INT 2F - AVATAR Serial Dispatcher - INSTALL IRQ4 HANDLER
AX = 1A43h
BX = 4156h ('AV')
ES:DI -> FAR handler for serial port using IRQ4
DS = data segment needed by handler
Return: AX = status/return value
0000h if no more room
1A43h if ASD not installed
else handle to use when uninstalling
Notes: (see AX=1A42h)
SeeAlso: AX=1A42h,AX=1A63h