MEMORY
In the past few decades, CPU processing speed as measured by the number of instructions executed per second has doubled every 18 months, for the same price. Computer memory has experienced a similar increase along a different dimension, quadrupling in size every 36 months, for the same price. Memory speed, however, has only increased at a rate of less than 10% per year. Thus, while processing speed increases at the same rate that memory size increases, the gap between the speed of the processor and the speed of memory also increases.
As the gap between processor and memory speeds grows, architectural solutions help bridge the gap. A typical computer contains several types of memory, ranging from fast, expensive internal registers (see Appendix A), to slow, inexpensive removable disks. The interplay between these different types of memory is exploited so that a computer behaves as if it has a single, large, fast memory, when in fact it contains a range of memory types that operate in a highly coordinated fashion. We begin the chapter with a high-level discussion of how these different memories are organized, in what is referred to as the memory hierarchy.
7.1 The Memory Hierarchy
Memory in a conventional digital computer is organized in a hierarchy as illustrated in Figure 7-1. At the top of the hierarchy are registers that are matched in speed to the CPU, but tend to be large and consume a significant amount of power. There are normally only a small number of registers in a processor, on the order of a few hundred or less. At the bottom of the hierarchy are secondary and off-line storage memories such as hard magnetic disks and magnetic tapes, in which the cost per stored bit is small in terms of money and electrical power, but the access time is very long when compared with registers. Between the registers and secondary storage are a number of other forms of memory that bridge the gap between the two.
hierarchy in the late 1990’s. Notice that Typical Cost, arrived at by multiplying Cost/MB ´ Typical Amount Used (in which “MB” is a unit of megabytes), is approximately the same for each member of the hierarchy. Notice also that access times vary by approximately factors of 10 except for disks, which have access times 100,000 times slower than main memory. This large mismatch has an important influence on how the operating system handles the movement of blocks of data between disks and main memory, as we will see later in the chapter.
7.2 Random Access Memory
In this section, we look at the structure and function of random access memory (RAM). In this context the term “random” means that any memory location can be accessed in the same amount of time, regardless of its position in the memory.
Figure 7-2 shows the functional behavior of a RAM cell used in a typical com-
puter. The figure represents the memory element as a D flip-flop, with additional controls to allow the cell to be selected, read, and written. There is a (bidirectional) data line for data input and output. We will use cells similar to the one shown in the figure when we discuss RAM chips. Note that this illustration does not necessarily represent the actual physical implementation, but only its functional behavior. There are many ways to implement a memory cell.
RAM chips that are based upon flip-flops, as in Figure 7-2, are referred to as static RAM (SRAM), chips, because the contents of each location persist as long as power is applied to the chips. Dynamic RAM chips, referred to as DRAMs, employ a capacitor, which stores a minute amount of electric charge, in which the charge level represents a 1 or a 0. Capacitors are much smaller than flip-flops, and so a capacitor based DRAM can hold much more information in the same area than an SRAM. Since the charges on the capacitors dissipate with time, the charge in the capacitor storage cells in DRAMs must be restored, or refreshed frequently.
DRAMs are susceptible to premature discharging as a result of interactions with naturally occurring gamma rays. This is a statistically rare event, and a system may run for days before an error occurs. For this reason, early personal computers (PCs) did not use error detection circuitry, since PCs would be turned off at the end of the day, and so undetected errors would not accumulate. This helped to keep the prices of PCs competitive. With the drastic reduction in DRAM prices and the increased uptimes of PCs operating as automated teller machines (ATMs) and network file servers (NFSs), error detection circuitry is now commonplace in PCs.
In the next section we explore how RAM cells are organized into chips.