Address Instruction

00 IN

01 STO 25

02 BRZ 09

03 IN

04 ADD 27

05 STO 27

06 LDA 25

07 SUB 26

08 BR 01

09 LDA 27

10 OUT

11 HLT

Address Contents

25 DAT ? 2 1 0 (decremented by 1 each time the loop is executed)

26 DAT 1

27 DAT 0

Assume now that the above program will only read 3 numbers. That is, the following numbers in this order will be placed, one at a time, in the In-basket: 2, 35, and 15, where 2 is the count of numbers that follow, and 35 and 15 are the numbers that are to be added. The first column in the table shows the order in which the instructions from the program will be executed. Trace the execution of these instructions and determine the contents of the PC before and after each instruction is executed. Also, write down in the table the contents of the In-basket; Accumulator; Memory locations 25, 26, and 27; and Out-basket after each instruction is executed. Memory location 25 controls the loop. It initially contains an unknown value (?), then 2, next 1, and finally 0. Memory location 26 always contains 1. It is used to decrease the loop count by 1. Memory location 27 is initialized with 0, and finally it stores 50, the sum of 35 and 15. The entry 00 > 01 in the PC column means that the PC is 00 when the instruction IN started and is changed to 01 when the instruction IN is finished.

The sequence in which instructions are executed	PC before > after	In-basket	Accumulator	Memory location 25	Memory location 26	Memory location 27	Out-basket
IN	00 > 01	2	2	?	1	0	?
STO 25					1
BRZ 09					1
IN					1
ADD 27					1
STO 27					1
LDA 25					1
SUB 26					1
BR 01					1
STO 25					1
BRZ 09					1
IN					1
ADD 27					1
STO 27					1
LDA 25					1
SUB 26					1
BR 01					1
STO 25					1
BRZ 09					1
LDA 27					1
OUT					1
HLT					1