Intel ARCHITECTURE IA-32 User Manual download pdf (Page 46)

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IA-32 Intel® Architecture Optimization

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execution units are not pipelined (meaning that µops cannot be

dispatched in consecutive cycles and the throughput is less than one per

cycle). The number of µops associated with each instruction provides a

basis for selecting instructions to generate. All µops executed out of the

microcode ROM involve extra overhead.

Execution Units and Issue Ports

At each cycle, the core may dispatch µops to one or more of four issue

ports. At the microarchitecture level, store operations are further divided

into two parts: store data and store address operations. The four ports

through which μops are dispatched to execution units and to load and

store operations are shown in Figure 1-4. Some ports can dispatch two

µops per clock. Those execution units are marked Double Speed.

Port 0. In the first half of the cycle, port 0 can dispatch either one

floating-point move µop (a floating-point stack move, floating-point

exchange or floating-point store data), or one arithmetic logical unit

(ALU) µop (arithmetic, logic, branch or store data). In the second half

of the cycle, it can dispatch one similar ALU µop.

Port 1. In the first half of the cycle, port 1 can dispatch either one

floating-point execution (all floating-point operations except moves, all

SIMD operations) µop or one normal-speed integer (multiply, shift and

rotate) µop or one ALU (arithmetic) µop. In the second half of the cycle,

it can dispatch one similar ALU µop.

Port 2. This port supports the dispatch of one load operation per cycle.

Port 3. This port supports the dispatch of one store address operation

per cycle.

The total issue bandwidth can range from zero to six µops per cycle.

Each pipeline contains several execution units. The µops are dispatched

to the pipeline that corresponds to the correct type of operation. For

example, an integer arithmetic logic unit and the floating-point

execution units (adder, multiplier, and divider) can share a pipeline.

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