2 - The Basics

Summary

This chapter reviews features that are found in all modern microprocessors: (i) instruction pipelining and (ii) a main memory hierarchy with caches, including the virtual-to-physical memory translation. It does not dwell on many details – that is what subsequent chapters will do. It provides solely a basis on which we can build later on.

Pipelining

Consider the steps required to execute an arithmetic instruction in the von Neumann machine model, namely:

1. 1. Fetch the (next) instruction (the one at the address given by the program counter).

2. 2. Decode it.

3. 3. Execute it.

4. 4. Store the result and increment the program counter.

In the case of a load or a store instruction, step 3 becomes two steps: calculate a memory address, and activate the memory for a read or for a write. In the latter case, no subsequent storing is needed. In the case of a branch, step 3 sets the program counter to point to the next instruction, and step 4 is voided.

Early on in the design of processors, it was recognized that complete sequentiality between the executions of instructions was often too restrictive and that parallel execution was possible. One of the first forms of parallelism that was investigated was the overlap of the mentioned steps between consecutive instructions. This led to what is now called pipelining.