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====== Int 31H, AH=01H, AL=00H ======

===== Version =====

0.9

===== Brief =====

Allocate DOS Memory Block

===== Input =====

  AX = 0100H
  BX = number of (16-byte) paragraphs desired
        
===== Return =====

  if function successful
  Carry flag = clear
  AX = real mode segment base address of allocated block
  DX = selector for allocated block
  
  if function unsuccessful
  Carry flag = set
  AX = error code
  0007H	memory control blocks damaged (also returned by DPMI 0.9 hosts)
  0008H	insufficient memory (also returned by DPMI 0.9 hosts).
  8011H	descriptor unavailable
  BX = size of largest available block in paragraphs

===== Notes =====

Allocates a block of memory from the DOS memory pool, i.e. memory below the 1 MB boundary that is controlled by DOS. Such memory blocks are typically used to exchange data with real mode programs, TSRs, or device drivers. The function returns both the real mode segment base address of the block and one or more descriptors that can be used by protected mode applications to access the block.

If the size of the block requested is greater than 64 KB (BX > 1000H) and the client is a 16-bit program, contiguous descriptors are allocated and the base selector is returned. The consecutive selectors for the memory block can be calculated using the value returned by the Get Selector Increment Value function (Int 31H Function 0003H). Each descriptor has a limit of 64 KB, except for the last which has a limit of blocksize MOD 64 KB.

If the DPMI host is 32-bit, the client is 16-bit, and more than one descriptor is allocated, the limit of the first descriptor will be set to the size of the entire block. Subsequent descriptors have limits as described in the previous Note. 16-bit DPMI hosts will always set the limit of the first descriptor to 64 KB even when running on an 80386 (or later) machine.

When the client is 32-bit, this function always allocates only one descriptor.

Client programs should never modify or free any descriptors allocated by this function. The Free DOS Memory Block function (Int 31H Function 0101H) will deallocate the descriptors automatically.

The DOS allocation function (Int 21H Function 48H) is used.

Refer to the rules for descriptor usage in Appendix D.
   
===== See also =====

   

===== Note =====

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

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