8086 Microprocessor code Procedures

Procedures

A procedure is a set of code that can be branched to and returned from in such a way that the code is as if it were inserted at the point from which it is branched to. The branch to procedure is referred to as the call, and the corresponding branch back is known as the return. The return is always made to the instruction immediately following the call regardless of where the call is located.

1 Calls, Returns, and Procedure Definitions

The CALL instruction not only branches to the indicated address, but also pushes the return address onto the stack. The RET instruction simply pops the return address from the stack. The registers used by the procedure need to be stored before their contents are changed, and then restored just before their contents are changed, and then restored just before the procedure is excited.

A CALL may be direct or indirect and intrasegment or intersegment. If the CALL is intersegment, the return must be intersegment. Intersegment call must push both (IP) and (CS) onto the stack. The return must correspondingly pop two words from the stack. In the case of intrasegment call, only the contents of IP will be saved and retrieved when call and return instructions are used.

Procedures are used in the source code by placing a statement of the form at the beginning of the procedure

Procedure name PROC Attribute and by terminating the procedure with a statement

Procedure name ENDP

The attribute that can be used will be either NEAR or FAR. If the attribute is NEAR, the RET instruction will only pop a word into the IP register, but if it is FAR, it will also pop a word into the CS register.

A procedure may be in:

1. The same code segment as the statement that calls it.

2. A code segment that is different from the one containing the statement that calls it, but in the same source module as the calling statement.

3. A different source module and segment from the calling statement.

In the first case, the attribute could be NEAR provided that all calls are in the same code segment as the procedure. For the latter two cases the attribute must be FAR. If the procedure is given a FAR attribute, then all calls to it must be intersegment calls even if the call is from the same code segment. For the third case, the procedure name must be declared in EXTRN and PUBLIC statements.

2 Saving and Restoring Registers

When both the calling program and procedure share the same set of registers, it is necessary to save the registers when entering a procedure, and restore them before returning to the calling program.

MSK PROC NEAR

PUSH AX

PUSH BX

PUSH CX

POP CX

POP BX

POP AX

RET

MSK ENDP

3 Procedure Communication

There are two general types of procedures, those operate on the same set of data and those that may process a different set of data each time they are called. If a procedure is in the same source module as the calling program, then the procedure can refer to the variables directly.

When the procedure is in a separate source module it can still refer to the source module directly provided that the calling program contains the directive

PUBLIC ARY, COUNT, SUM

EXTRN ARY: WORD, COUNT: WORD, SUM: WORD

4 Recursive Procedures

When a procedure is called within another procedure it called recursive procedure. To make sure that the procedure does not modify itself, each call must store its set of parameters, registers, and all temporary results in a different place in memory

Eg. Recursive procedure to compute the factorial