What Is a Processor and How Does It Work

So first let’s bring in the CPU it means Central Processing Unit, the CPU is the brains of the computer. Let’s also bring in memory,  the RAM, this is where the CPU accesses stored information it needs.

Now the CPU also has a built-in memory, this is called the Cache. The cache is considerably smaller than RAM, with sizes ranging in the order of 32 kilobytes to 8 megabytes. The purpose of the cache is to give the CPU information it needs immediately.

The CPU and RAM are separate objects, so when the CPU needs information, it takes time, a  very small amount of time to read the data from the memory.

This time can add considerable delay to computer operation. With the cache being right on the CPU reduces this time to almost nothing. The reason why you don’t need much cache storage is that it just needs to store little bits of important information that the CPU will need to use soon or has been using a lot of recently.

There are various methods implemented to determine what goes on to the cache. What should be kept on the cache and when it should be written back on to the RAM. In a typical CPU, there are various levels of cache each with different read and write times and sizes, for the sake of simplicity we’ll assume a single cache for our CPU.

So now with the basic components out of the way. Let’s get into how the computer operates. When a CPU executes an instruction there are five basic steps that need to be completed:

•  Fetch- Get the instruction from the memory and store it in the cache in some cases.
• Decode- Get the appropriate variables needed for the execution of the instruction.
• Execute- Compute the result of the instruction.
• Memory- For instructions that require a memory read/write operation to be done.
• Write Back (WB)- Write the results of the instruction back into memory.

Nearly every instruction goes through the first three and final steps, only certain instructions go through the memory steps such as load and stores but for the sake of simplicity, we’ll assume every instruction requires all five steps. Now each step takes one clock cycle, this translates to a CPI, clock cycles per instruction, of five.

As a note: Most modern processors can execute billions of clock cycles per second, for example- a 3.4 GHz processor can execute 3.4 billion clock cycles per second. Now a CPI of 5 is very inefficient, meaning the resources of the CPU are wasted.

This is why pipelining was introduced, bringing asynchronous operation into computing. Pipelining essentially makes it so each step can be executed in a different clock cycle. One instruction per clock cycle, a CPI of 1.

Essentially what pipelining does is take the segmented instruction and execute them in each clock cycle, since the segmented steps are smaller than the size and less complex than a normal instruction, you can do the steps of other instructions in the same clock cycle. For example- if a step for one instruction is fetching the data you could begin decoding another, executing another, etc. Since the hardware involved for those steps is not being blocked.

Superscalar pipeline adds to this performance further. Think of pipelines as a highway, now typical lanes in the highway can execute one instruction per clock cycle. With superscalar processors, you add more lanes to the highway, for example- a 2 wide superscalar also referred to as a dual-issue machine, has a theoretical CPI of 1/2 instructions per clock cycle.

There are various other methods implemented to make the processor CPI more efficient. such as:-

1. Loop unrolling
2. The very long instruction word
3. VLIWs- which are essentially multiple instructions wrapped into one larger instruction.
4. Compiler Scheduling- allowing for out-of-order execution and etc.

There are also many issues that come along with pipelining that decrease CPI such as:-

1. Data Hazards
2. Memory Hazards
3. Structural Hazard and etc.