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Int 31H Function 0300H
Simulate Real Mode Interrupt
![[0.9]](../0.9.gif)
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
| 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 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.