CPU
The central processing unit (CPU)
The CPU in modern computers is the embodiment of the “mill” in Babbage’s difference engine. The term central processing unit originated way back in the mists of computer time when a single massive cabinet contained the circuitry required to interpret machine level program instructions and perform operations on the data supplied. The central processing unit also completed all processing for any attached peripheral devices. Peripherals included printers, card readers, and early storage devices such as drum and disk drives. Modern peripheral devices have a significant amount of processing power themselves and off-load some processing tasks from the CPU. This frees the CPU up from input/output tasks so that its power is applied to the primary task at hand.
Early computers only had one CPU and could only perform one task at a time.
We retain the term CPU today, but now it refers to the processor package on a typical motherboard.
There is really nothing to see here other than the processor package itself. The processor package is a chip containing the processor(s) sealed inside a metal container and mounted on a small printed circuit (PC) board. The package is simply dropped into place in the CPU socket on a motherboard and secured with a locking lever arrangement. A CPU cooler attaches to the processor package. There are several different physical sockets with specific numbers of contacts, so getting the correct package to fit the motherboard socket is essential if you build your own computers.
How the CPU works
Let’s look at the CPU in more detail. Figure 2 is a conceptual diagram of a hypothetical CPU so that you can visualize the components more easily. The RAM and system clock are shaded because they are not part of the CPU and are only shown for clarity. Also, no connections between the CPU clock and the control unit to the CPU components are drawn in. Suffice it to say that signals from the clock and the control unit are an integral part of every other component.
This design does not look particularly simple, but the reality is even more complicated. This figure is sufficient for our purposes without being overly complex.
The arithmetic logic unit (ALU) performs the arithmetic and logical functions that are the work of the computer. The A and B registers hold the input data, and the accumulator receives the result of the operation. The instruction register contains the instruction that the ALU is to perform.
For example, when adding two numbers, one number is placed in the A register and the other in the B register. The ALU performs the addition and puts the result in the accumulator. If the operation is a logical one, the data to be compared is placed into the input registers. The result of the comparison, a 1 or 0, is put in the accumulator. Whether this is a logical or arithmetic operation, the accumulator content is then placed into the cache location reserved by the program for the result.
There is another type of operation performed by the ALU. The result is an address in memory, and it is used to calculate a new location in memory to begin loading instructions. The result is placed into the instruction pointer register.
Instruction register and pointer
The instruction pointer specifies the location in memory containing the next instruction to be executed by the CPU. When the CPU completes the execution of the current instruction, the next instruction is loaded into the instruction register from the memory location pointed to by the instruction pointer.
After the instruction is loaded into the instruction register, the instruction register pointer is incremented by one instruction address. Incrementing allows it to be ready to move the next instruction into the instruction register.
Cache
The CPU never directly accesses RAM. Modern CPUs have one or more layers of cache. The CPU’s ability to perform calculations is much faster than the RAM’s ability to feed data to the CPU. The reasons for this are beyond the scope of this article, but I will explore it further in the next article.
Cache memory is faster than the system RAM, and it is closer to the CPU because it is on the processor chip. The cache provides data storage and instructions to prevent the CPU from waiting for data to be retrieved from RAM. When the CPU needs data — and program instructions are also considered to be data — the cache determines whether the data is already in residence and provides it to the CPU.
Get viswatech’s stories in your inbox
Join Medium for free to get updates from this writer.
If the requested data is not in the cache, it’s retrieved from RAM and uses predictive algorithms to move more data from RAM into the cache. The cache controller analyzes the requested data and tries to predict what additional data will be needed from RAM. It loads the anticipated data into the cache. By keeping some data closer to the CPU in a cache that is faster than RAM, the CPU can remain busy and not waste cycles waiting for data.
Our simple CPU has three levels of cache. Levels 2 and 3 are designed to predict what data and program instructions will be needed next, move that data from RAM, and move it ever closer to the CPU to be ready when needed. These cache sizes typically range from 1 MB to 32 MB, depending upon the speed and intended use of the processor.
The Level 1 cache is closest to the CPU. In our CPU, there are two types of L1 cache. L1i is the instruction cache, and L1d is the data cache. Level 1 cache sizes typically range from 64 KB to 512 KB.
Memory management unit
The memory management unit (MMU) manages the data flow between the main memory (RAM) and the CPU. It also provides memory protection required in multitasking environments and conversion between virtual memory addresses and physical addresses.
CPU clock and control unit
All of the CPU components must be synchronized to work together smoothly. The control unit performs this function at a rate determined by the clock speed and is responsible for directing the operations of the other units by using timing signals that extend throughout the CPU.
Random access memory (RAM)
Although the RAM, or main storage, is shown in this diagram and the next, it is not truly a part of the CPU. Its function is to store programs and data so that they are ready for use when the CPU needs them.
How it works
CPUs work on a cycle that is managed by the control unit and synchronized by the CPU clock. This cycle is called the CPU instruction cycle, and it consists of a series of fetch/decode/execute components. The instruction, which may contain static data or pointers to variable data, is fetched and placed into the instruction register. The instruction is decoded, and any data is placed into the A and B data registers. The instruction is executed using the A and B registers, with the result put into the accumulator. The CPU then increases the instruction pointer’s value by the length of the previous one and begins again.
The need for speed
Although the basic CPU works well, CPUs that run on this simple cycle can be used even more efficiently. There are multiple strategies for boosting CPU performance, and we look at two of them here.
Supercharging the instruction cycle
One problem early CPU designers encountered was wasted time in the various CPU components. One of the first strategies for improving CPU performance was overlapping the portions of the CPU instruction cycle to utilize the various parts of the CPU more fully.
For example, when the current instruction has been decoded, the next one is fetched and placed into the instruction register. As soon as that has occurred, the instruction pointer is updated with the next instruction’s memory address.
This design looks nice and smooth, but factors such as waiting for I/O can disrupt the flow. Not having the proper data or instructions in the cache requires the MMU to locate the correct ones and move them to the CPU, and that can take some time. Certain instructions also take more CPU cycles to complete than others, interfering with smooth overlapping.
Nevertheless, this is a powerful strategy for improving CPU performance.
Hyperthreading
Another strategy to improve CPU performance is hyperthreading. Hyperthreading makes a single processor core work like two CPUs by providing two data and instruction streams. Adding a second instruction pointer and instruction register to our hypothetical CPU, as shown in Figure 5, causes it to function like two CPUs, executing two separate instruction streams during each instruction cycle. Also, when one execution stream stalls while waiting for data — again, instructions are also data — the second execution stream continues processing. Each core that implements hyperthreading is the equivalent of two CPUs in its ability to process instructions.
