Example of an assembler question for the cs2 exam

3 a) Explain briefly the effect of the following instructions on a PIC microprocessor. [4]

i) bcf 2,3

ii) comf 21,1

iii) incf 11,0

iv) andlw B'11001111'

Answer

i) clear bit 3 of register 2

ii) reg[21]:= ~ reg[21]

iii) w:= reg[11]+1

iv) w:=w and 207

b) Provide a diagram of the datapaths and functional units in a PIC microprocessor

labeling your buses. [4]

reg

c) With reference to your diagram supplied for part b, identify which

buses are active for the following instructions [4]

i) addwf 12,1

ii) retlw 7

answer

i. phase 1: const bus addresses registers, r, a,b busses feed alu; phase 2: const bus addresses registers, result bus goes to content of registers

ii. const bus fed through b bus to alu, then via result bus to w reg. Simultaneously stack popped into pc

d) Translate the following pseudo code into PIC assembler [8]

State assumptions made about your parameter passing mechanism.

--- Return true if the array is sorted in ascending order

procedure isSorted(a:array[1..4] of byte):boolean;

variable x:byte;

begin

x:= a[1];

for i:= 1 to 4 do

if a[i]<x then return false

x:=a[i]

enddo

return true

end

assume parameter passed in w

movwf a ; save parameter

movwf fsr

movfw indir

movwf x ; x:=a[i]

movlw 4

movwf I ; initialise count

again

movfw indir ; get array element

subwf x,0

btfsc status,carry

retlw 0 ; if a[i]<x return false

incf fsr,1 ; address next array value

movfw indir

movwf x ; x gets next array value

decfsz i

goto again

retlw 1 ; return true