DOS/32 Advanced DOS Extender - Programmer's Reference

2.26 - DPMI function 0303h - Allocate Real Mode Callback Address

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.


In: AX = 0303h
DS:ESI = selector:offset of protected mode procedure to call
ES:EDI = selector:offset of 32h-byte buffer for real mode register data structure to be used when calling callback routine.

if successful:
CF clear
CX:DX = segment:offset of real mode callback

if failed:
CF set
AX = error code

8015h - callback unavailable


a) A descriptor may be allocated for each callback to hold the real mode SS descriptor. Real mode callbacks are a limited system resource. A client should use the Free Real Mode Callback Address DPMI function 0304h to release a callback that it is no longer using.

b) The contents of the real mode register data structure is not valid after the function call, but only at the time of the actual callback.


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