.::Microprogramming::.

The Pentium primarily uses a microprogramming approach to the control unit design.

Because it is a CISC-based processor, the more complex instruction set must be converted into simpler microcode instructions.

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Control ROM

• Contains the microcode which controls the sequence of operations performed by the processor.

Control Unit

• Has direct control over both pipelines. Microcode is designed to use both pipelines, so instructions requiring microcode execute faster.

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Instruction Decode Unit

• Decodes the object code from the Prefetch Unit into microcode.
• Contains two parallel decoders that issue the next two sequential instructions into the execution pipeline.
• Some instructions are decoded into one to four micro-operations (or uops).
• More complex instructions are decoded into sequences of preprogrammed uops. These are obtained from the microcode instruction sequencer.
• The instruction decoder can generate up to six uops per clock cycle.

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Partially hardwired

• To improve performance, certain heavily used instructions are still hardwired.
• For example, the MOV and ALU operations are hardwired, which allows them to operate more quickly.
• In general, these simple instructions execute in one clock cycle. The exceptions are the ALU memory to register (3 clock cycles) and register to memory (2 clock cycles) instructions. Sequencing hardware is used to allow them to function as simple instructions.