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Int 31H Function 0303H
Allocate Real Mode Callback Address
![[0.9]](../0.9.gif)
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.
Returns
if function successful
Carry flag = clear
CX:DX = segment:offset of real mode callback
if function unsuccessful
Carry flag = set
AX = error code
| 8015H | callback unavailable |
Notes
- DPMI hosts must provide a minimum of 16 callback
addresses per client.
- 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 function (Int 31H Function 0304H) to release a callback
that it is no longer using.
- For further information on writing real mode callback procedures,
see that page.
- 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.