Int 31H Function 0303H

Allocate Real Mode Callback Address [0.9]

Returns a unique real mode segment:offset, known as a "real mode callback," that will transfer control from real mode to a protected mode procedure. Callback addresses obtained with this function can be passed by a protected mode program to a real mode application, interrupt handler, device driver, or TSR, so that the real mode program can call procedures within the protected mode program or notify the protected mode program of an event.

Call With

AX = 0303H
DS:(E)SI = selector:offset of protected mode procedure to call
ES:(E)DI = selector:offset of 32H-byte buffer for real mode register data structure to be used when calling callback routine.


if function successful
Carry flag = clear
CX:DX = segment:offset of real mode callback

if function unsuccessful
Carry flag = set
AX = error code
8015Hcallback unavailable


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