Transfers control to procedure, return address is (IP) is pushed to
stack. 4-byte address may be entered in this form:
1234h:5678h, first value is a segment second
value is an offset (this is a far call, so CS is also pushed to stack).
ORG 100h ; for COM file.
ADD AX, 1
RET ; return to OS.
p1 PROC ; procedure declaration.
MOV AX, 1234h
RET ; return to caller.
C Z S O P A