Memory

Tyrell: If we give them a past, we create a cushion for their emotions and, consequently, we can control them better.
Deckard: Memories. You're talking about memories.
—the movie Blade Runner
In this chapter, you will learn everything you need to know about memory in embedded systems. In particular, you will learn about the types of memory you are likely to encounter, how to test memory devices to see if they are working properly, and how to use Flash memory.
6.1. Types of Memory
Many types of memory devices are available for use in modern computer systems. As an embedded software engineer, you must be aware of the differences between them and understand how to use each type effectively. In our discussion, we will approach these devices from a software viewpoint. As you are reading, try to keep in mind that the development of these devices took several decades and that there are significant physical differences in the underlying hardware. The names of the memory types frequently reflect the historical nature of the development process and are often more confusing than insightful.
Most software developers think of memory as being either random-access (RAM) or read-only (ROM). But, in fact, there are subtypes of each and even a third class of hybrid memories. In a RAM device, the data stored at each memory location can be read or written, as desired. In a ROM device, the data stored at each memory location can be read at will, but never written. In some cases, it is possible to overwrite the data in a ROM-like device. Such devices are called hybrid memories because they exhibit some of the characteristics of both RAM and ROM. Figure 6-1 provides a classification system for the memory devices that are commonly found in embedded systems.
6.2. Memory Testing
One of the first pieces of serious embedded software you are likely to write is a memory test. Once the prototype hardware is ready, the designer would like some reassurance that she has wired the address and data lines correctly and that the memory chips are working properly. At first this might seem like a fairly simple assignment, but as you look at the problem more closely you will realize that it can be difficult to detect subtle memory problems with a simple test. In fact, as a result of programmer naiveté, many embedded systems include memory tests that would detect only the most catastrophic memory failures. Some of these might not even notice that the memory chips have been removed from the board!