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Example - 06proc.asm - Procedures


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Example - 06proc.asm

; ---------------------------------------------------------------

;  A general purpose time delay procedure.

;  The delay is controlled by the value in AL.

;  When the procedure terminates, the CPU registers are
;  restored to the same values that were present before 
;  the procedure was called. Push, Pop, Pushf and Popf
;  are used to achieve this.  In this example one procedure 

;  is re-used three times.  This re-use is one of the main 
;  advantages of using procedures.

;------ The Main Program ----------------------------------------
	MOV	AL,8	; A short delay.
	CALL	30	; Call the procedure at address [30]

	MOV	AL,10	; A middle sized delay.
	CALL	30	; Call the procedure at address [30]

	MOV	AL,20	; A Longer delay.
	CALL	30	; Call the procedure at address [30]

JMP	Start	; Jump back to the start.

; ----- Time Delay Procedure Stored At Address [30] -------------
	ORG	30	; Generate machine code from address [30]

	PUSH	AL	; Save AL on the stack.
	PUSHF		; Save the CPU flags on the stack.
	DEC	AL	; Subtract one from AL.
	JNZ	REP	; Jump back to Rep if AL was not Zero.

	POPF		; Restore the CPU flags from the stack.
	POP	AL	; Restore AL from the stack.

	RET		; Return from the procedure.
; ---------------------------------------------------------------
; ---------------------------------------------------------------


15)	Re-do the traffic lights program and use this procedure 
	to set up realistic time delays.  02tlight.asm

16)	Re-do the text input and display program with procedures.
	Use one procedure to input the text and one to display it. 

; ---------------------------------------------------------------

You can copy this example program from the help page and paste it into the source code editor.


A value is placed into the AL register before calling the time delay procedure. This value determines the length of the delay.


Call the procedure at address [30]. This alters the instruction pointer IP to [30] and the program continues to run from that address. When the CPU reaches the RET command it returns to the address that it came from. This return address is saved on the stack.


This is a region in memory where values are saved and restored. The stack uses the Last In First Out rule. LIFO. The CALL command saves the return address on the stack. The RET command gets the saved value from the stack and jumps to that address by setting IP.

ORG 30

Origin at address [30]. ORG specifies at what RAM address machine code should be generated. The time delay procedure is stored at address [30].


Save the value of AL onto the stack. The CPU stack pointer SP points to the next free stack location. The push command saves a value at this position. SP is then moved back one place to the next free position. In this simulator, the stack grows towards address Zero. A stack overflow occurs if the stack tries to fill more than the available memory. A stack underflow occurs if you try to pop an empty stack.


Save the CPU flags in the status register SR onto the stack. This ensures that the flags can be put back as they were when the procedure completes. The stack pointer is moved back one place. See the Push command. NOTE: Items must be popped in the reverse order they were pushed.


Subtract one from AL. This command sets the Z flag if the answer was Zero or the S flag if the answer was negative.


Jump Not Zero to the address that Rep marks. Jump if the Z flag is not set.


Restore the CPU flags from the stack. Increase the stack pointer by one.


Restore the AL register from the stack. This is done by first moving the stack pointer SP forward one place and copying the value at that stack position into the AL register. A stack underflow occurs when an attempt is made to pop more items off the stack than were present. NOTE: Items must be popped in the reverse order they were pushed.


Return from the procedure to the address that was saved on the stack by the CALL command. Procedures can re-use themselves. This is called recursion. It is a powerful technique and dangerous if you don't understand what is happening! Accidental or uncontrolled recursion causes the stack to grow until it overwrites the program or overflows.

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© C Neil Bauers 2003