Saturday, November 15, 2014

Previous GATE Questions on Microprocessors and Memory Mapping with Solutions (1987 - Till Date)


1.       The program given below is run on an 8085 based microcomputer system. Determine the contents of the registers: PC, SP, B, C, H, L after a halt instruction is executed.
Answer:  PC = 3001, SP = 0FFE, B = 20H, C = 0BH, H = 2FH and L = 38H


1.       In an 8085 microprocessor system with memory mapped I/O,
            a.       I/O devices have 16 bit addresses
            b.      I/O devices are accessed using IN and OUT instructions
            c.       There can be a maximum of 256 input devices and 256 output devices
            d.      Arithmetic and logic operations can be directly performed with the I/O data.
Answer:  A

2.       The following program is run on 8085 microprocessor:
 At the completion of the execution of the program, the program counter of the 8085 contains _____________ and the stack pointer contains _______________.
Answer: SP = 0FFC, PC = 200A

3.   Figure shows the memory circuit of 8085 microprocessor.
            a.       What is the total size of the memory in the circuit?
            b.      What are the beginning and ending addresses of the memory in chip 1?
            c.       What are the beginning and ending addresses of the memory in chip 2?
            d.      Are the memory chips in the circuit ROM or RAM?
            e.      How will you replace the two NAND gates I the circuit with one 3 to 8            decoder without changing the memory size or the memory addresses? Assume          that the decoder has one active high enable E1 and one active low enable E2.


1.       In a microprocessor, wait states are used to
            a.       Make the processor wait during a DMA operation
            b.      Make the processor wait during an interrupt processing
            c.       Make the processor wait during a power shutdown
            d.      Interface slow peripherals to the processor


2.        A microprocessor has five address lines [A4 – A0] and eight data lines [D7 – D0]. An input device A, an output device B, a ROM and a RAM are memory mapped to the microprocessor at the addresses as shown in figure. Devices A and B have four addressable registers each/ RAM has 8 bytes and ROM has 16 bytes.
            a.       Indicate the address lines to be connected to each device and memory.
            b.      Obtain the minimum sum of product expression for the chip select (CS)            function of each device/memory.


1.       When a CPU is interrupted, it
            a.       Stops executing of instructions
            b.      Acknowledges interrupt and branches of subroutine
            c.       Acknowledges interrupt and continues
            d.      Acknowledges interrupt and waits for the next instruction from the interrupt   device
Answer: B

2.       A ‘DMA’  transfer implies
            a.       Direct transfer of data between memory and accumulator
            b.      Direct transfer of data between memory and I/O devices without use of µP
            c.       Transfer of data exclusively within µP registers
            d.      A fast transfer of data between µP and I/O devices
Answer: B

3.       An ‘Assembler’ of a microprocessor is used for
            a.       Assembly of processors in a production line
            b.      Creation of new program’s using different modules
            c.       Translation of a program form assembly language to machine language
            d.      Translation of a higher level language into English text
Answer: C

4.   A hypothetical CPU has a parallel address bus, a parallel data bus, a RD and WR active LOW signals. Two ROMs of size 4K words each and two RAMs of sizes 16K and 8K words, respectively, are to be connected to the CPU. The memories are to be so connected that they fill the address space of the CU as per the memory map shown in the figure. Assuming tat chip select signals are active LOW.
            a.       What is the number of lines in the address bus of the CPU?
            b.      Determine the values of address X, Y, Z and W as decimal numbers.
            c.       Using a 2 to 4 decoder and some additional gates, draw a circuit for the           decoding logic.


1.       The total number of memory accesses involved  (inclusive of the op-code fetch), when an 8085 processor executes the instruction LDA 2003 is
            a.       1
            b.      2
            c.       3
            d.      4
Answer: D
               Solution :      

2.       The following sequence of instructions are executed by an 8085 microprocessor:
1000    LXI SP, 27FFH
1003    CALL 1006
1006    POP H
The contents of the stack pointer (SP) and the HL register pair on completion of execution of these instructions are
            a.       SP = 27FF,  HL = 1003  
            b.      SP = 27FD,  HL = 1003 
            c.       SP = 27FF,  HL = 1006 
            d.      SP = 27FD,  HL = 1006
Answer: C
               Solution :      


1.       In 8085 µP system, the RST instruction will cause an interrupt
            a.       Only if an interrupt service routine is not being executed
            b.      Only if a bit in the interrupt mask is made 0
            c.       Only if interrupts have been enabled by an EI instruction
            d.      None of the above

