第5章アルゴリズムのハードウェアによる実現：コンピューターシステム設計

We have five items on our wish list for an ideal memory and, since there is no single technology that excels in all five, we must combine several technologies to come close to the ideal memory system. This ideal memory system must be fast, dense, persistent, large in capacity, and inexpensive. The technologies that we currently use have the following characteristics:

• Technology 1: very fast but expensive, less dense, and volatile

• Technology 2: faster and denser, but volatile and moderately expensive

• Technology 3: persistent and inexpensive but slow

• Technology 4: persistent and very inexpensive but very slow

We need to get the best of all of them, and the best way to combine them must ensure that we have higher capacity from technologies 3 and 4 but make technologies 1 and 2 closer to the processor so that they can respond faster to the processor. The obvious way to do this is to use an arrangement of storage available on a computer system in the form of a triangle as shown in Figure 5.20. We call this design memory hierarchy.

Table 5.3 gives the names of the technologies that we discuss later in this section. The processor is connected to the first cache using a set of wires called the bus. If the processor does not find what it wants, it goes to the next level, and so on.

Memory is volatile, at least for now, with research exploring other technologies for persistent memories, speed, storage, and expense. This covers technologies 1 and 2 and, therefore, they are closer to the processor. Technologies 1 and 2 cover two types of memories that both include random access memory (RAM), which allows the processor to access any part of the memory in any order. Technology 2 is called DRAM, and technology 1 is called SRAM. Let us explore each one in turn.

As you now know, the memory stores instructions and data, which are presented as 1s and 0s. One type of memory, dynamic random access memory (DRAM), consists of a large number of capacitors. A capacitor is a very small electrical component that stores an electrical charge. A capacitor can be in one of two states: either it holds a charge, in which case we say that a 1 is stored in this capacitor, or it does not hold a charge, which means a 0. With millions of capacitors, we can store a large number of 1s and 0s. This is what you find in the specs of your laptop; when you say that you have 32GB of RAM, it means there are about 32 billion bytes in memory. Each byte consists of 8 bits. Each bit requires a capacitor.

Capacitors have a not-so-great characteristic though. When a charge is left on a capacitor for some time, the capacitor starts discharging and loses its charge. This means we lose the data stored in memory. Because of this, there is circuitry built inside the DRAM that, every few milliseconds, checks the capacitors and adds a charge to them. We call this the refresh cycle. It is done dynamically, hence the name dynamic RAM or DRAM.

The capacitors are not standalone by themselves. Transistors are used with them to help organize those capacitors into rows (also called word lines) and columns (also called bit lines) for addressing specific bits. Figure 5.21 shows a simplified view of DRAM. The cell, which stores 1 bit, is made up of the capacitor and some transistors.

What is shown in Figure 5.21 is called a bank. Every few banks form a chip. Every few chips form a rank. A small memory board that contains several memory banks is a dual in-line memory module (DIMM). Several DIMMs form a channel. This organization is shown in Figure 5.22. The reason for having this organization is twofold. First, if we have one huge 2-D array of memory cells, it is too complex, slow, and power hungry, so dividing it into parts makes those parts simpler and that means faster and less power hungry. Second, if there are several memory addresses that need to be accessed and they fall into different banks, for example, the memory can respond in parallel.

You must have heard the term 64-bit machines, right? Most of our computer systems nowadays are 64 bit. One of the definitions of this term is that the connection between the processor and the memory has 64-bit width; its memory can send the data to, or receive data from, the processor in chunks of 64 bits.

The curve that we saw in Figure 5.19 shows the slow speed increase of the DRAM, which affects the performance of the overall system. If the processor must go to the memory for every instruction and every piece of data, the overall system performance is very low; therefore, computer designers speed things up by using a faster technology together with the DRAM. This faster technology, technology 1 in Figure 5.20 is called the SRAM.