XMS35.txt

  1. XMS v3.5 API
  
   XMS v3.5 has been created to allow accessing extended memory beyond the 4 GB
  barrier. 

  To achieve this, the XMS v3.0 API has been extended:
  
  AH=0C8h: query free super-extended memory. Returns in EAX largest free block
           in kB, in EDX the total amount (in kB) of free super-extended memory.
           AX=0 indicates an error.
  
  AH=0C9h: allocate block of super-extended memory. Expects in EDX the
           requested amount of memory in kB. Returns AX=0 if an error occured,
           else AX is 1 and the handle of the block is in DX.

  AH=0CCh: lock a (super-extended) memory block. Expects handle in DX. Returns
           64-bit physical address of locked block in EDX:EBX. Returns AX=0
           if an error occured.

  XMS function 00 (Get Version) will return ax=0350h, that is version 3.50.


  2. XMS v3.51 API
  
  XMS v3.51 extends the v3.50 API by another function:

  AH=0CBh: super-extended block move function. Register setup is like XMS
           function 0Bh. However, the structure that holds source and
           destination of the memory block to copy has been extended by two
           1-byte fields that must be filled with bits 32-39 of the source
           and destination address.

           sxms_move struct
           len           dd  ?   ;  +0: block length in bytes
           src_handle    dw  ?   ;  +4: source handle
           src_offset    dd  ?   ;  +6: offset into source
           dst_handle    dw  ?   ; +10: destination handle
           dst_offset    dd  ?   ; +12: offset into destination
           src_high      db  ?   ; +16: NEW: bits 32-39 of source offset
           dest_high     db  ?   ; +17: NEW: bits 32-39 of destination offset
           sxms_move ends


  3. BIOS Interrupt 15h, AX=E820h

   Since the memory beyond the 4 GB limit must be managed exclusively,
  Int 15h, ax=E820h should be intercepted in a way that all memory blocks with
  addresses >= 100000000h are changed from "available" to "reserved".


  4. BIOS Interrupt 15h, AH=87h

   In V86 mode, the XMM's 'move extended memory' functions (AH=0Bh & AH=0CBh)
  will need the help of the Expanded Memory Manager (EMM), since privileged
  code has to be executed. The only EMMs that currently support accessing
  memory beyond 4 GB are Jemm386/JemmEx v5.80+. Their Int 15h API has been
  exhanced as well.
  
  Register setup for Int 15h, AH=87h: 
   
  - AH: 87h
  - EAX[bits 16-31]: F00Fh
  - CX: F00Fh
  - ECX[bits 16-31]: size of block in words
  - DS:SI: same as the standard ( pointing to a GDT ), descriptors 2 & 3
           defining address bits 0-31 of source/destination region.
  - DX: address bits 32-47 of the source region.
  - BX: address bits 32-47 of the destination region.

   If the call succeeded, the carry flag is cleared and register AH is 0.
   If an error occured ( for example, CPU doesn't support PSE ), the carry
   flag is set and AH is != 0.

  Japheth