Answer:  C
                  Solution :      

2.       The decoding circuit is shown in figure, has been used to generated the active low chip select signal for a microprocessor peripheral (The address lines are designated as A0 to A7 for I/O addresses).
            a.       60H – 63H
            b.      A4 – A7H
            c.       30 – 33H
            d.      70 – 73H

Answer:  A
                  Solution :

3.   The following instructions have been executed by an 8085 µP
For which address will the next instruction be fetched?

            a.       6019
            b.      6379
            c.       6979
            d.      None of the above

Answer:  C
                  Solution :      

4.   A signed integer has been stored in a byte using the 2’s complement format. We wish to store the same integer in a 16 bit word. We should

Answer: C
                Solution :

5.   Match the following, while moving data between registers of the 8085 and the stack

Answer:  1 - c,  2 - d
                  Solution :      

6.   An 8085 µP uses a 2 MHz crystal. Find the time taken by it to execute the following delay subroutine, inclusive of the call instruction in the calling program.
       You are given that a CALL instruction takes 18 cycles of the system clock, PUSH requires 12 cycles and conditional jump takes 10 cycles if the jump is taken and 7 cycles if it is not. All other instructions used above take (3n + 1) clock cycles, where n is the number of accesses to the memory, inclusive of the opcode fetch.
Answer: 1.842 mSec
                Solution :  


1.       An I/O processor control the flow of information between
            a.       Cache memory and I/O devices
            b.      Main memory and I/O devices
            c.       Two I/O devices
            d.      Cache and main memory
Answer: B
                 Solution :     

2.       An instruction used to set the carry flag in a computer can be classified as
            a.       Data transfer
            b.      Arithmetic
            c.       Logical
            d.      Program control
Answer: C
                 Solution :    

3.   Write a short assembly language program, without using any arithmetic instruction, to store hexadecimal 5D in the flag register of 8085 microprocessor. Data in other registers of the processor must not alter upon executing this program.
                 Solution :          


1.       If CS = A15’A14A13 is used as the chip select logic of a 4K RAM in an 8085 system, then its memory range will be
            a.       3000 – 3FFFFH
            b.      7000 – 7FFFH
            c.       5000 – 5FFFH and 6000 – 6FFFH
            d.      6000 – 6FFFH and 7000 – 7FFFH
Answer: D
                 Solution :    

2.   An 8085 assembly language program  is given below.

Answer:  (a) this program will found the highest number present in the given memory location, finally that highest number will present in the accumulator.
(b) (i)  A = 2B,  B = 2B,  C = 00, HL = 2002
      (ii) CY  = 1 and ZF = 1
      (iii)  [2000] = 18H,  [2001] = 10H, [2002] = 2BH and [2100] = 2BH
                  Solution :      


1.       In the 8085 microprocessor, the RST 6 instruction transfers the program execution to the following location
            a.       30H
            b.      24H
            c.       48H
            d.      60H
            Answer: A
                 Solution :

2.       The number of hardware interrupts (which require an external signal to interrupt) present in an 8085 microprocessor are
            a.       1
            b.      4
            c.       5
            d.      13
Answer: C
                 Solution :

3.   The contents of register (B) and Accumulator (A) of 8085 microprocessor are 49H and 3AH respectively. The contents of A and the status of carry flag (CY) and sign flag (S) after executing SUB B instruction are
            a.       A = F1 , CY = 1 ,  S = 1
            b.      A = 0F, CY = 1,  S = 1
            c.       A = F0, CY = 0,  S = 0
            d.      A = 1F, CY = 1,  S = 1
Answer: A
                 Solution :

4.   The program and machine code for an 8085 microprocessor are given by
The starting address of the above program is 7FFFH. What would happen if it is executed from 8000H?

