Introduction to Assembly Language Programming for Microcontrollers

1. Sequential Program Design

Example 1: Adding two unsigned double-byte numbers.

Assume the addend is stored in internal RAM at 40H (high byte), 41H (low byte), and the adder is stored at 50H (high byte), 51H (low byte), with the sum stored in 40H and 41H units.

The program is as follows:

ORG 0000H

JMP START

ORG 0100H

START: CLR C ; Clear Cy

MOV R0, ＃41H ; Load the address of the addend into data pointer R0

MOV R1, ＃51H ; Load the address of the adder into data pointer R1

MOV A, @R0 ; Load the content of the low byte of the addend into A

ADD A, @R1 ; Add the two low bytes

MOV @R0, A ; Store the sum of the low byte into the low byte of the addend

DEC R0 ; Point to the high byte of the addend

DEC R1 ; Point to the high byte of the adder

MOV A, @R0 ; Load the high byte of the addend into A

ADDC A, @R1 ; Add the two high bytes with Cy

MOV @R0, A ; Store the sum of the high byte into the high byte of the addend

END

2. Branch Program Design

• Implementing branches: Conditional transfer instructions

  Accumulator zero check instructions JZ/JNZ

  Comparison transfer instruction CJNE

  Bit control transfer instructions JC/JNC/JB/JNB/JBC

• Control each branch: Unconditional transfer instructions

  LJMP/AJMP/SJMP/JMP

Loop programs generally consist of four main parts:

(1) Initialization: Prepares for the loop program, such as setting loop counts, and pre-setting values for variables and address pointers.

(2) Processing: The part of the program that is executed repeatedly, it is the entity of the loop program and the main part of the loop program.

(3) Loop control: This part modifies loop variables and control variables, and checks if the loop should end, until the end condition is met, at which point it exits the loop.

(4) End part: This part mainly analyzes, processes, and stores the results of the loop program.

Example: Convert a byte’s hexadecimal number in a certain internal RAM unit into two ASCII codes, storing the result in two consecutive internal RAM units.

Assuming a byte’s hexadecimal number is in internal RAM unit 40H, and the result is stored in units 50H and 51H, parameter passing can be done using the stack, and the program is as follows:

MAIN: MOV R1, ＃50H ; R1 is the pointer for storing results

MOV A, 40H ; Ais the hexadecimal number to be converted

SWAP A ; Convert the high half byte first

PUSH ACC ;Push onto the stack

LCALL HEASC ; Call the subroutine to convert the content of the low half byte to ASCII code;

Subroutine HEASC for conversion

POP ACC

MOV @R1, A ; Store the conversion result of the high half byte

INC R1

PUSH 40H

LCALL HEASC

POP ACC

MOV @R1, A ; Store the conversion result of the low half byte

END

HEASC: MOV R0, SP

DEC R0

DEC R0 ; R0 points to the address of the hexadecimal parameter

XCH A, @R0 ; Retrieve the parameter to be converted

ANL A, ＃0FH ; Retain the low half byte

ADD A, ＃2 ; Modify the A value

MOVC A, @A+PC ; Lookup table

XCH A,@R0; Return the result to the stack

RET

TAB: DB 30H, 31H, 32H, …