## A Level January, 2013 A4-R4: COMPUTER SYSTEM ARCHITECTURE

TOTAL TIME: 3 HOURS TOTAL MARKS: 100
(PART ONE – 40; PART TWO – 60)
PART ONE

1. Each question below gives a multiple choice of answers. Choose the most appropriate one and enter in the “tear-off” answer sheet attached to the question paper, following instructions therein. (1x10)
1.1 Conversion of hexadecimal number 6B2 to its binary number equivalent is
A) 111100011011
B) 011011010101
C) 011010110010
D) 011011000010
1.2 Which of the following registers is used to keep track of the address of the next instruction?
B) Instruction Register
C) Program Counter
D) Memory Data Register

1.3 The meaning of the instruction ISZ Y (increment and skip if zero) is
A) Increment the content of location Y by one, if the result is zero, skips the next instruction.
B) Increment the locations Y by one if it is zero then skip the next instruction
C) Increment the content of accumulator by one, if it is zero then skip to the location Y
D) None of the above
1.4 A better way to describe the operation of multiplexers is by means of a
A) Function Table
B) Truth Table
C) Symbol Table
D) Relation Table
1.5 Simplified form for Boolean Algebraic Expression F= ABC+ABC’+A’C (where A’ means
complement of A) is
A) BC+A’C
B) AB+A’C
C) AC+AC’
D) None of the above
1.6 A memory unit has 64 K words and each word is of 8 bits. How many address lines will be
required in this case?
A) 16
B) 19
C) 6
D) 3
1.7 An arithmetic shift-left is equivalent to
A) Multiplying the number by 2
B) Dividing the number by 2
C) Changing the sign of the number
D) Reversing the number
1.8 In which addressing mode the operand is included in the instruction format?
1.9 An instruction at address 016 with I=0, Opcode is of LDA and address part= 078, memory
content at 078 = C8D2 and AC= B8E7. What will be the contents of AC, after the execution?
A) C8D2
B) A842
C) B8E7
D) F649
1.10 Solving the given Karnaugh’s map results in expression
A’ represents the complement of A.
A) A’+B
B) A+B’
C) A+B
D) A’+B’

2. Each statement below is either TRUE or FALSE. Choose the most appropriate one and ENTER in the “tear-off” sheet attached to the question paper, following instructions therein. (1x10)

2.1 Representation of the decimal number -16 in binary using two’s complement is 11110010.
2.2 Making modifications are difficult in micro programmed control.
2.3 Control unit of the basic computer consists of two decoders, two sequence counters and a
number of control logic gates.
2.4 During cycle stealing by DMA controller CPU remains inactive.
2.5 A decoder is a sequential circuit.
2.6 A.A’=0
2.7 The + symbol appearing in the following statement has the same meaning.
P+Q: R1 R2+R3 where P and Q are two binary variables of a control function.
2.8 Synchronous devices use same clock pulses.
2.9 Handshaking is a method of transmitting control signals between two independent units like
CPU and I/O interface.
2.10 A stack-organized computer uses one address instruction format.

3. Match words and phrases in column X with the closest related meaning/word(s)/phrase(s) in column Y. Enter your selection in the “tear-off” answer sheet attached to the question paper, following instructions therein. (1x10)

#### X                                                                  Y

3.1 STA instruction                                                                     A. Content Addressable
Memory
3.2 2n input and one output                                                       B. Combinatorial Circuit
3.3 Allows the execution of a program of size larger                C. D7
than the available memory
3.4 This changes by only one bit as it sequences from               D. CISC
one number to the next
3.5 Flip Flop                                                                                E. Moves information from
Accumulator to memory
3.6 A computer with large number of instructions                     F. Gray Code
3.7 It is used to increase the speed of processing                      G. Virtual memory
3.8 Determines the type of instruction in the decode                H. 1110001
phase of the instruction cycle
3.9 Associative Memory                                                           I. Cache Memory
3.10 For every operation code the control unit issues              J. 16
K. Multiplexer
L. A sequence of micro
operations
M. Auxiliary memory

4. Each statement below has a blank space to fit one of the word(s) or phrase(s) in the list below. Enter your choice in the “tear-off” answer sheet attached to the question paper, following instructions therein. (1x10)

A. 10010111                          B. Control Function                                  C. CLA
D. Memory mapped              E. Z=0                                                       F. Adder-Subtractor
G. Flip-flops                          H. 217                                                       I. CMA
J. Interrupt driven                  K. SIMD                                                   L. 01100001
M. MIMD

4.1 The storage elements employed in clocked sequential circuits are called ________.
4.2 Binary Coded Decimal representation of decimal number 97 is ________.
4.3 If the word size of memory is 2 bytes and address is of 16 bits, then the size of memory is
________ bytes.
4.4 P is a ________ in the register transfer operation P: R2 R1.
4.5 Including an exclusive-OR gate with each full-adder results in ________.
4.6 The condition tested for Branch if not zero(BNZ) is ________.
4.7 ________ represents an organization that includes many processing units under the
supervision of a common control unit.
4.8 In assembly language instruction field for complement accumulator is ________.
4.9 ________ I/O is better than programmed I/O.
4.10 ________ I/O does not distinguished between memory and I/O addresses.
A4-R4 Page 5 of 5 January, 2013
PART TWO
5.
a) Perform the operation 13250 – 72532 of two unsigned decimal numbers using 10’s
complement. Explain each step clearly.
b) The content of a 4-bit register is initially 1101. The register is shifted six times to the right
with the serial input being 101101. What is the content of the register after each shift?
c) Draw the logic diagram of a 2-to-4 line decoder using only NOR gates. Include an enable
input. Write the complete truth table also.
(4+4+7)
6.
a) Given the Boolean function F= xy’z+x’y’z+xyz. Simplify the algebraic expression using
Boolean algebra and draw the logic diagram from the simplified expression. Find out the
reduction in number of gates.
b) The 8-bit registers AR, BR, CR and DR initially have the following values:
AR=11110010 BR= 11111111 CR=10111001 DR=11101010
Determine the 8-bit values in each register after the execution of the following sequence of
micro operations
AR AR+BR Add BR to AR
CR CR ^ DR, BR BR+1 AND DR to CR, increment BR
AR AR – CR Subtract CR from AR
(9+6)
7.
a) With the help of flow chart explain how the control unit determines that a given instruction is
a memory reference instruction, register reference instruction or input output instruction.
b) The memory unit of a computer has 256K words of 32 bits each. The computer has an
instruction format with four fields: an operation code field, a mode field to specify one of 7
addressing modes, a register address field to specify one of 60 processor registers and a
memory address. Specify the instruction format and the number of bits in each field if the
instruction is in one memory word.
(9+6)
8.
a) Explain the following terms in the context of Cache Memory:
i) Locality of reference
ii) Hit/Miss Ratio
iii) Mapping
b) How many 128x8 RAM chips are needed to provide a memory capacity of 2048 bytes?
Explain clearly.
c) Differentiate between programmed and interrupt driven I/O.
(6+4+5)
.
9.
a) Write 8086 assembly language instructions to evaluate the arithmetic expression 5+(6-2)
leaving the result in ax using (i) 1 register, (ii) 2 registers.
b) Write short notes on:
i) DMA
ii) Asynchronous Data Transfer
(8+7)