR R0,ArmErrorBlock ; ARM error block is word, string, &00 SWI "OS_GenerateError" ; &2B .ArmErrorBlock EQUD err_num32bit ; error numbers < 256 are the same as 8-bit error numbers EQUS err_string$ EQUB 0
Errors can also be returned to the caller:
ADR R0,ArmErrorBlock ; Point R0 to error block CMPVC R0,#1<<31 ; These two instructions CMNVC R0,#1<<31 ; set the Overflow flag MOV PC,LINK ; Return to caller .ArmErrorBlock EQUD err_num32bit ; error numbers < 256 are the same as 8-bit error numbers EQUS err_string$ EQUB 0
RISC-V
ECALL &AC000F EQUD err_num32bit ; error numbers < 256 are the same as 8-bit error numbers EQUS err_string$ EQUB 0
BBC BASIC
In BASICs that support the ERROR command (BASIC V and some earlier versions):
ERROR err_num,err_string$
