Returns and procedures definitions in 8086, Computer Engineering

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Q. Returns and Procedures definitions in 8086?

8086 microprocessor supports RET and CALL instructions for procedure call. CALL instruction not only branches to indicate address but also pushes return address onto the stack. Additionally it also initialized IP with address of the procedure. RET instructions basically pops the return address from stack. 8086 supports two types of procedure call. These are NEAR and FAR calls.

NEAR procedure call is also called as Intrasegment call as called procedure is in same segment from that call has been made. Hence only IP is stored as return address. IP can be stored on stack as:

912_Returns and Procedures definitions in 8086.png

Please note that the growth of stack is towards stack segment base. So stack becomes full on an offset 0000h. Also for push operation, we decrement SP by 2 as stack is a word stack (word size in 8086 = 16 bits) whereas memory is byte organised memory.

FAR procedure call which is also known as intersegment call is a call made to separate code segment. So the control will be transferred outside current segment. So both IP and CS need to be stored as return address. These values on stack after the calls looks as below:

597_Returns and Procedures definitions in 80861.png

When 8086 executes FAR call it first stores contents of code segment register followed by contents of IP on to the stack. A RET from NEAR procedure. Pops the 2 bytes in IP. RET from the FAR procedure pops 4 bytes from the stack.


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