The machine interrupt vector for Int 24H (memory locations 0000:0090H through 0000:0093H) contains the address of the routine that receives control when a critical error (usually a hardware error) is detected. This address is also copied into locations 12H through 15H of the program segment prefix (PSP) when a program is loaded but before it begins executing, and is restored from the PSP (in case it was modified by the application) as part of MS-DOS's termination handling.
This interrupt should never be issued directly.
Notes:
On entry to the critical-error interrupt handler, bit 7 of register AH is clear (0) if the error was a disk I/O error; otherwise, it is set (1). BP:SI contains the address of a device-driver header from which additional information can be obtained. Interrupts are disabled. The registers will be set up for a retry operation, and an error code will be in the lower half of the DI register, with the upper half undefined.
The lower byte of DI contains:
00H write-protect error 01H unknown unit 02H drive not ready 03H unknown command 04H data error (CRC) 05H bad 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 reserved 0EH reserved 0FH invalid disk change (MS-DOS version 3 only)
Note that these are the same error codes returned by the device driver in the request header. Also, upon entry, the stack is set up as shown in Figure 8-8, page 149.
When a disk I/O error occurs, MS-DOS automatically retries the operation before issuing a critical-error Int 24H. The number of retries varies in different versions of MS-DOS, but is typically in the range three to five.
Int 24H handlers must preserve the SS, SP, DS, ES, BX, CX, and DX registers. Only Int 21H Functions 01H—0CH and 59H can be used by an Int 24H handler; other function calls will destroy the MS-DOS stack and its ability to retry or ignore an error.
When the Int 24H handler issues an IRET, it should return an action code in AL that will be interpreted by DOS as follows:
0 ignore the error 1 retry the operation 2 terminate the program 3 [3.0+] fail the function call in progress
If the Int 24H handler returns control directly to the application program rather than to MS-DOS, it must restore the program's registers, removing all but the last three words from the stack, and issue an IRET. Control returns to the instruction immediately following the function call that caused the error. This option leaves MS-DOS in an unstable state until a call to an Int 21H function higher than Function 0CH is made.
Example:
See Chapter 8.