Int 24h - Critical Error Handler

invoked when a critical (usually hardware) error is encountered; should never be called directly

Critical Error Handler is invoked with those resgisters as parameters:

bit	Dos	type and processing flags
7		0 : disk I/O error 1 : if block device, bad FAT image in memory if char device, error code in DI
6		unused
5	3+	1 if Ignore allowed, 0 if not
4	3+	1 if Retry allowed, 0 if not
3	3+	1 if Fail allowed, 0 if not
2		disk area of error 00 = DOS area 01 = FAT 10 = root dir 11 = data area
1
0		1 if write, 0 if read

Drive number if AH bit 7 clear

Pointer to device driver header (BP:[SI+4] bit 15 set if char device)

DI value	Dos version	low byte contains error code if AH bit 7 set
00h		write-protection violation attempted
01h		unknown unit for driver
02h		drive not ready
03h		unknown command given to driver
04h		data error (bad CRC)
05h		bad device driver request structure length
06h		seek error
07h		unknown media type
08h		sector not found
09h		printer out of paper
0Ah		write fault
0Bh		read fault
0Ch		general failure
0Dh	3+	sharing violation
0Eh	3+	violation
0Fh		invalid disk change
10h	3+	FCB unavailable
11h	3+	sharing buffer overflow
12h	4+	code page mismatch
13h	4+	out of input
14h	4+	insufficient disk space

Handler must return:

AL	meaning
00h	ignore error and continue processing request
01h	retry operation
02h	terminate program through the equivalent of INT 21h,4Ch (INT 20h for DOS 1.x)
03h	fail system call in progress

SS,SP,DS,ES,BX,CX,DX preserved

Notes & Comments

the only DOS calls the handler may make are INT 21/AH=01h-0Ch,30h,59h
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 a critical error occurs inside the critical error handler, the DOS call is automatically failed (DOS 3+)