Chapter 4, Problem 41E

Explanation of Solution

Trace out for given “Example 4.3”:

Tracing the program is the process of documenting the state of the various aspects of the execution of the program over assembly instruction. Every instruction has the process like fetch, store, and decode which are used to determine the desired outcome to execute the program.

This tracing has address, label, instruction, RTN, and values of each registers. The trace out for given code “example 4.3” is as follows:

Instruction: Load X

Step	RTN	PC	IR	MAR	MBR	AC
(initial values)		100	-	-	-	-
Fetch	MARßPC	100	-	100	-	-
	IRßM[MAR]	100	110C	100	-	-
	PCßPC+1	101	110C	100	-	-
Decode	MARßIR[11-0]	101	110C	10C	-	-
	(Decode IR[15-12]	101	110C	10C	-	-
Get operand	MBRßM[MAR]	101	110C	10C	000C	-
Execute	ACßAC-MBR	101	110C	10C	000C	000C

Instruction: Subt Y

Step	RTN	PC	IR	MAR	MBR	AC
(initial values)		101	110C	10C	000C	000C
Fetch	MARßPC	101	110C	101	000C	000C
	IRßM[MAR]	101	410D	101	000C	000C
	PCßPC+1	102	410D	101	000C	000C
Decode	MARßIR[11-0]	102	410D	101	000C	000C
	(Decode IR[15-12]	102	410D	10D	000C	000C
Get operand	MBRßM[MAR]	102	410D	10D	0014	000C
Execute	ACßAC-MBR	102	410D	10D	0014	FFF8

Instruction: Skipcond 400

Step	RTN	PC	IR	MAR	MBR	AC
(initial values)		102	410D	10D	0014	FFF8
Fetch	MARßPC	102	410D	102	0014	FFF8
	IRßM[MAR]	102	8400	102	0014	FFF8
	PCßPC+1	103	8400	102	0014	FFF8
Decode	MARßIR[11-0]	103	8400	400	0014	FFF8
	(Decode IR[15-12]	103	8400	400	0014	FFF8
Get operand	Not necessary	103	8400	400	0014	FFF8
Execute	Do nothing	103	8400	400	0014	FFF8

Instruction: Jump Else

Step	RTN	PC	IR	MAR	MBR	AC
(initial values)		103	8400	400	0014	FFF8
Fetch	MARßPC	103	8400	103	0014	FFF8
	IRßM[MAR]	103	9108	103	0014	FFF8
	PCßPC+1	104	9108	103	0014	FFF8
Decode	MARßIR[11-0]	104	9108	108	0014	FFF8
	(Decode IR[15-12]	104	9108	108	0014	FFF8
Get operand	Not necessary	104	9108	108	0014	FFF8
Execute	PCßIR[11-0]	108	9108	108	000C	FFF8

Instruction: Load Y

Step	RTN	PC	IR	MAR	MBR	AC
(initial values)		108	9108	108	000C	FFF8
Fetch	MARßPC	108	9108	108	000C	FFF8
	IRßM[MAR]	108	110D	108	000C	FFF8
	PCßPC+1	109	110D	108	000C	FFF8
Decode	MARßIR[11-0]	109	110D	10D	000C	FFF8
	(Decode IR[15-12]	109	110D	10D	000C	FFF8
Get operand	MBRßM[MAR]	109	110D	10D	0014	FFF8
Execute	ACßMBR	109	110D	10D	0014	0014

Instruction: Subt X

Step	RTN	PC	IR	MAR	MBR	AC
(initial values)		109	110D	10D	0014	0014
Fetch	MARßPC	109	110D	109	0014	0014
	IRßM[MAR]	109	410C	109	0014	0014
	PCßPC+1	10A	410C	109	0014	0014
Decode	MARßIR[11-0]	10A	410C	10C	0014	0014
	(Decode IR[15-12]	10A	410C	10C	0014	0014
Get operand	MBRßM[MAR]	10A	410C	10C	000C	0014
Execute	ACßAC-MBR	10A	410C	10C	000C	0008

Instruction: Store Y

Step	RTN	PC	IR	MAR	MBR	AC
(initial values)		10A	410C	10C	000C	0008
Fetch	MARßPC	10A	410C	10A	000C	0008
	IRßM[MAR]	10A	210D	10A	000C	0008
	PCßPC+1	10B	210D	10A	000C	0008
Decode	MARßIR[11-0]	10B	210D	10D	000C	0008
	(Decode IR[15-12]	10B	210D	10D	000C	0008
Get operand	Not necessary	10B	210D	10D	000C	0008
Execute	MBRßAC	10B	210D	10D	0008	0008
(Changes Y)	M[MAR]ßMBR	10B	210D	10D	0008	0008

Instruction: Halt

Step	RTN	PC	IR	MAR	MBR	AC
(initial values)		10B	210D	10D	0008	0008
Fetch	MARßPC	10B	210D	10B	0008	0008
	IRßM[MAR]	10B	7000	10B	0008	0008
	PCßPC+1	10C	7000	10B	0008	0008
Decode	MARßIR[11-0]	10C	7000	000	0008	0008
	(Decode IR[15-12]	10C	7000	000	0008	0008
Get operand	Not necessary	10C	7000	000	0008	0008
(Changes Y)	Terminate program	10C	7000	000	0008	0008

Explanation:

From above tables, the register values is based on instruction and each register value being modified according to the micro operation. The new values are shaded in above table which is based on instruction execution.