8085 MICROPROCESSOR SHORT QUESTIONS AND ANSWER GATE,IES,PSU FUNDAMENTAL Q BY SATYANARAYAN PRADHAN

MICROPROCESSOR AND MICROCONTROLLER QUESTION WITH ANSWER 8085 PROVIDED BY ASST. PROF,SATYANARAYAN PRADHAN GANDHI ENGINEERING COLLEGE, BHUBANESWAR ELECTRONICS AND COMMUNICATION ENGINEERING DEPT.

1. What is Microprocessor? Give the power supply & clock frequency of 8085

     

     Ans:-A microprocessor is a multipurpose, programmable logic device that reads

binary instructions from a storage device called memory accepts binary data as

input and processes data according to those instructions and provides result as

output. The power supply of 8085 is +5V and clock frequency in 3MHz. 

2. List few applications of microprocessor-based system. 

  Ans:-

It is used:

i. For measurements, display and control of current, voltage, temperature, pressure, etc. 

ii. For traffic control and industrial tool control.

iii. For speed control of machines.

3. What are the functions of an accumulator?

The accumulator is the register associated with the ALU operations and

sometimes I/O operations. It is an integral part of ALU. It holds one of data to be

processed by ALU. It also temporarily stores the result of the operation performed

by the ALU.

4. List the 16 – bit registers of 8085 microprocessor.

Stack pointer (SP) and Program counter (PC).

5. List the allowed register pairs of 8085.

1. B-C register pair
2. D-E register pair
3. H-L register pair

6. Mention the purpose of SID and SOD lines

SID (Serial input data line):

It is an input line through which the microprocessor accepts serial

data.

SOD (Serial output data line):

It is an output line through which the microprocessor sends output

serial data. 

7. What is an Opcode?

The part of the instruction that specifies the operation to be performed is

called the operation code or opcode

8. What is the function of IO/M signal in the 8085?

It is a status signal. It is used to differentiate between memory locations

and I/O operations. When this signal is low (IO/M = 0) it denotes the

memory related operations. When this signal is high (IO/M = 1) it denotes

an I/O operation.

9. What is an Operand?

The data on which the operation is to be performed is called as an

Operand.

10. Control signals used for DMA operation are

HOLD & HLDA. 

11. What is meant by interrupt?

Interrupt is an external signal that causes a microprocessor to jump to a

specific subroutine. 

12. Explain priority interrupts of 8085.

The 8085 microprocessor has five interrupt inputs. They are TRAP, RST

7.5, RST 6.5, RST 5.5, and INTR. These interrupts have a fixed priority of

interrupt service. If two or more interrupts go high at the same time, the 8085 will service

them on priority basis. The TRAP has the highest priority followed bye RST 7.5,

RST 6.5, RST 5.5. The priority of interrupts in 8085 is shown in the table. 

Interrupts	Priority
TRAP	1
RST 7.5	2
RST 6.5	3
RST 5.5	4
INTR	5

13. What is a microcomputer?

A computer that is designed using a microprocessor as its CPU is called

microcomputer

14. What is the signal classification of 8085

All the signals of 8085 can be classified into 6 groups

• Address bus
• Data bus
• Control and status signals
• Power supply and frequency signals
• Externally initiated signals
• Serial I/O ports

15. What are operations performed on data in 8085

The various operations performed are

Store 8-bit data

Perform arithmetic and logical operations

Test for conditions

Sequence the execution of instructions

Store data temporarily during execution in the defined R/W

memory locations called the stack

16. Steps involved to fetch a byte in 8085

i. The PC places the 16-bit memory address on the address bus

ii. The control unit sends the control signal RD to enable the memory

chip

iii. The byte from the memory location is placed on the data bus

iv. The byte is placed in the instruction decoder of the microprocessor and

the task is carried out according to the instruction

17. How many interrupts does 8085 have, mention them

The 8085 has 5 interrupt signals; they are INTR, RST7.5, RST6.5, RST5.5

and TRAP. 

18. What is an instruction?

An instruction is a binary pattern entered through an input device to

command the microprocessor to perform that specific function.

19. What is the use of ALE

The ALE is used to latch the lower order address so that it can be available

in T2 and T3 and used for identifying the memory address. During T1 the ALE

goes high, the latch is transparent ie, the output changes according to the input

data, so the output of the latch is the lower order address. When ALE goes low the

lower order address is latched until the next ALE.

20. Explain the signals HOLD, READY and SID

HOLD indicates that a peripheral such as DMA controller is requesting the

use of address bus, data bus and control bus. READY is used to delay the

microprocessor read or write cycles until a slow responding peripheral is ready to

send or accept data. SID is used to accept serial data bit by bit.

21. Explain the different instruction formats with examples

The instruction set is grouped into the following formats

One byte instruction

Two byte instruction

Three byte instruction

MOV C,A

MVI A,39H

JMP 2345H

                                        GANDHI ENGINEERING COLLEGE, BHUBANESWAR

                           ELECTRONICS AND COMMUNICATION ENGINEERING DEPT.

1. On which model is based the basic architecture of a digital computer?
ANS;

The basic architecture of a digital computer is based on Von Neumann model.