Answer: (a) if the program executes from 8000H, it will find an infinite loop with only one instruction "JMP 8000", an unconditional jump to memory location 8000.
(b) Number of machine cycles required for CALL 2000H and LDA 2000H are 5 and 4 respectively.
(c) XRA  A   and  SUB  A
                 Solution :   


1.       An 8085 microprocessor based system uses a 4K x8 bit RAM whose starting address is AA00H. The address of the last byte in this RAM is
            a.       0FFFH
            b.      1000H
            c.       B9FFH
            d.      BA00H
Answer: C
           Solution :


2.   Consider the following sequence of instructions for an 8085 microprocessor based system.
                a.   If the program execution begins at the location FF00H, write down the          sequence of instructions which are actually executed till a HLT instruction.        Assume all Flags are initially RESET.
                b.   Which of the three ports (PORT1, PORT2 and PORT3) will be loaded with             data, and what is the bit pattern of the data?
Answer: (b) port 2 with data 03H
                Solution :      


1.       Consider the following assembly language program.
MVI B, 87H
START:           JMP NEXT
MVI B, 00H
NEXT:            XRA B
The execution of the above program in an 8085 microprocessor will result in
            a.       An output of 87H at PORT1
            b.      An output of 87H at PORT2
            c.       Infinite looping of the program execution with accumulator data remaining at 00H
            d.      Infinite looping of the program execution with accumulator data alternating     between 00H and 87H
Answer: B
                 Solution :

2.   An 8085 microprocessor operating at 5 MHz clock frequency executes the following routine.
START:           MOV A, B
            a.       Determine the total number of machine cycles required to execute the routine till the JMP instruction is executed for the first time.
            b.      Determine the time interval between two consecutive (MEMW)’ signals.
            c.       If the external logic controls the READSY line so that three WAIT states are introduced in the I/O WRITE machine cycle, determine the time interval between      two consecutive (MEMW)’ signals.
Answer: (a)  21   (b)  41  (c) 44
                  Solution :    


1.   In an 8085 microprocessor, the instruction CMP B has been executed while the content of the accumulator is less than that of register B. As a result,
            a.       Carry flag will be set but Zero flag will be reset
            b.      Carry flag will be reset but Zero flag will be set
            c.       Both Carry flag and Zero flag will be reset
            d.      Both Carry flag and Zero flag will be set
Answer: A
                 Solution :


1.       The 8255 programmable peripheral interface is used as described below.
            (i)                  An A/D converter is interfaced to a microprocessor through an 8255. The   conversion is initiated by a signal from the 8255 on port C. A signal on port C. A            signal on port C causes data to be strobed into port A.
            (ii)                Two computers exchange data using a pair of 8255s. Port A works as a      bidirectional data port supported by appropriate handshaking signals.
            The appropriate modes of operation of the 8255 for (i) and (ii) would be
                        a.       Mode 0 for (i) and Mode 1 for (ii) 
                        b.      Mode 1 for (i) and Mode 2 for (ii)
                        c.       Mode 2 for (i) and Mode 0 for (ii)
                        d.      Mode 2 for (i) and Mode 1 for (ii)
Answer: B
                  Solution : 

2.   The number of memory cycles required to execute the following 8085 instructions
            (i)                  LDA 3000H
            (ii)                LXI D, F0F1H
Would be
            a.       2 for (i) and 2 for (ii) 
            b.      4 for (i) and 3 for (ii)
            c.       3 for (i) and 3 for (ii)
            d.      3 for (i) and 4 for (ii)
Answer: B
                  Solution : 

3.   Consider the sequence of 8085 instructions given below.
LXI H, 9258H
Which one of the following is performed by this sequence of instructions?
            a.       Contents of location 9258 are moved to the accumulator
            b.      Contents of location 9258 are compared with the contents of the accumulator
            c.       Contents of location 9258 are complemented and stored in location 9258
            d.      Contents of location 5892 are complemented and stored in location 5892.
Answer: C
                  Solution :

4.   It is desired to multiply the numbers 0AH by 0BH and store the result in the accumulator. The numbers are available in registers B and C respectively. A part of the 8085 program for this purpose is given below:
MVI A, 00H
LOOP: ..............
The sequence of instructions to complete the program would be
            a.       JNZ LOOP,          ADD B,                      DCR C
            b.      ADDB,                 JNZ LOOP,    DCR C
            c.       DCR C,                 JNZ LOOP,    ADD B
            d.      ADD B,                DCR C,                       JNZ LOOP
Answer: D
                  Solution :


Statement for linked Answer Questions 1 & 2 :
Consider an 8085 microprocessor system.
1.       The following program starts at location 0100H.
LXI H, 0701
MVI A, 20H
THE Contents of accumulator when the program counter reaches 0109H is
            a.       20H
            b.      02H
            c.       00H
            d.      FFH
Answer: C
2.       If in addition, following code exists from 0109H onwards.
What will be the result in the accumulator after executing the last instruction?
            a.       40H
            b.      20H
            c.       60H
            d.      42H
Answer: C
                 Solution (1 & 2):  


