Knowing that there are alternative methods to

implement our steps we can reexamine the previous

assembly language program.

Assm language

# ~ Comment

CLR $000F 3 6 extended mode

LDX #$0000 3 3 to use indexed mode the index register needs to be initialized.

CLR $0F,X 2 7 indexed mode

LDAA $0E 2 3 direct mode

LDAA $000E 3 4 extended mode could have been used however since address of switches is between $0000 and $00FF, a full 16 bit address is unnecessary.

LDAA $0E,X 2 5 index mode (in this case it is assumed that the index register was previously initialized by the earlier LDX instruction.

ECE 3010 ELEMENTS OF ELEC. MACHINES AND DIGITAL SYSTEMS CHAPTER 3

B. YOSHIDA, P.ENG 2013 V1.3.4 3-47

Assm language

# ~ Comment

CMPA #$00 2 2 immediate mode used since the value used for the comparison is a constant.

BEQ off 2 4

LDAB #$01 2 2 immediate mode used since the value used is a constant.

STAB $0F 2 4 Direct mode

STAB $000F 3 3 extended mode could have been used however since address of switches is between $0000 and $00FF, a full 16 bit address is unnecessary.

STAB $0F,X 2 6 index mode (in this case it is assumed that the index register was previously initialized by the earlier LDX instruction.

JMP $0003 3 3 Extended mode

ECE 3010 ELEMENTS OF ELEC. MACHINES AND DIGITAL SYSTEMS CHAPTER 3

B. YOSHIDA, P.ENG 2013 V1.3.4 3-48

Assm language

# ~ Comment

JMP $03,X 2 4 index mode (in this case it is assumed that appropriate value was placed in the index register by the earlier LDX instruction.

LDAA #$00 2 2 Immediate mode

CLRA 1 2 Implied mode - alternative method to load $00 into accumulator A

STAA $0F 2 4 Direct mode

STAA $000F 3 5 extended mode could have been used however since address of switches is between $0000 and $00FF, a full 16 bit address is unnecessary.

STAB $0F,X 2 6 index mode (in this case it is assumed that the index register was previously initialized by the earlier LDX instruction.

ECE 3010 ELEMENTS OF ELEC. MACHINES AND DIGITAL SYSTEMS CHAPTER 3

B. YOSHIDA, P.ENG 2013 V1.3.4 3-49

Assm language

# ~ Comment

JMP $0003 3 3 Extended mode

JMP $03,X 2 4 index mode (in this case it is assumed that the index register was previously initialized by the earlier LDX instruction.

Based on the alternatives available we can see that

different addressing modes / instruction choices will affect

the amount of memory required to store the program as

well as the amount of time required to execute the

program.

Example E3.5 - Modifying the simple program Modify the door ajar light system to indicates if it is a

front door or rear door which is open. Also determine

how much memory is required for your program, and

how long it will take to respond to a change in a door if

the microprocessor is running at 1MHz.

ECE 3010 ELEMENTS OF ELEC. MACHINES AND DIGITAL SYSTEMS CHAPTER 3

B. YOSHIDA, P.ENG 2013 V1.3.4 3-50