2. What is meant by distributed processing?
ANS:

Distributed processing involves the use of several microprocessors in a single

computer system. For example, for such a system, the first microprocessor may control
keyboard activities, the second controls storage devices like disk drives, the third controls
input/ output operations, while the fourth may act as the main system processor.

3. What is the technology used in microprocessor?
ANS:
NMOS technology is used in microprocessors.

4. How does the microprocessor communicate with the memory and input / output
devices?
ANS:

The microprocessor communicates with the memory and input / output
devices via three buses, viz., data bus, address bus and control bus.

5. What are the different jobs that the CPU is expected to do at any given point of time?
ANS:

The CPU may perform a memory read or write operation, an I/O read or write

operation or an internal activity.
6. What is mnemonic?
ANS:

It is very difficult to understand a program if it is written in either binary or
hex code. Thus the manufactures have devised a symbolic code for each instruction,
called a mnemonic.

7. What is machine language programming?
ANS:

Programming a computer by utilizing hex or binary code is known as machine

language programming.
8. How many different instructions microprocessor 8085 has? What is an instruction set?
ANS:

8085 microprocessor has a total 74 different instructions for performing

different operations or tasks.

The entire different instructions that a particular microprocessor can handle is

called its instruction set.
9. What an instruction consists of?
ANS:

An instruction consists of an operation code (called opcode) and the address of

the data (called operand), on which the opcode operates.

Operation code(op code)	Address of data (operand)
Field1	field 2

10. In how many groups can the signals of 8085 be classified?
ANS:

The signals of 8085 can be classified into seven groups according to their

functions. These are:

(a) Power supply and frequency signals
(b) Data and Address buses
(c) Control bus
(d) Interrupt signals
(e) Serial I/O signals
(f) DMA signals
(g) Reset signals

11. What is the operating frequency of 8085?
ANS;

8085 operates at a frequency of 3 MHZ, and the maximum frequency of

operation is 500 KHz.

The versions 8085 A-2 operates at a maximum frequency of 5 MHz

12. What is the purpose of CLK signal of 8085?
ANS;

The CLK (out) signal obtained from pin 37 of 8085 is used for synchronizing

external devices.
13. What are the temporary registers of 8085?
ANS:
The temporary registers of 8085 are temporary data register W and Z registers.
These registers are not available to the programmer, but 8085 uses them internally to
hold temporary data during execution of some instructions.
14. In what other way HL pair can be used?
ANS:

HL register pair can be used as a data pointer or memory pointer.

15. Describe the Status Register of 8085?
ANS:

Explain the Flag registers and the structure.

16. What is the purpose of increment/decrement address latch register?
ANS:

These 16-bit registers increment/decrement the contents PC or SP when

instructions related to them are executed.
17. What is an instruction?
ANS:

An instruction is a command which asks the microprocessor to perform a

specific task or job.
18. What is meant by instruction set?
ANS:

The entire different instructions that a particular microprocessor can handle is

called its instruction set.
19. What are the different types of data transfer operations possible?
ANS:

The different types of data transfer operations possible are cited below;

 Between two registers.
 Between a register and a memory location.
 A data byte can be transferred between a register and a memory location.
 Between an I/O device and the accumulator.
 Between a register pair and the stack.
The term ‘data transfer’ is a misnomer – actually data is not transferred, but copied from
source to destination.

20. Explain the two instructions (a) LDAX and (b) STAX
ANS;

The instruction LDAX indicates that the contents of the designated register
pair point to a memory location and copies the content of the memory location into the
accumulator.
As an Example: let D=40H, E= 50H and memory location 4050H =AB H. Then LDAX D
transfers the contents of memory location 4050H to the accumulator. Thus, after the
execution of instruction, ACC = AB H.
Register contents                                     Memory                             Register contents after
Before execution                                                    location                                          instruction

A	XX	XX	F						A	AB	XX	F
D	40	50	E			4050	AB		D	40	50	E

STAX stands for ‘store accumulator indirect’. The contents of the accumulator are copied

into the memory location specified by the contents of the register pair.

As an example, let D=40 H and E = 50 H and ACC = AB H. Then STAX D stores the

accumulator contents in the memory location 4050 H.

21. What happens when HLT is executed in software?

ANS:

All the buses go into tri-state on execution of HLT instruction.

22. Explain DAA instruction with example?

ANS:

It stands for ‘decimal adjust accumulator’.

Execution of DAA instruction converts the contents of the accumulator into

two BCD values. The system utilities the AC flag (not accessible by the programmer, but

is used internally for DAA operation) for this conversion by the following the procedure

stated below,

(a) If the lower order 4-bits (D3 – D0) of the accumulator is greater than 910 or if the AC

flag is set, then this instruction (i.e., DAA) adds 0610 to the low-order 4-bits.

(b) If the higher order 4-bits (D7 - D4) of the accumulator is greater than 910 or if the CY

flag is set, then this instruction (i.e., DAA) adds 6010 to the high –order 4 – bits.

EXAMPLE: Discussed in the class

23. What the instruction DCR M stands for?

ANS:

Here the memory location M is pointed to by the content of HL register pair.

