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INT 0x21 - 0x34 - Get Address to DOS Critical Flag

INDOS (Undocumented DOS 2.0+)


reg value descr
AH 0x34


reg descr
ES:BX address of a byte indicating whether a DOS call is in progress. No DOS calls should be made if set.

Notes & Comments

  • AKA the INDOS flag, this critical section flag may be checked from within an interrupt handler before requesting a DOS service. It is a semaphore that is non-zero when DOS is busy, and zero otherwise.
  • though this flag indicates whether a DOS interrupt is active, it should not be used alone to determine DOS is safe for re-entry; Here's the standard rule for safe DOS entry: if INT 28 is active or this flag and the critical error flag are clear then it is safe to call DOS
  • this interrupt should be used only during TSR initialization; the returned pointer should be used thereafter
  • this flag is cleared after a critical error (INT 24)
    • ES:BX-1 points to the critical error flag for DOS 3.x+
    • ES:BX+1 points to the critical error flag for DOS 2.x
    • ES:BX-1AA points to the critical error flag for COMPAQ DOS 3.0

INT 21,5D in DOS 3.x+ can be used to locate the critical error flag


back2root/ibm-pc-ms-dos/interrupts/int_21/int_21_34.txt · Dernière modification : 2024/04/08 08:23 de