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Int 31H Function 0301H
Call Real Mode Procedure With Far Return Frame
![[0.9]](../0.9.gif)
Simulates a FAR CALL to a real mode procedure. The called
procedure must return by executing a RETF (far return)
instruction.
Call With
AX = 0301H
BH = flags
| Bit | Significance |
|---|
| 0 | reserved for historical reason, must be zero |
| 1-7 | reserved, must be zero |
CX = number of words to copy from protected mode to
real mode stack
ES:(E)DI = selector:offset of real mode register data
structure in the following format:
| Offset | Length | Contents |
|---|
| 00H | 4 | DI or EDI |
| 04H | 4 | SI or ESI |
| 08H | 4 | BP or EBP |
| 0CH | 4 | reserved, ignored |
| 10H | 4 | BX or EBX |
| 14H | 4 | DX or EDX |
| 18H | 4 | CX or ECX |
| 1CH | 4 | AX or EAX |
| 20H | 2 | CPU status flags |
| 22H | 2 | ES |
| 24H | 2 | DS |
| 26H | 2 | FS |
| 28H | 2 | GS |
| 2AH | 2 | IP |
| 2CH | 2 | CS |
| 2EH | 2 | SP |
| 30H | 2 | SS |
Returns
if function successful
Carry flag = clear
ES:(E)DI = selector:offset of modified real mode register
data structure
if function unsuccessful
Carry flag = set
AX = error code
| 8012H | linear memory unavailable (stack) |
| 8013H | physical memory unavailable (stack) |
| 8014H | backing store unavailable (stack) |
| 8021H | invalid value (CX too large) |
Notes
- 32-bit programs must use ES:EDI to point to the real mode
register data structure. 16-bit programs should use
ES:DI.
- The CS:IP in the real mode register data structure
specifies the address of the real mode procedure to call.
- If the SS:SP fields in the real mode register data
structure are zero, a real mode stack will be provided by
the DPMI host. Otherwise, the real mode SS:SP will be
set to the specified values before the interrupt handler
is called.
- Values placed in the segment register positions of the
data structure must be valid for real mode; i.e. the
values must be paragraph addresses and not selectors.
- All general register fields in the data structure are
DWORDs so that 32-bit registers can be passed to real
mode. Note, however, that 16-bit hosts are not required
to pass the high word of 32-bit general registers or the
FS and GS segment registers to real mode even when
running on an 80386 or later CPU.
- The target real mode procedure must return with the stack
in the same state as when it was called. This means that
the real mode code may switch stacks while it is running,
but must return on the same stack that it was called on
and must exit with a RETF (far return) and should not
clear the stack of any other parameters that were passed
to it on the stack.
- When this function returns, the real mode register data
structure will contain the values that were returned by
the real mode procedure.
- It is the caller's responsibility to remove any
parameters that were pushed on the protected mode stack.