Online HelpPC 2.10 Quick Reference Utility Copyright 1991 David Jurgens
INT 24 - Critical Error Handler
no input data
on entry to INT 24:
AH = bit 7 of register AH is set to one if other than disk error
= bit 7 of register AH is set to zero if disk error
AL = failing drive number
AH = bits 0-2 indicate affected disk area and read/write status
│5│4│3│2│1│0│ AH
│ │ │ │ │ └──── read = 0, write = 1
│ │ │ └─┴───── 00=DOS, 01=FAT, 10=DIR, 11=data area
│ │ └──────── 0=FAIL not allowed, 1=FAIL allowed
│ └───────── 0=RETRY not allowed, 1=RETRY allowed
└────────── 0=IGNORE not allowed, 1=IGNORE allowed
DI = error code in lower byte
BP:SI = pointer to device header control block where additional
information about the error can be found
on exit:
AL = action code determining exit behavior
= 00 ignore error
= 01 retry
= 02 terminate through INT 23
= 03 fail system call (DOS 3.x+)
Error Codes in low order byte of DI:
00 write protect error
01 unknown unit
02 drive not ready
03 unknown command
04 data error (bad CRC)
05 bad request structure length
06 seek error
07 unknown media type
08 sector not found
09 printer out of paper
0A write fault
0B read fault
0C general failure
Users Stack at Entry to Error Handler:
(top of stack)
IP DOS code next instruction pointer
CS
FLAGS DOS's flags
AX -\
BX \
CX \
DX \
SI |- User's registers at entry to INT 21 function
DI /
BP /
DS /
ES -/
IP User code next instruction pointer
CS
FLAGS
- on entry registers are setup for a retry operation
- user routine must issue an IRET or simulate an IRET
- INT 21,0 through INT 21,C and INT 21,59 can safely be invoked
from the handler. Other calls may destroy DOS's stack
- handler must preserve register SS,SP,DS,ES,BX,CX,DX
- choosing ignore can show side effects, since it causes DOS
to continue as if it the call were successful
- if an improper action code is specified in DOS 3.x it is changed:
if IGNORE is invalidly specified, action is converted to FAIL
if RETRY is invalidly specified, action is converted to FAIL
if FAIL is invalidly specified, action is converted to ABORT
- IGNORE requests are converted to FAIL for FAT and DIR disk
errors and network critical errors
- if the user routine wishes to handle the error instead of passing
it to DOS, it must restore the user program registers from the
stack and remove all but the last 3 words from the stack (FLAGS,
CS, IP) and issue an IRET
- do not execute this interrupt directly
- INDOS flag is cleared on INT 24 (see INT 21,34)
[helppc.bosbyte.nl] critical error handler