Simulate Real Mode Interrupt [0.9]

Simulates an interrupt in real mode. The function transfers control to the address specified by the real mode interrupt vector. The real mode handler must return by executing an IRET.

Call With

AX = 0300H
BL = interrupt number
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, should be zero
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 (reserved, ignored)
2CH	2	CS (reserved, ignored)
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 is ignored by this function. The appropriate interrupt handler will be called based on the value passed in BL.

• 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.

• The flags specified in the real mode register data structure will be pushed on the real mode stack's IRET frame. The interrupt handler will be called with the interrupt and trace flags clear.

• 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 handler 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 return with an IRET.

• When this function returns, the real mode register data structure will contain the values that were returned by the real mode interrupt handler.

• It is the caller's responsibility to remove any parameters that were pushed on the protected mode stack.

Copyright © 1996 Updated Apr 1996