Call Real Mode Procedure With Far Return Frame [0.9]

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

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.

Copyright © 1996 Updated Apr 1996