building assembler programs chapter five dr. gheith abandah1

Building Assembler Programs

Chapter Five

Dr. Gheith Abandah 1

Outline

• Building Structured Programs• Conditional Branching and Working with Bits• Subroutines • Generating Time Delays and Intervals• Indirect Addressing and PCL• Using PCL Register for Lookup Table• Flashing LEDs Example


Building Structured Programs

• If you don’t watch it, assembly programs become spaghetti programs.

• It is better to develop structured programs; programs structured into routines with specific functions.

• Start by representing programs using:– Flow diagrams– State Diagrams



Flow Diagrams

Rectangle for process or action

Diamond for decision


State Diagrams

Some machines tend to move from one state to another when a time period is completed or aspecific event occurs.

Conditional Branching and Working with Bits

; Testing and manipulating single bits

movlw 00 ;clear all bits in port A and B

movwf porta

movwf portb

loop bcf portb, 3 ;preclear port B, bit 3

btfss porta, 3

bsf portb, 3 ;but set it if button pressed

;

bcf portb, 4 ;preclear port B, bit 4

btfss porta, 4

bsf portb, 4 ;but set it if button pressed

goto loop

end


Subroutines


Generating Time Delays and Intervals

…

call delay500

…

;500ms delay (approx)

;100 calls to delay5

delay500

movlw D'100'

movwf delcntr2

del2

call delay5

decfsz delcntr2, 1

goto del2

return

;Delay of 5ms approx.

;Instr. Cycle T = 5us.

;200 iterations * 5T

delay5

movlw D'200'

movwf delcntr1

del1

nop ;1T

nop ;1T

decfsz delcntr1,1 ;1T

goto del1 ;2T

return


Indirect Addressing and PCL


Using PCL Register for Lookup Table

....

movf sample_no,0

call table

movwf portb

....

table addwf pcl

retlw 23

retlw 3f

retlw 47

retlw 7f

retlw 0a2

retlw 1f

retlw 03

retlw 67

retlw 0c5

retlw 32


W = 5

W = 1f

Flashing LEDs Example – Page 1;

;specify SFRs

pcl equ 02

status equ 03

porta equ 05

trisa equ 05

portb equ 06

trisb equ 06

;

pointer equ 10

delcntr1 equ 11

delcntr2 equ 12

;


Flashing LEDs Example – Page 2org 00

;Initialise

start bsf status,5 ;select memory bank 1

movlw B'00011000'

movwf trisa ;setup port A

movlw 00

movwf trisb ;all port B bits output

bcf status,5 ;select bank 0

;


Flashing LEDs Example – Page 3;The “main” program starts here

movlw 00 ;clear all bits in port A

movwf porta

movwf pointer ;also clear pointer

loop movf pointer,0 ;move pointer to W register

call table

movwf portb ;move W register to port B

call delay

incf pointer,1

btfsc pointer,3 ;test if pointer = 8

clrf pointer ;if yes, clear to start over

goto loop


Flashing LEDs Example – Page 4;Introduces delay of 500ms approx, for 800kHz clock

delay movlw D'100'

movwf delcntr2

outer movlw D'200'

movwf delcntr1

inner nop

nop

decfsz delcntr1,1

goto inner

decfsz delcntr2,1

goto outer

return


Flashing LEDs Example – Page 5;Lookup Table

table addwf pcl

retlw 23

retlw 3f

retlw 47

retlw 7f

retlw 0a2

retlw 1f

retlw 03

retlw 67

;

end


Summary• It is important to devise good structures for programs

as they are developed. Flow and state diagrams can help with this.

• A number of techniques assist in producing clear and well-structured programs. These include subroutines, look-up tables, macros and the use of Include files.

• The full 16F84A instruction set can be applied to build big and sophisticated programs.

• More complex programs require greater expertise in the use of simulation techniques.


building assembler programs chapter five dr. gheith abandah1

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