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Concept
Prefetch input queue
Summary
Fetching the instruction opcodes from program memory well in advance is known as prefetching and it is served by using a prefetch input queue (PIQ). The pre-fetched instructions are stored in a queue. The fetching of opcodes well in advance, prior to their need for execution, increases the overall efficiency of the processor boosting its speed. The processor no longer has to wait for the memory access operations for the subsequent instruction opcode to complete. This architecture was prominently used in the Intel 8086 microprocessor.
Pipelining was brought to the forefront of computing architecture design during the 1960s due to the need for faster and more efficient computing. Pipelining is the broader concept and most modern processors load their instructions some clock cycles before they execute them. This is achieved by pre-loading machine code from memory into a prefetch input queue.
This behavior only applies to von Neumann computers (that is, not Harvard architecture computers) that can run self-modifying code and have some sort of instruction pipelining. Nearly all modern high-performance computers fulfill these three requirements.
Usually, the prefetching behavior of the PIQ is invisible to the programming model of the CPU. However, there are some circumstances where the behavior of PIQ is visible, and needs to be taken into account by the programmer.
When an x86 processor changes mode from real mode to protected mode and vice versa, the PIQ has to be flushed, or else the CPU will continue to translate the machine code as if it were written in its last mode. If the PIQ is not flushed, the processor might translate its codes wrong and generate an invalid instruction exception.
When executing self-modifying code, a change in the processor code immediately in front of the current location of execution might not change how the processor interprets the code, as it is already loaded into its PIQ. It simply executes its old copy already loaded in the PIQ instead of the new and altered version of the code in its RAM and/or cache.
Official source
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