WAEC SSCE - Computer Studies - 2018 (Essay)

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(a) What is a register?

A register is a small, very high-speed temporary storage location built inside the Central Processing Unit (CPU). It holds a single piece of data, an instruction or an address that the processor is currently working on, so that it can be accessed almost instantly during processing.

(b) Two types of register:

• Accumulator (ACC)
• Program Counter (PC)

Other acceptable examples: Memory Address Register (MAR), Memory Data/Buffer Register (MDR/MBR), Instruction Register (IR), Index Register.

(c) Three functions of registers:

1. They temporarily hold data and instructions that are being processed by the CPU.
2. They store the memory address of the next instruction or of the data to be fetched (for example the Program Counter and Memory Address Register).
3. They hold the intermediate and final results of arithmetic and logic operations (for example the Accumulator).

(d) Three differences between a register and main memory:

(Any three of the above differences are acceptable.)

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(a) A logic gate is an electronic circuit that receives one or more binary inputs, 0 or 1, and produces a single binary output according to a specified logical rule.

(b)

1. Standard logic gate: AND gate.
2. Alternative logic gate: NAND gate.

(c)(i) Logic symbol for an XOR gate

The XOR gate gives an output of 1 only when the two input values are different.

R = P ⊕ Q

(c)(ii) Truth table for the XOR logic gate

Inputs		Output
P	Q	R = P ⊕ Q
0	0	0
0	1	1
1	0	1
1	1	0

(d) Uses of logic gates:

1. They are used in arithmetic and logic units, such as binary adders in computers.
2. They are used in control and decision-making circuits, for example traffic-light controllers and burglar alarms.

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(a)(i) Define the term computer system.

A computer system is a complete set of interconnected components (hardware, software, data and people) that work together to accept data as input, process it under the control of stored instructions, and produce information as output, as well as store the data and results for future use.

(a)(ii) Major components of the computer system, with one example each:

• Hardware - the physical parts of the computer. Example: keyboard, monitor or CPU.
• Software - the programs and instructions that direct the hardware. Example: Microsoft Windows (operating system) or MS Word.
• Data/Information - the raw facts fed in and the processed results. Example: a list of students' scores.
• People (Users/Liveware) - the human beings who operate and use the system. Example: a computer operator or programmer.

(Where "components" is taken narrowly as hardware units: Input unit - keyboard; Central Processing Unit - microprocessor; Output unit - monitor; Storage unit - hard disk.)

(b)(i) Two activities involved in transferring data into information:

• Processing the data (for example sorting, calculating, classifying or summarising it).
• Organising and presenting the results in a meaningful, useful form (for example arranging into a report or table).

Other acceptable activities: collecting/inputting data, editing/validating, and storing.

(b)(ii) Acronyms in full:

• EDSAC - Electronic Delay Storage Automatic Calculator.
• EDVAC - Electronic Discrete Variable Automatic Computer.

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(a) Definitions

(i) An array: An array is a data structure that holds a fixed number of data items of the same data type under a single variable name, with each item (element) identified by an index or subscript. For example, one array can store many students' scores, referred to as SCORE(1), SCORE(2), and so on.

(ii) A syntax error: A syntax error is a programming error that occurs when a statement breaks the grammatical rules of the programming language (for example a misspelt keyword, a missing bracket or a wrong statement structure). Such errors are detected by the translator (compiler or interpreter) and prevent the program from running until corrected.

(b) QBASIC program: multiples of three between 1 and 20 into a one-dimensional array

The multiples of three between 1 and 20 are 3, 6, 9, 12, 15 and 18 (six values).

REM Store multiples of 3 between 1 and 20 in an array
CLS
DIM M(6)
K = 0
FOR N = 1 TO 20
    IF N MOD 3 = 0 THEN
        K = K + 1
        M(K) = N
    END IF
NEXT N
PRINT "Multiples of three are:"
FOR I = 1 TO K
    PRINT M(I)
NEXT I
END

The loop tests each number from 1 to 20; when a number divides exactly by 3 (N MOD 3 = 0) it is stored in the next array position and later displayed.

(c) Convert \(D5FA_{16}\) to binary

Replace each hexadecimal digit with its 4-bit binary equivalent:

\[D = 1101,\quad 5 = 0101,\quad F = 1111,\quad A = 1010\]

\[\therefore D5FA_{16} = 1101\,0101\,1111\,1010_2 = 1101010111111010_2\]

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(a)(i) What is booting?

Booting is the process of starting up a computer, during which the operating system is loaded from secondary storage into main memory (RAM) so that the computer becomes ready for use. It begins when power is switched on and the system runs its start-up instructions.

(a)(ii) Two differences between the types of booting (cold vs warm booting):

(Any two differences are acceptable.)

(b) What is a peripheral?

A peripheral is any device that is connected to and works with the computer but is not part of the central processing unit. Peripherals are used to put data into the computer, get results out, or store data (for example keyboard, printer, monitor and external disk).

(c) One example each:

• (i) Input device: keyboard (also mouse, scanner).
• (ii) Output device: monitor (also printer, speaker).
• (iii) Storage device: flash drive (also hard disk, CD/DVD).
• (iv) A component found inside the system unit: the motherboard (also CPU, RAM, power supply).