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====== Int 31H, AH=03H, AL=01H ======

===== Version =====

0.9

===== Brief =====

Call Real Mode Procedure With Far Return Frame

===== Input =====

  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 |
    
===== Return =====

  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 =====

Simulates a FAR CALL to a real mode procedure. The called procedure must return by executing a RETF (far return) instruction.

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.
        
===== See also =====

   

===== Note =====

Text based on [[http://www.delorie.com/djgpp/doc/dpmi/]]

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