Question

Write a program in 68K assembly code that adds an odd parity to each ASCII character. Your code must satisfy the following specifications:

1. Define the following 64 characters in the SRC address.SRC:DC.B 'Computing and Software Systems, University of Washington Bothell'

2. Define the 64-byte space.DST:DC.B 64

3. Read each of the 64 characters, (i.e., each byte) into D0, check the number of 1s in it, set 1 to the MSB with "ORI.B #$80, D0" to create an odd parity, and save it to the 64-byte space defined above.

4. You need to use ROL.B to count the number 1s in D0. Check with SCS.B if a carry occurs. For this purpose use D1.

5. Use D0 - D4 as well as A0 - A1 registers as follows:

D0: reads the next character from SRC.

D1: checks with "SCS.B D1" if a carry was generated

D2: counts the number of 1s of the character D0 has.

D3: counts down from 64 to 0, (i.e., used for a loop to read the 64 characters).

D4: counts down from 8 to 0, (i.e., used for a loop to examine the lowest 8 bits in D0).

A0: points to the next character to read into D0

A1: points to the next space to write the character (with/without a parity) from D0

6. You also need to use "LSR.B #1, D2" to check if the number of 1s in D2 is even.

ORG $1000

START: ; first instruction of program

MOVEA #SRC, A0 ; A0: points to the next charcater to read into D0

MOVEA #DST, A1 ; A1: points to the space to write D0's content

; ADD YOUR ODD PARITY GENERATION CODE HERE

* Put program code here

SIMHALT ; halt simulator

* Put variables and constants here

SRC:

DC.B 'Computing and Software Systems, University of Washington Bothell' ;64 bytes

DST:

DS.B 64

END START ; last line of source

Answer 1

Solution:

         ORG      $8000

DATA     EQU      $9000

RESULT   EQU      $9002

   CLR.B    D1             ;D1 calculates parity

       MOVE.W   DATA,D0        ;D0 holds data

       MOVE.W   #16,D2         ;D2 counts 16 bit

LOOP     LSR.W    #1,D0          ;shift low bit into carry

       BCC      ZERO           ;if bit is zero, go on

       NOT.B    D1             ;bit is 1, change parity

ZERO     SUB.W    #1,D2          ;decrement count

   BNE      LOOP           ;loop until count is zero

       MOVE.B   D1,RESULT ;store parity in RESULT

       TRAP     #14            ;return to MON68K

        END