1.       An I/O peripheral device shown n figure (b) below is to be interfaced to an 8085 microprocessor. To select the I/O device in the address range D4H – D7H, its chip-select (CS’) should be connected to the output of the decoder shown in the figure (a) below :
            a.       Output 7
            b.      Output 5
            c.       Output 2
            d.      Output 0
Answer: B
                Solution :

2.       Following is the segment of a 8085 assembly language program
                        LXI SP, EFFFH
                        CALL 3000H
            3000H :  LXI H, 3CF4H
                          PUSH PSW
                          POP PSW
On the completion of RET instruction, the contents of SP is
            a.       3CF0H
            b.      3CF8H
            c.       EFFDH
            d.      EFFFH
Answer: B
                Solution :     


1.       An 8255 chip is interfaced to an 8085 microprocessor system as an I/O mapped I/O as shown in the figure. The address lines A0 and A1 of the 8085 are used by the 8255 chip to decode internally its three ports and the control register. The address lines A0 to A7 as well as the IO/M’ signal are used for address decoding. The range of addressees for which the 8255 chip would get selected is 

            a.       F8H – FBH
            b.      F8H – FCH
            c.       F8H – FFH
            d.      F0H – F7H
            Answer: C
               Solution :

Statement for the linked answer questions 2 & 3 :
An 8085 assembly language program is given below.
Line 1  MVI A, B5H
             2         MVI B, 0EH
             3         XRI 69H
             4         ADD B
             5         ANI 9BH
             6         CPI 9FH
             7         STA 3010H
             8         HLT

2.       The contents of the accumulator just after execution of the ADD instruction in line 4 will be
            a.       C3H
            b.      EAH
            c.       DCH
            d.      69H
Answer: B

3.   After execution of line 7 of the program, the status of the CY and Z flags will be ….. respectively.
            a.       0, 0
            b.      0, 1
            c.       1, 0
            d.      1, 1
Answer: C
               Solution (2 & 3): 


1.       An 8085 executes the following instructions

2710 LXI H, 30A0H
2713 DAD H
2714 PCHL

All addresses and constants are in Hexa decimal. Let PC be the contents of the program counter and HL be the contents of HL register pair just after executing PCHL.
Which of the following statements is correct?

            a.       PC = 2715H  and HL = 30A0H 
            b.      PC = 30A0H  and HL = 2715H
            c.       PC = 6140H  and HL = 6140H
            d.      PC = 6140H and HL = 2715H
Answer: C
                Solution :



1.       In a microprocessor, the service routine for a certain interrupt starts from a fixed location of memory which cannot be externally set, but the interrupt can be delayed or rejected. Such an interrupt is
            a.       non-maskable and non-vectored
            b.      maskable and non-vectored
            c.       non-maskable and vectored
            d.      maskable and vectored
Answer: D
               Solution :


1.       For the 8085 assembly language program given below, the content of the accumulator after the execution of the program is

3000    MVI A, 45H
3002    MOV B, A
3003    STC
3004    CMC
3005    RAR
3006    XRA B

            a.       00H
            b.      45H
            c.       67H
            d.      E7H
Answer: C
               Solution :


1.       An 8085 assembly language program is given below. Assume that the carry flag is initially unset. The contents of the accumulator after the execution of the program is

MVI A, 07H

            a.       8CH
            b.      64H
            c.       23H
            d.      15H
Answer: C
               Solution :


1.       For 8085 microprocessor, the following program is executed.
    MVI A,05H
             MVI B, 05H
          DCR B
              JNZ PTR
          ADI 03H
At the end of the program, accumulator contains
            a.       17H
            b.      20H
            c.       23H
            d.      05H
Answer: A
               Solution :


1.       For the 8085 microprocessor, the interfacing circuit to input 8 – bit digital data (DI0 – DI7) from an external device is shown in figure. The instruction for correct data transfer is 
            a.       MVI A, F8H
            b.      IN F8H
            c.       OUT F8H
            d.      LDA F8F8H
Answer: D
                 Solution :

2.       An 8085 microprocessor executes “STA 1234H” with starting address location 1FFEH (STA copies the contents of the accumulator to the 16 bit address location). While the instruction is fetched and executed, the sequence of values written at the address pins A15 -  A8 is
Answer: A
                Solution :

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