CPU Organization and Instruction Set (Registers, Addressing Modes) : MCQs & PYQs with Explanation

Option B

Pipelining is a technique in which a processor overlaps multiple stages of instruction execution. While one instruction is being executed, another instruction can be fetched and a third one can be decoded at the same time.
This means different stages like Fetch → Decode → Execute are performed simultaneously for different instructions, increasing CPU efficiency.

• Superscaling means executes multiple instructions in parallel using multiple pipelines, not stage overlapping.

Option A

In assembly language, addressing mode specifies how the operand is provided to the instruction.

In Immediate Addressing Mode, the operand is a constant value that is directly given in the instruction itself. It is not stored in a register or memory location.
In instruction MOV R1, #35H, #35H represents a constant hexadecimal value (35H). The # symbol indicates that the value is immediate data. This is immediate addressing mode because the value is directly provided in the instruction.

Option A

IA-32 (Intel Architecture, 32-bit) is a widely used computer architecture known for its support of a wide range of data types such as bytes (8 bits), words (16 bits), double words (32 bits), and quad words (64 bits).
It defines various data sizes and supports complex instruction sets for handling these different types efficiently, making it versatile across many computing applications.

Option B

An ‘m’ bit microprocessor means that it can process m bits of data at a time. This directly relates to the size of its data register, which must be capable of holding m bits.

Option D

Execution Time = (Number of Instructions) / (Instructions Per Second)
Number of instructions for program P= 2n²
Given, n=10000
Number of instructions = 2*(10000)² = 2*10⁸
Speed of computer is given as 10⁶ instructions per second.
Execution Time = (2*10⁸) / (10⁶) = 200 seconds

Option A

The Memory Address Register (MAR) is used to hold the address of the data that needs to be fetched from or stored to memory, including secondary memory locations during data transfer operations. 
When the CPU wants to fetch data from secondary memory, it places the address of the required data in the MAR, which then facilitates the transfer via the system bus.

• PC (Program Counter) keeps track of the next instruction.
• IR (Instruction Register) holds the current instruction.
• MBR (Memory Buffer Register) temporarily holds the data being transferred to or from memory.

Option C

A zero address instruction format means that the instruction does not explicitly specify any operands. Instead, all operations are performed using a stack, where operands are implicitly taken from the top of the stack.
In a stack-organized architecture, instructions like ADD automatically use the top elements of the stack, so no address fields are required.

Option A

In a one-address instruction format, one operand is explicitly specified, while the other operand is assumed to be in the Accumulator (AC). Operations are performed using the accumulator, and intermediate results are often stored in memory.
The given expression is Y=(A-B)/(C+D*E).
To evaluate this expression, we first compute the denominator (C+D*E), store it temporarily, then compute the numerator (A-B), divide the numerator by the stored denominator, and finally store the result in Y.

Execution Table:

Option B

In microprocessors like 8085, interrupts can be maskable or non-maskable. A Non-Maskable Interrupt (NMI) cannot be disabled or ignored by the processor and has the highest priority.
TRAP is a non-maskable interrupt, meaning:

• It cannot be masked (disabled).
• It is used for critical events like power failure.

Option C

In an instruction format, the total instruction size is fixed, and it must accommodate:

• Opcode to identify the instruction.
• Register operands.
• Immediate operand.

The instruction length is 32 bits. To represent 56 instructions, 6 bits are required for the opcode. 
Since there are 32 registers, each register needs 5 bits, so two registers require 10 bits. The remaining bits for the immediate operand are 32-(6+10) = 16 bits. As the immediate value is unsigned, its maximum value is 2¹⁶-1 = 65535.

Option D

The Program Counter (PC) is a special register in a processor that stores the address of the next instruction to be executed.
Normally, the Program Counter increases sequentially after each instruction. But some instructions change the normal flow of execution by loading a new address into the Program Counter.
The instructions that directly affect the Program Counter are:

• CALL
• JUMP (JMP)

These instructions transfer control from one location of the program to another.

• The CALL instruction is used to transfer program control to a subroutine or function.
• The JUMP instruction is used to transfer program control directly from one location to another location in the program.

Option C

In a computer, the operation code (opcode) is the part of an instruction that specifies which operation the CPU should perform, such as addition, subtraction, or data transfer.
If an opcode contains n bits, then each bit can have two possible values either 0 or 1. Therefore, the total number of different combinations possible with n bits is 2ⁿ.
Each unique combination can represent a different operation. Hence, a computer can perform 2ⁿ different operations.
For example, if value of n is 3 then total operations = 2³ = 8.

Option B

When the CPU starts fetching or decoding the next instruction while the current instruction is still executing, this overlapping process is called Pipelining. It helps the CPU work faster by using its time efficiently.

Option C

In PCB (Printed Circuit Board) manufacturing, different technologies are used to mount electronic components on the board. The cost of production depends on the manufacturing process, equipment required, assembly complexity, and component handling.
Through Hole Technology (THT) is considered the least expensive and simplest technology for basic PCB design and assembly, especially for small-scale production and educational projects.
In this technology, the leads of components are inserted through holes drilled in the PCB and then soldered on the opposite side.
In Surface Mount Technology (SMT), the components mounted directly on PCB surface and cost is high.

Option A

The register that keeps track of the address of the next instruction to be executed is the Program Counter (PC). After one instruction is executed, the PC is automatically updated to point to the next instruction in memory.