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~)