A new Multiplex Interrupt (INT 2Fh) is issued by OS/2 to signal either of the following two events:

• Switching the DOS application to the background (AX = 4001H)
  
• Switching the DOS application to the foreground (AX = 4002H)

A DOS application that receives this signal must hook the Multiplex Interrupt vector. That is, when the application is started, it must save the current INT 2Fh vector and set this vector to point to the application's interrupt handler.

When the notification is received, the application must save all registers, perform whatever processing is required, restore all registers, and issue the IRET instruction to return to the operating system. Only the following forms of processing are supported:

• Modifying application or video memory (or both) 
  Issuing ROM BIOS video service calls (INT 10h)
  
  Issuing EGA Register Interface calls (INT 10h)
  
  Programming the EGA video card directly

  Note: When an application is being switched to the background, and the application's INT 2Fh handler uses the EGA Register Interface to save the EGA registers, these registers are restored automatically when the application is returned to the foreground.

  An application can receive notification that it is being switched to the background at any time. Code sequences that are sensitive to interruption can be protected with CLI/STI instructions. When the switch notification occurs, the application (other than its INT 2Fh handler) is frozen until it is returned to the foreground.

  When an application's INT 2Fh handler receives notification with a value in AH other than 40H, the application must issue the JMP FAR instruction to branch to the previous INT 2Fh vector.

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