Int 21H, AH=50H

2 and higher

SET CURRENT PROCESS ID (SET PSP ADDRESS)

AH = 50h
BX = segment of PSP for new process

DOS uses the current PSP address to determine which processes own files and memory; it corresponds to process identifiers used by other OSs

under DOS 2.x, this function cannot be invoked inside an INT 28h handler without setting the Critical Error flag

under MS-DOS 3.0+ and DR DOS 3.41+, this function does not use any of the DOS-internal stacks and may thus be called at any time, even during another INT 21h call

some Microsoft applications such as Quick C 2.51 use segments of 0000h and FFFFh and direct access to the SDA (see #01687 at AX=5D06h) to test whether they are running under MS-DOS rather than a compatible OS; although one should only call this function with valid PSP addresses, any program hooking it should be prepared to handle invalid addresses

this function is supported by the OS/2 compatibility box

this call was undocumented prior to the release of DOS 5.0

AH=26h,AH=51h,AH=62h

Text based on Ralf Brown Interrupt List Release 61