Jawaharlal Nehru Technological University Hyderabad B.Tech II COMPUTER ORGANIZATION AND MICROPROCESSOR Previous Question Paper (September - 2021 - R18)

A computer is a digital system that processes data according to a set of instructions. Its basic operational concepts revolve around the interaction between the Central Processing Unit (CPU), memory, and input/output (I/O) devices.

The fundamental operations include:

• Input: The computer accepts data and instructions from external devices like keyboards or sensors.
• Processing: The CPU performs arithmetic and logical operations on the input data. This involves fetching instructions from memory, decoding them, and executing them.
• Storage: Data and instructions are stored in memory (RAM) for immediate access or in secondary storage (HDD/SSD) for long-term retention.
• Output: The processed results are communicated to the user through output devices like monitors or printers.

The flow of information is managed by the control unit, which coordinates the activities of all components. The system operates through the fetch-decode-execute cycle, where instructions are retrieved from memory, interpreted, and then carried out by the arithmetic logic unit (ALU) or control unit.

Block Diagram of a Digital Computer:

The CPU acts as the brain, containing the ALU for data manipulation and the control unit for system coordination. The memory unit stores both the program instructions and the data being processed. The input and output units provide the interface between the computer and the external environment, allowing for data exchange via system buses (data, address, and control buses).

The 8086 microprocessor instruction set is categorized into several functional groups. Each instruction follows a specific execution cycle involving fetch, decode, and execute phases.

1. Data Transfer Instructions: These move data between registers, memory, and I/O ports. Example: MOV AX, BX copies the contents of BX to AX.

2. Arithmetic Instructions: These perform operations like addition, subtraction, multiplication, and division. Example: ADD AL, 05H adds 5 to the AL register and updates flags.

3. Bit Manipulation Instructions: These include logical (AND, OR, XOR, NOT) and shift/rotate operations. Example: AND BL, 0FH masks the upper four bits of the BL register.

4. String Instructions: These operate on blocks of memory. Example: MOVSB moves a byte from the source index (SI) to the destination index (DI).

5. Program Execution Transfer Instructions: These change the flow of the program using jumps, calls, and returns. Example: JMP label performs an unconditional jump to the specified address.

6. Processor Control Instructions: These manipulate the processor flags or state. Example: STC sets the carry flag to 1.

The execution of these instructions involves the 8086 Bus Interface Unit (BIU) fetching the opcode from memory into the instruction queue, while the Execution Unit (EU) decodes and performs the operation. For instance, in an ADD instruction, the EU retrieves the operands, performs the binary addition in the ALU, updates the status flags in the FLAGS register, and stores the result in the destination operand.