Jawaharlal Nehru Technological University Hyderabad B.Tech IV MICROPROCESSORS AND MICROCONTROLLERS Previous Question Paper (September - 2022 - R16)

The 8086 architecture is divided into two independent functional units: the Bus Interface Unit (BIU) and the Execution Unit (EU). This separation allows for instruction pipelining, where the BIU fetches instructions while the EU executes them, increasing overall throughput.

The Bus Interface Unit (BIU) is responsible for all external bus operations. It contains the segment registers (CS, DS, SS, ES), the Instruction Pointer (IP), the address adder, and a 6-byte instruction prefetch queue. The BIU fetches instructions from memory, reads/writes data to ports and memory, and calculates the 20-bit physical address using the formula:

The Execution Unit (EU) fetches instructions from the BIU's queue and executes them. It contains the Arithmetic Logic Unit (ALU), the General Purpose Registers (AX, BX, CX, DX), Pointer and Index registers (SP, BP, SI, DI), and the Flag register. The control circuitry in the EU decodes instructions and generates the necessary control signals to perform operations.

The 8086 architecture is characterized by its 16-bit data bus and 20-bit address bus, allowing it to access 1 MB of memory. The pipelining mechanism ensures that the processor does not wait for memory access cycles during the execution of instructions.

Assembler directives are commands provided to the assembler program rather than the processor. They guide the assembler in the process of assembling the source code into machine code, defining data, allocating memory, and managing segments, but they do not generate executable machine instructions.

Common assembler directives include:

• DB (Define Byte): Allocates one or more bytes of memory and initializes them with specified values.
• DW (Define Word): Allocates one or more 16-bit words of memory.
• EQU (Equate): Assigns a specific value or constant to a label, which cannot be redefined later.
• ORG (Origin): Sets the starting address for the subsequent code or data in the memory segment.
• END: Indicates the end of the source program to the assembler.
• ASSUME: Tells the assembler which segment register to use for a specific logical segment.