INT 21 - European MSDOS 4.0 - SEND SIGNAL
AH = 8Dh
AL = signal number (see AH=8Ch)
BH = signal argument
BL = action
00h send to entire command subtree
01h send only to specified process
DX = Process ID
Return: CF clear if successful
CF set on error
AX = error code (01h,06h)(see AH=59h)
Desc:   invoke the exceptional-condition handler for the specified process
Note:   error 06h may be returned if one or more of the affected processes
have an error handler for the signal

See also: 218C

218D - European MSDOS 4.0 - SEND SIGNAL