8086 assembler tutorial for beginners (part 8)

Procedures

Procedure is a part of code that can be called from your program in order to make some specific task. Procedures make program more structural and easier to understand. Generally procedure returns to the same point from where it was called.

The syntax for procedure declaration:
name PROC

      ; here goes the code
      ; of the procedure ...

RET
name ENDP

name - is the procedure name, the same name should be in the top and the bottom, this is used to check correct closing of procedures.

Probably, you already know that RET instruction is used to return to operating system. The same instruction is used to return from procedure (actually operating system sees your program as a special procedure).

PROC and ENDP are compiler directives, so they are not assembled into any real machine code. Compiler just remembers the address of procedure.

CALL instruction is used to call a procedure.

Here is an example: 


ORG    100h

CALL   m1

MOV    AX, 2

RET                   ; return to operating system.

m1     PROC
MOV    BX, 5
RET                   ; return to caller.
m1     ENDP

END


The above example calls procedure m1, does MOV BX, 5, and returns to the next instruction after CALL: MOV AX, 2.

There are several ways to pass parameters to procedure, the easiest way to pass parameters is by using registers, here is another example of a procedure that receives two parameters in AL and BL registers, multiplies these parameters and returns the result in AX register: 


ORG    100h

MOV    AL, 1
MOV    BL, 2

CALL   m2
CALL   m2
CALL   m2
CALL   m2

RET                   ; return to operating system.

m2     PROC
MUL    BL             ; AX = AL * BL.
RET                   ; return to caller.
m2     ENDP

END


In the above example value of AL register is update every time the procedure is called, BL register stays unchanged, so this algorithm calculates 2 in power of 4,
so final result in AX register is 16 (or 10h).


Here goes another example,
that uses a procedure to print a Hello World! message: 


ORG    100h

LEA    SI, msg        ; load address of msg to SI.

CALL   print_me

RET                   ; return to operating system.

; ==========================================================
; this procedure prints a string, the string should be null
; terminated (have zero in the end),
; the string address should be in SI register:
print_me     PROC

next_char:
    CMP  b.[SI], 0    ; check for zero to stop
    JE   stop         ;

    MOV  AL, [SI]     ; next get ASCII char.

    MOV  AH, 0Eh      ; teletype function number.
    INT  10h          ; using interrupt to print a char in AL.

    ADD  SI, 1        ; advance index of string array.

    JMP  next_char    ; go back, and type another char.

stop:
RET                   ; return to caller.
print_me     ENDP
; ==========================================================

msg    DB  'Hello World!', 0   ; null terminated string.

END


"b." - prefix before [SI] means that we need to compare bytes, not words. When you need to compare words add "w." prefix instead. When one of the compared operands is a register it's not required because compiler knows the size of each register.


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