Digital Circuit Design with an Introduction to CPLDs and FPGAs

Chapter 9: Memory Devices


This chapter is an introduction to computer memory devices. We discuss the random-access memory (RAM), read-only memory (ROM), row and column decoders, memory chip organization, static RAMs (SRAMs) dynamic RAMs (DRAMs), volatile, nonvolatile, programmable ROMs (PROMs), Erasable PROMs (EPROMs), Electrically Erasable PROMs (EEPROMs), flash memories, and cache memory.

9.1 Random-Access Memory (RAM)

Random access memory (RAM) is the best known form of computer memory. RAM is considered "random access" because we can access any memory cell directly if we know the row and column that intersect at that cell. In a typical RAM, the access time is independent of the location of the data within the memory device, in other words, the time required to retrieve data from any location within the memory device is always the same and can be accessed in any order. RAM is volatile, that is, data are lost when power is removed.

Another type of memory is the serial access memory (SAM). This type of memory stores data as a series of memory cells that can only be accessed sequentially (like a cassette tape). If the data is not in the current location, each memory cell is checked until the required data is found. SAM works very well for memory buffers, where the data is normally stored in the order in which it will be used such as the buffer memory on a video card. One advantage of SAM over RAM is that the former is nonvolatile memory, that is, stored...