Starting to Program – An Introduction to Assembler

Chapter Four

Dr. Gheith Abandah 1

Outline

• Introduction• The PIC 16 Series Instruction Set• Byte-oriented file register operations• Bit-oriented file register operations• Literal and control operations• Instruction Formats• Operation Types• MPASM Assembler• Example

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Introduction

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Micro-controller Programming Options:

1. Machine Code00 0111 0001 0101

2. Assembly Language: needs an assembleraddwf NUM, w

3. High-Level Language: needs a compilerfor (i=0; i<10; i++) sum += a[i];

Development Process

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ALU Data Flow

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The PIC 16 Series Instruction Set

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• 35 instructions• Three Groups:

1. Byte-oriented file register operations2. Bit-oriented file register operations3. Literal and control operations

• Operation Types:– Arithmetic – Logic– Data movement – Control– Misc

1. Byte-oriented file register operations

• Format: op f, d– op: operation– f: number of file or register– d: destination (0: working register, 1: file register)

• Example:addwf PORTA, 0

Adds the contents of the working register and register PORTA, puts the result in the working register.

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2. Bit-oriented file register operations

• Format: op f, b– op: operation– f: number of file or register– b: bit number, 0 through 7

• Example:bsf STATUS, 5

Sets to 1 Bit 5 of register STATUS.

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3. Literal and control operations

• Format: op k– op: operation– k: literal, an 8-bit if data or 11-bit if address

• Examples:addlw 5

Adds to the working register the value 5. call 9

Calls the subroutine at address 9.

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Instruction Formats

Arithmetic Operations

Mnemonic Operands Description Cycles Status Affected

ADDWF f, d Add W and f 1 C,DC,ZCOMF f, d Complement f 1 ZDECF f, d Decrement f 1 ZINCF f, d Increment f 1 Z

SUBWF f, d Subtract W from f 1 C,DC,ZADDLW k Add literal and W 1 C,DC,ZSUBLW k Subtract W from literal 1 C,DC,Z

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Logic Operations

Mnemonic Operands Description Cycles Status Affected

ANDWF f, d AND W with f 1 Z

IORWF f, d Inclusive OR W with f 1 Z

XORWF f, d Exclusive OR W with f 1 Z

ANDLW k AND literal with W 1 Z

IORLW k Inclusive OR literal with W 1 Z

XORLW k Exclusive OR literal with W 1 Z

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Data Movement Operations

Mnemonic Operands Description Cycles Status Affected

MOVF f, d Move f 1 ZMOVWF f Move W to f 1SWAPF f, d Swap nibbles in f 1

MOVLW k Move literal to W 1

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Control Operations

Mnemonic Operands Description Cycles Status Affected

DECFSZ f, d Decrement f, Skip if 0 1 (2)INCFSZ f, d Increment f, Skip if 0 1 (2)BTFSC f, b Bit Test f, Skip if Clear 1 (2)BTFSS f, b Bit Test f, Skip if Set 1 (2)CALL k Call subroutine 2

GOTO k Go to address 2RETFIE - Return from interrupt 2RETLW k Return with literal in W 2

RETURN - Return from Subroutine 2Dr. Gheith Abandah 14

Miscellaneous OperationsMnemonic Operands Description Cycles Status

AffectedCLRF f Clear f 1 ZCLRW - Clear W 1 ZNOP - No Operation 1

RLF f, d Rotate Left f through Carry 1 C

RRF f, d Rotate Right f through Carry 1 C

BCF f, b Bit Clear f 1BSF f, b Bit Set f 1

CLRWDT - Clear Watchdog Timer 1 TO',PD'SLEEP - Go into standby mode 1 TO',PD'

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Assembler Format

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Assembler Directives

Assembler directive Summary of action

list Implement a listing option

#include Include additional source file

org Set program origin

equ Define an assembly constant; this allows us to assign a value to a label

end End program block

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Number Representation in MPASM

Radix Example

Decimal D’255’

Hexadecimal H’8d’ or 0x8d

Octal O’574’

Binary B’01011100’

ASCII ‘G’ or A’G’

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Example: move push button to LED

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Example – Page 1;***************************************************

;ELECTRONIC PING-PONG DATA MOVE

;This program moves push button switch values from

;Port A to the leds on Port B

;TJW 21.2.05 Tested 22.2.05

;***************************************************

;

;Configuration Word: WDT off, power-up timer on,

; code protect off, RC oscillator

;

list p=16F84A

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Example – Page 2;

;specify SFRs

status equ 03

porta equ 05

trisa equ 05

portb equ 06

trisb equ 06

;

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Example – Page 3;

org 00

;Initialise

start bsf status,5 ;select memory bank 1

movlw B’00011000’

movwf trisa ;set port A direction

movlw 00

movwf trisb ;all port B bits output

bcf status,5 ;select bank 0

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Example – Page 4;

;The "main" program starts here

clrf porta ;clear all bits in ports A

loop movf porta,0 ;move port A to W register

movwf portb ;move W register to port B

goto loop

end

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Summary• Assembler is a programming language that is part of the toolset

used in embedded systems programming. It comes with its own distinct set of rules and techniques.

• It is essential to adopt and learn an IDE when developing programs. The MPLAB IDE is an excellent tool for PIC microcontrollers, both for learners and professionals. And it can’t be beaten on price!

• While some people are eager to get programs into the hardware immediately, it is extremely useful to learn the features of a simulator. The simulator in MPLAB allows the user to test program features with great speed, and is an invaluable learning tool.

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