| AH | = | type and processing flags (see #02544) | ||
| AL | = | drive number if AH bit 7 clear | ||
| BP:SI |
 |
device driver header (see #01646 at INT 21/AH=52h) (BP:[SI+4] bit 15 set if character device) | ||
| DI | = | low byte contains error code if AH bit 7 set (see #02545) | ||
| STACK | DWORD | return address for INT 24 call | ||
| WORD | flags pushed by INT 24 | |||
| WORD | original AX on entry to INT 21 | |||
| WORD | BX | |||
| WORD | CX | |||
| WORD | DX | |||
| WORD | SI | |||
| WORD | DI | |||
| WORD | BP | |||
| WORD | DS | |||
| WORD | ES | |||
| DWORD | return address for INT 21 call | |||
| WORD | flags pushed by INT 21 | |||
| AL | = | action code (see #02546) |
| SS | - | preserved |
| SP | - | preserved |
| DS | - | preserved |
| ES | - | preserved |
| BX | - | preserved |
| CX | - | preserved |
| DX | - | preserved |
If the handler returns to the application by popping the stack, DOS will be in an unstable state until the first call with AH > 0Ch.
For DOS 3.1+, IGNORE (AL=00h) is turned into FAIL (AL=03h) on network critical errors.
If IGNORE specified but not allowed, it is turned into FAIL.
If RETRY specified but not allowed, it is turned into FAIL.
If FAIL specified but not allowed, it is turned into ABORT.
(DOS 3.0+) if a critical error occurs inside the critical error handler, the DOS call is automatically failed (AL set to 03h and the INT 24 call skipped).
The initial critical error handler is located in the kernel and always results in FAIL. However, by default it is replaced by the critical error handler in the command interpreter when it loads. This can be suppressed (e.g. for BBS systems) by loading the primary DOS 3.00+ COMMAND.COM shell with the undocumented option /F 'Fail'. This syntax is also supported with PTS/DOS 6.51+, S/DOS 1.0+, and DR-OpenDOS 7.02+ (1997-11-13) COMMAND.COM, as well as 4DOS.COM/NDOS.COM. OpenDOS 7.01+ COMMAND.COM (1997-03-21) introduced this under /N, which is still supported by newer releases (although it has a slightly different meaning now).