ECEN 3213 Spring 2001 Quiz 3 28 March 2001

Solution

Consider the following Program:

count equ $0001

list equ $0002

temp equ $0003

org $E000

start ldaa #$00

ldx #$000F

staa count

begin ldab $1004 ;load value from port E into Register B

stab temp ; store value into memory

inx ; increment Register X for storing a value

inca ; increment count of number of values read.

staa count

staa list,x ; store read value into memory.

ldaa count

cmpa #100 ; check for count hitting 100.

blo start

stop ; count hit 100, all done.

org $FFFE

dc.w start

end

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The program above assembled with no errors. However, it ran forever, not stopping after reading 100 values on Port E. Also, when the memory was inspected as I stepped through it, I noticed it was NOT saving the correct value on Port E.

1. (35pts) What single statement needs to be changed, and to what does it need to be changed, to get the program to properly stop after 100 values have been read?

Ans: Change blo start to blo begin

2. (35pts) What single statement needs to be changed, and to what does it need to be changed, to get the program to properly store the input value into the memory?

Ans: Change staa list,x to stab list,x

3. (30pts) Why does it work Properly to have “list equ $0002”? In other words, why doesn’t the indexed addressing cause a value to be accidentally confused with temp, which is in location $0003?

Ans: Because the indexed store (ldab list,x) starts at( list + x ) which is (2 + 15) because x is initialized with ldx #$000F.