Questions -

Question 1: A digital computer has a memory unit with 16 bits per word. The instruction set consists of 122 different operations. All instructions have an operation code part (opcode) and an address part (allowing for only one address). Each instruction is stored in one word of memory.

1. How many bits are needed for the opcode?

2. How many bits are left for the address part of the instruction?

3. What is the maximum allowable size for memory?

4. What is the largest unsigned binary number that can be accommodated in one word of memory?

Question 2: Consider an instruction Add 1000. Memory contents, Accumulator (AC), and R1 Register contents are depicted in the below figure: (such as memory address 1000 contains a value 1400, memory address 1100 contains a value 400 etc. All the numbers used here are decimals.)

Assuming R1 is implied in the indexed addressing mode, determine the actual value loaded into the accumulator (AC) using the following addressing modes:

1. Immediate

2. Direct

3. Indirect

4. Indexed

Question 3: More registers appears to be a good thing, in terms of reducing the total number of memory accesses a program might require. Using the arithmetic expression S = (A+B)-(C+D), support this statement. [Hints: First, determine the number of memory accesses necessary using MARIE and the two registers for holding memory data values (AC and MBR). Then perform the same arithmetic computation for a processor that has more than three registers (for example, R1, R2, R3, R4) to hold memory data values.]

Question 4: Assemble the MARIE program below.

Hex Address     Label               Instruction

100                  Start               LOAD A

101                                         ADD B

102                                         STORE D

103                                         CLEAR

104                                         OUTPUT

105                                          ADDI D

106                                         STORE B

107                                           HALT

108                   A,                  HEX 00FC

109                   B,                    DEC 14

10A                   C,                  HEX 0108

10B                   D,                  HEX 0000

a) List the hexadecimal code for each instruction.

b) Draw the symbol table.

c) What is the value stored in the AC when the program terminates?

Rationale - This assessment task will assess the following learning outcome/s:

• be able to investigate the internal operation of the Central Processing Unit (CPU) and describe how it is used to execute instructions.
• be able to investigate and describe in detail the essential elements of computer organisation including internal bus, memory, Input/Output ( I/O) organisations and interfacing standards and discuss how these elements function.

This assessment task covers topics on CPU operation and Instruction Set Architecture, and has been designed to ensure that you are engaging with the subject content on a regular basis.