Intel 8088 Instruction Queue
The Intel 8088 is a variant of the Intel 8086 microprocessor, introduced on June 1, 1979. One of the distinctive features of both the Intel 8086 and 8088 is their use of an instruction queue, specifically known as the Prefetch Input Queue. This component plays a crucial role in the performance and efficiency of these early microprocessors.
Prefetch Input Queue in Intel 8088
The prefetch input queue in the Intel 8088 is a four-byte queue, in contrast to the six-byte queue of the Intel 8086. This queue allows the microprocessor to fetch instructions from memory ahead of their execution, storing them temporarily. This mechanism is part of a broader strategy known as instruction pipelining, which is designed to improve the efficiency and throughput of the CPU by overlapping the fetching, decoding, and execution stages of instruction processing.
Functionality
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Instruction Fetching: The CPU fetches instructions from the system memory. In the case of the Intel 8088, the instructions are fetched in 8-bit chunks due to its 8-bit external data bus, even though it processes instructions internally as 16-bit.
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Queue Storage: These fetched instructions are stored in the prefetch input queue. For the Intel 8088, this queue can hold up to four bytes of instructions at a time. This contrasts with the Intel 8086, where the queue can hold up to six bytes, reflecting the 8086's 16-bit external data bus.
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Instruction Decoding and Execution: As long as there are bytes in the queue, the CPU's instruction decoder can continuously decode and execute instructions without waiting for new fetches from memory. This reduces the number of wait states that the CPU experiences, thereby enhancing its overall performance.
Impact on Performance
The design and implementation of the prefetch input queue significantly influence the microprocessor's performance. By fetching instructions in advance, the Intel 8088 minimizes the idle time that would otherwise occur as the processor waits for new instructions to be fetched from memory. This prefetch mechanism is particularly beneficial when executing instructions sequentially, as it ensures a steady supply of instructions available for decoding and execution.
Instruction Pipelining
The prefetch input queue is a fundamental component of instruction pipelining, a technique where multiple instruction phases overlap, allowing the next instruction to be fetched while the current one is being decoded or executed. This pipelining approach helps in maximizing the utilization of the CPU's resources, improving the instruction throughput.
The instruction queue of the Intel 8088, although smaller than that of the Intel 8086, plays a similar vital role in enabling the pipelining process. By prefetching instructions, the CPU keeps the pipeline stages full, thereby reducing the time wasted waiting for new instructions.
Relationship with Other Components
The Intel 8088's prefetch input queue works in conjunction with other components, such as the Intel 8087 floating-point coprocessor, which also utilizes instruction queuing mechanisms. The interplay between these components facilitates complex computations and efficient processing, especially in systems where the 8088 serves as the primary microprocessor.
Enhanced Instruction Set
The prefetch input queue supports the extensive instruction set of the Intel 8086/8088 family, enabling these processors to handle a wide variety of computational tasks efficiently. This instruction set includes operations for arithmetic, logic, control, and data transfer, all of which benefit from the queue's ability to prefetch and streamline instruction execution.