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====== Int 31H, AH=0DH, AL=02H ======

===== Version =====

1.0

===== Brief =====

Serialize on Shared Memory

===== Input =====

  AX = 0D02H
  SI:DI = shared memory block handle
  DX = option flags

^ Bit ^ Significance ^
| 0 | 0 = suspend client until serialization available     1 = return immediately with error if serialization not available |
| 1 | 0 = exclusive serialization requested    1 = shared serialization requested |
| 2-15 | reserved, must be zero |

===== Return =====

  if function successful
  Carry flag = clear
  
  if function unsuccessful
  Carry flag = set
  AX = error code
  8004H	deadlock (host detected a deadlock situation)
  8005H	request cancelled with Int 31H Function 0D03H
  8017H	lock count exceeded
  8018H	exclusive serialization already owned by another client
  8019H	shared serialization already owned by another client
  8023H	invalid handle

===== Notes =====

Requests serialization of a shared memory block. Successful serialization symbolizes ownership or right of access to a block, and can be used by DPMI clients to synchronize the inspection or modification of a shared memory block.

For each client, the DPMI host maintains four different local (virtual machine) serialization counts (exclusive, shared, pending shared, and pending exclusive) for each shared memory block, as well as a global serialization count. The global serialization count is only updated when the sum of a virtual machine's exclusive and shared serialization counts goes from 0 to 1 (serialize) or 1 to 0 (free).

A successful exclusive serialization blocks any serialization request (exclusive or shared) for the same block by another virtual machine. Exclusive serialization should be regarded as "ownership for writing," and should only be requested if the client intends to modify the block. A successful shared serialization will only block requests for exclusive serialization by another client. Shared serialization can be thought of as "read-only access," and should used when the client only intends to inspect the block and will not change its contents.

Setting bit 0 of DX to 1 when the serialization request is made allows a client to determine whether a shared memory area is serialized without being suspended. Clients which "poll" for the availability of a resource in this manner are encouraged to yield the CPU with Int 2FH Function 1680H at appropriate intervals.

A serialization call that causes a client to be suspended can be canceled by a client interrupt service routine (such as a keyboard or timer interrupt handler) requesting the Free Serialization function (Int 31H Function 0D03H). In such cases, the original serialization request will return with the Carry flag set and AX = 8005H.

A client that has been suspended while waiting for serialization of a shared memory block can still service interrupts. Some hosts may need to reissue the serialization request on behalf of the client after the interrupt service routine returns, but this event will be invisible to the client.

Hosts are not required to detect deadlock. Clients that terminate and stay resident in order to function as resident service providers, executing in the context of other clients, must be careful to avoid deadlocks and incorrect sequencing in acquiring and releasing resources.

===== See also =====

   

===== Note =====

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

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