EER-018
Introduction to Digital Computers
Assignment #9

1. 

 

org

$3FF    ;initialize one data value in memory

data_v:

fcb

$67

 

org

$300   ;put program in memory at $300

start:

LDX

#$C0

 

LDA

$3FF

 

STA

,X

 

LSRA

 

 

INCX

 

 

STA

,X

 

INCX

 

 

INCA

 

 

STA

,X

 

STOP

 

 

ORG

$7FE

 

FDB

START    ; to set the reset address

Modify the program above so that it accomplishes the same thing, but uses only 8 instructions. (Two instructions can be eliminated. ) Hint: considering using the indexed, 8-bit offset address mode.

2. Write a program that counts the number of zeros in memory location $C0 and stores the result in location $C1.

Example input and output in memory:

$C0  $0C       0000 1100 <-- 6 zeros, 2 ones
$C1  $06

Assume that the value of the accumulator, A, is $AA  before each of the following 6805 instructions are executed. Give the final value of the accumulator and the CCR flags, N, Z, C. Assume the C flag is initially set to 1.

instruction:

COMA

ROLA

LSLA 

NEGA

accumulator:

 

 

 

 

CCR flags:

 

 

 

 
 

3. The following instructions occur in a sequence, one after the other. What are the final values of the A and X register?

LDA #$57

ORA #$82

DECA

CLRA

TAX
 

4. Assume that there are 2 data values in memory starting at location $C2. Write a program to move the 2 values to the memory locations starting at location $DF.

5. Repeat problem 4, but now assume that there are 20 values to be moved. Hint – index mode is useful - the offset can be the difference between the two addresses.

 

6.  Analyze the program shown below by filling in the worksheet that follows for each instruction that is fetched and executed. 
 

Address

Label

Instruction

Operand

Comment

$0200

START:

LDA 

#$FF

; Load A immediate

$0202

 

INCA

 

; Increment A

$0203

 

LDX

#$D0

; Load X immediate

$0205

 

ADC 

#2

; Add 2 to A with carry

$0207

 

STA 

,X

; Store A in memory using indexed mode

$0208

 

STX

1,X

; Store X in memory using indexed mode


 

Accumulator (A)

Cond. Codes 
1  1  1  H  I  N  Z  C

Index Register (X)

Program Counter (PC)

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Memory
Address (fill in)

Memory
Contents

 

 

 

 

 

 
 

 

 

 
 

 

 

 
 

 

 

 

7. Hand assemble the following short program and give the resulting addresses and machine code.
 

 

org

$300    ; put program in memory starting at location $300

start:

LDA

#$50

loop:

DECA

 

 

BNE

loop

 

STOP

 

  8. Give the final values of the A and X registers and the memory locations shown when the program below terminates.
 

 

org

$3FF    ;initialize one data value in memory

 

data_v:

fcb

4

 

 

org

$300   ;put program in memory at $300

Memory

start:

LDX

#$C0

address: contents

LDA

data_v

$C0

again:

STA

,X

$C1

 

DECA

 

$C2

 

BEQ

 done

$C3

 

INCX

$C4

 

BRA

again

 

 done

STOP

 

 

 

ORG

$7FE

 

 

FDB

START    ; to set the reset address

 

9. Write a program that checks memory locations $C0, $C1, $C2 and $C3 and clears the memory location if the data is an even number. If the number is odd, it is left untouched.

For example: 

Before:

$C0  $0C       even
$C1  $07        odd
$C2  $F3        odd
$C3  $80        even

After:

$C0  $00       
$C1  $07       
$C2  $F3       
$C3  $00