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

There are several reasons for the slow speed of getting the data from DRAM to processor. One is the much slower speed of DRAM technology relative to the processor. The second reason is that going off the chip that contains the processor and to the bus to reach the DRAM memory is a slow process. To overcome this, we need to have a faster memory technology inside the chip together with the processor.

The solution is to use static random access memory (SRAM) which keeps data in the computer’s memory for as long as the machine is powered on. This means it does not need a refresh like the DRAM and is designed with a faster technology than DRAM. However, SRAMs are bigger in area; a single bit requires a large area in the chip as it needs four to six transistors, which is much larger than the capacitor in DRAM. Moreover, inside the chip we do not have a lot of space due to the existence of the processor itself. So, SRAM is a small, fast memory inside the chip that is connected to the processor from one side and to the DRAM off-chip from the other side, as shown in Figure 5.23. The DRAM is in the range of 8–64GB, while the SRAM starts from the KB range to a few MBs.

One important distinction between the cache memory and the DRAM is that the former is programmer-transparent (i.e., its use cannot be managed by the programmer), but the latter is not. Your laptop has 32GB of RAM, which is the size of the DRAM, but you may not know how much cache your processor has. However, if you know how the cache works, you can write more efficient programs as we learn when we talk about locality.

In Figure 5.23, SRAM is more commonly referred to as cache memory, which is the memory that allows for high-speed retrieval of data. Let us see how the cache and the DRAM memory work together. From now on, whenever we say cache we mean the SRAM, and whenever we say memory, we mean the DRAM.

Suppose the processor executes a program that accesses array A consecutively. The processor starts with A[0] and asks the cache memory whether it has A[0]. Initially the cache is empty, so it does not have the needed data which is called a cache miss. The cache then gets the data from the memory. But instead of just getting A[0], it gets A[0], A[1], A[2], …, A[x]. The number of extra elements the cache brings depends on the design of the cache. In most processors available now, the cache usually brings 64 bytes from the memory. So, if array A is an array of integers, that is, each element is 4 bytes in length, then the cache brings 16 elements from the memory, from A[0] to A[15]. The processor gets the A[0] it wants, and the extra elements brought from memory, which are in the cache. Now, if the processor wants A[1], it finds it right away in the cache, which is called a cache hit. However, if the processor instead needs A[17], then we have another cache miss and the cache gets to the memory again to bring several elements, including A[17].

Both the SRAM and DRAM, or cache and memory, are volatile—whenever there is a power perturbation or the machine runs out of battery, everything in the cache and the memory is gone. And we cannot build a full-fledged computer with volatile memory only; we need persistent storage too.

Storage exists in computer systems to ensure that data continues to exist even after the computer is powered off. Storage, presented as technologies 3 and 4 in Figure 5.20, has few characteristics that differ from DRAM and SRAM (technologies 1 and 2). The first, and most important one, is that they are persistent—they are non-volatile. The second characteristic is that they are slower in speed than DRAM and SRAM but have lower costs and higher capacities. It is to be noted that technologies 3 and 4 do not have to exist together in a computer system; you can have a computer with either or both.

Besides the storage that exists inside the computer system, there is a lot of storage in the cloud. That is, storage does not exist in your computer, but you can access it through the Internet. This storage is managed by big tech companies. For example, we have Azure from Microsoft, AWS from Amazon, Google Drive from Google, and so on. These companies are serving millions of users and are isolating users’ data from each other. There are techniques to make each user access cloud storage and even software in the cloud.

Now, it is time to give them names. Technology 3 is called a solid state disk (SSD) and technology 4 is called hard disk drive (HDD). What are the differences and where does the commonly encountered term “flash drive” fit in? Let us start with the older technology first.