Instruction pipelining and Queue Operation in 8086

Instruction pipelining

Instruction pipelining is a technique used in the 8086 microprocessor to improve its performance by executing multiple instructions simultaneously. The 8086 microprocessor has a five-stage instruction pipeline that allows it to overlap the execution of multiple instructions, thereby increasing its throughput.

The five stages of the instruction pipeline in the 8086 microprocessor are:

Instruction Fetch (IF) - In this stage, the instruction is fetched from the memory and loaded into the instruction register.

Instruction Decode (ID) - In this stage, the instruction is decoded to determine the operation to be performed and the operands required.

Execution (EX) - In this stage, the instruction is executed, and the result is stored in a temporary register.

Memory Access (MA) - In this stage, the operands required for the instruction are retrieved from the memory.

Write Back (WB) - In this stage, the result of the instruction is written back to the register or memory.

The instruction pipelining technique in the 8086 microprocessor allows multiple instructions to be executed simultaneously, thereby improving the performance of the processor. For example, while the processor is executing an instruction in the EX stage, the next instruction can be fetched and decoded in the IF and ID stages, respectively. This overlapping of instruction execution allows the processor to execute multiple instructions in a single clock cycle.

However, instruction pipelining can also introduce performance issues such as pipeline stalls and hazards. A pipeline stall occurs when the pipeline cannot continue execution due to a dependency between instructions. Hazards occur when one instruction relies on the result of a previous instruction that has not yet completed execution.

To address these issues, the 8086 microprocessor implements techniques such as instruction reordering and register renaming. These techniques allow the processor to continue execution even when there are dependencies between instructions, thereby improving its performance.

Queue Operation in 8086

In 8086 microprocessors, the queue operation is used to improve the performance of the memory access operation. The queue is a set of two memory locations that can hold the recently accessed memory addresses. The purpose of the queue is to allow faster access to recently used memory locations, thereby reducing the number of memory cycles required to access data.

When a memory read operation is performed, the queue stores the memory address of the data being read. If the next memory access is a read operation and the requested data is in the queue, then the data can be retrieved from the queue rather than accessing the memory, thus reducing the time required to fetch the data.

The queue operation is particularly useful in improving the performance of string operations such as MOVS, CMPS, and SCAS, where multiple memory accesses are required. By utilizing the queue, the number of memory accesses required can be reduced, resulting in faster string operations.

The queue operation in the 8086 microprocessor is controlled by the processor itself and does not require any specific programming or instructions. The queue size is two and the queue can store both code and data addresses. The queue can be disabled by setting the appropriate control bit in the control register.

Overall, the queue operation in the 8086 microprocessor is an effective technique to improve the performance of memory access operations, particularly in string operations.