Here is a program to do that: ORG 20H MOV R0,#20H ; starting address MOV A,#F0H ; value to fill L1: MOV @R0,A INC R0 CJNE R0,#30H,L1 MOV R0,#30H ; starting address MOV A,#E0H ; left shifted value L2: MOV @R0,A INC R0 CJNE R0,#40H,L2 END The program: A. Fills memory addresses 20h to 2Fh with value F0h B. Fills memory addresses 30h to 3

1. Subroutine
MCS-51 Microcontroller
By Arkhom JODTANG
Civil Aviation Training Center, Thailand

2. • Subroutines are a small part of program that
able to call for use in multiple times.
Subroutine1
Subroutine2
Subroutine3

3. • Subroutines are
contended as the
part of main
program.
Main Program
Subroutine1
Subroutine2
ORG 00H
END

4. Writing Subroutine
• ACALL instruction to call subroutine
– Use LCALL for longer then 2 kByte distance
• Label, such as
– Delay:
– Multipli4X4Bytes:
– CheckOnes:
• RET instruction
– RET will make return to the next instruction (After
ACALL)

5. Recording the Return Address
• Stack will be use to record the return to main
program address.

6. Sample Program
ORG 00H
MOV R0, #30H
Loop:
ACALL DecimalAdj
ACALL WriteData
INC R0
INC A
CJNE R0,#70H, Loop
SJMP $
; --------------------------------
DecimalAdj:
CLR C
DA A
RET
WriteData:
MOV @R0, A
RET
END

7. Sample Program
ORG 0000H
Start: MOV P2, #11101111B
ACALL DELAY_100ms
MOV P2, #11010111B
ACALL DELAY_100ms ; Note:
MOV P2, #10111011B ; P2 Connect with Active LOW LED Display
ACALL DELAY_100ms
MOV P2, #01111101B
ACALL DELAY_100ms
JMP Start
DELAY_100ms:
MOV R7, #64H
DELAY_1ms_1:
MOV R6, #0E6H
DELAY_1ms_2:
NOP
NOP
DJNZ R6, DELAY_1ms_2
DJNZ R7, DELAY_1ms_1
RET

8. Sample Program
• ; Program to paint memory area with integer number.
• ; Then use subroutine to check, if found letter (A-F)
• ; then change that byte to #60H
• ORG 00H
• MOV A, #0H
• MOV R0, #30H
• L_Paint:
• MOV @R0, A
• ACALL ADJUST
• INC R0
• INC A
• CJNE A, #60H, L_Paint
• SJMP $
• ADJUST:
• PUSH Acc
• MOV A,@R0
• ANL A,#0FH
• SUBB A,#09H
• JC SKIP_Adjust
• MOV @R0,#60H
• SKIP_Adjust:
• POP Acc
• RET
• END

9. Sample Program
; Program DIGITAL count-up to Port 1 (LED); Push SW1 (P0.0) to start ; R2 Control the speed by press D3 to Increase, Push D4
to Decrease speed
ORG 0000H
MOV R2, #0FFH
Start: MOV P1, #000H
MOV R0, #00H
in : MOV P1, R0
JNB P0.3, SKIP
MOV A, R2
SUBB A, #10H ; Increase speed of Delay
MOV R2, A
JMP SKIP
JNB P0.4, SKIP
MOV A, R2
ADD A, #10H ; Decrease speed of Delay
MOV R2, A
SKIP:
ACALL DELAY1
INC R0
CJNE R0, #0255D, in
JMP Start
DELAY1:
MOV A, R2
MOV R7, A
DELAY_1ms_1:
MOV R6, #0E6H
DELAY_1ms_2:
NOP
NOP
DJNZ R6, DELAY_1ms_2
DJNZ R7, DELAY_1ms_1
RET
END

10. Sample Question
• Program to
A. Fill value #F0 to #FFH to memory address 20h to 2Fh
B. Also Fill left shifted value of item A to to memory address 30h to 3Fh