Answer1

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WHAT'S YOUR ENGINEERING QUOTIENT?

Problem 8—An 8085 microprocessor uses a 2MHz crystal. Find the time taken by it to execute the following delay subroutine, including the CALL instruction in the calling program.

DELAY:
PUSH PSW
MVI A,64H
DELAYLOOP:
NOP
DCR A
JNZ LOOP
POP PSW
RET

The CALL and RET instructions each take 18 cycles of the system clock, PUSH and POP take 12 cycles each, and conditional jump takes 10 cycles if the jump is taken and 7 cycles if is not. NOP takes 1 cycle. All other instructions used above take (3 ı N + 1) clock cycles, where N is the number of accesses to memory, not including the opcode fetch.

Answer 8

The various timings are shown with the help of the following table:

Instruction Number of Cycles Comments

CALL
18

PUSH
12

MVI A,64H
4
3N + 1 where N = 1 (one memory access)

NOP
100
100 ı 1 cycle each

DCR A
100
100 ı 1 cycle each

JNZ (taken)
990
The loop is executed 64H ı 1 = 99 times,
for a total of 99 ı 10 cycles.

JNZ (not taken)
7
When the condition is false

POP
12

RET
18

Total:

1261

The 2MHz crystal frequency is divided by 2 inside the 8085. Hence the operating frequency is 1MHz, and the time for 1 cycle is 1 ıs.

The time for 1261 cycles is therefore 1261 ıs = 1.261 ms.

Contributor: Naveen PN

03-02

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