On executing DCR M, the content of the memory location is decremented by 01. Thus, if

the memory location, as pointed to by HL register pair is AB H, then DCR M makes the

content of that memory location to be AA H. 

25. What is stack?

ANS:

A stack is a group of memory locations in the R/ W.

26. How the stack is intialised?

ANS:

The stack is initialised by a 16 – bit register, called the stack pointer (SP)

register.

27. What type of memory is the stack?

ANS:

Stack is a ‘last-in first-out’ or LIFO type memory. This means that data which

is pushed last into stack is popped out of it first.

28. How the stack is initialised?

ANS:

The stack is initialised by means of the stack pointer. The software instruction

is like this: LXI SP, 0033H.

It means that the stack is initiated at the memory location 3300 by the stack pointer.

29. By how many memory locations SP is decremented / incremented when PUSH/ POP

instruction is performed?

ANS:

SP is decremented/ incremented by the two memory locations when PUSH/

POP is executed.

30. What is a subroutine?

ANS:

A subroutine is a group of instructions, written separately from the main

program, which performs a function that is required repeatedly in the main program.

31. Why a subroutine is used in a program?

ANS:

Since a subroutine is called more than once by the main program, thus, use of

subroutines saves subroutine, hexa-decimal converter subroutine, display subroutine is

called by the main program, the more is the saving of memory space.

32. Explain nesting of subroutines.

ANS:

The process of a subroutine calling a second subroutine and the second

subroutine in its turn calling a third one and so on is called nesting of subroutine.

Theoretically speaking, the number of subroutines that can be called by this

process is infinite but, in practice it is limited by the size of memory.

EXAMPLE:

33. Write a program which will store the contents of accumulator and flag register at

2000H and 2001H memory locations respectively. (Using PUSH & POP instructions)

ANS:

The following is the program for the above, using PUSH – POP instructions.

LXI SP, 4000 H: Accumulator

PUSH PSW: Accumulator and flag register contents pushed into stack

POP B: Accumulator content goes to B and flag register content goes C register

MOV A, B: Content of B taken to accumulator

STA 2000H: Accumulator content stored into memory location 2000 H

MOV A, C: Content of C moved to accumulator

STA 2001H: Accumulator content stored into memory location 2001 H

HLT: Program halted

34. What is the function of S0 & S1.

S1	S0	OPERATION
0	0	HALT
0	1	WRITE
1	0	READ
1	1	FETCH

35. Explain different between JMP and CALL instructions?

ANS:

WEKIPIDIA

36. Why do you use XRA A instruction?

ANS:

The XRA A instruction is used to clear the contents of the Accumulator

and store the value 00H.

37. Compare CALL and PUSH instructions?

ANS:

CALL	PUSH
When CALL is executed themicroprocessor automatically stores the16-bit address oF the instruction next toCALL on the stack.	The programmer uses the instructionPUSH to save the contents of the registerpair on the stack.
When CALL is executed the stack pointeris decremented by two.	When PUSH is executed the stackpointer register is decremented by two.

38. Compare RET and POP instructions?

ANS:

RET	POP
RET transfers the contents of the top twolocations of the stack to the PC.	POP transfers the contents of the top twolocations of the stack to the specified registerpair
When RET is executed the SP is incrementedby two.	When POP is executed the SP is incrementedby two.
Has 8 conditional RETURN instructions.	No conditional POP instructions.

39. Write the Evolution of Microprocessors?

ANS:

SEARCH WEKIPEDIA

40. What is memory capacity? How it can be expressed?

ANS;

Memory capacity is specified with the product of address (i.e. 2n. Where ‘n’ is

the number of address bit) and data line.

(a) Generally memory capacity can be expressed as,

Memory capacity = Address * Data

(b) The standard expressions are, 1 KB, 2 KB, 16 KB etc.

41. How memory address lines are calculated for a given memory locations?

ANS:

The following procedure are used for calculation of memory address line

i.e. log (2x) = log (memory locations)

=> x log (2) = log (memory locations)

=> x = log (memory location) / log (2)

42. What is memory capacity? How it can be expressed?

ANS:

Memory capacity is specified with the product of address (i.e. 2n. Where ‘n’ is

the number of address bit) and data line.

(a) Generally memory capacity can be expressed as,

Memory capacity = Address * Data

(b) The standard expressions are, 1 KB, 2 KB, 16 KB etc.

43. Write the comparison between static RAM & Dynamic RAM.

ANS:

SEARCH WEKIPEDIA

44. Explain in brief the difference Read Only Memory.

ANS:

The different kinds of Read only Memory is given below,

(a) PROM (b) EPROM (c) EEPROM (d) MASKED ROM (e) FLASH MEMORY

Write two points for each ROM.

45. (Write an assembly language program)

Ten number 8-bit data are stored starting from memory location 2100H. Transfer this

entire block of data to memory location starting from 3100H.

ANS:

MVI B, 0A H

LXI H, 2100 H

LXI D, 3100 H

LOOP: MOV A, M

STAX D

INX H

INX D

DCR B

JNZ LOOP

HLT

ASST. PROF. SATYANARAYAN PRADHAN

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