Memory
This chapter introduces the basic concepts, terminology, and roles of memory in digital systems. The material presented here will not delve into the details of the device physics or low-level theory of operation. Instead, the intent of this chapter is to
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Describe the basic architecture and terminology for semiconductor-based memory systems. Describe the basic architecture of non-volatile memory systems. Describe the basic architecture of volatile memory systems. Design a VHDL behavioral model of a memory system.
10.1 Memory Architecture and Terminology The term memory is used to describe a system with the ability to store digital information. The term semiconductor memory refers to systems that are implemented using integrated circuit technology. These types of systems store the digital information using transistors, fuses, and/or capacitors on a single semiconductor substrate. Memory can also be implemented using technology other than semiconductors. Disk drives store information by altering the polarity of magnetic fields on a circular substrate. The two magnetic polarities (north and south) are used to represent different logic values (i.e., 0 or 1). Optical disks use lasers to burn pits into reflective substrates. The binary information is represented by light either being reflected (no pit) or not reflected (pit present). Semiconductor memory does not have any moving parts, so it is called solid-state memory and can hold more information per unit area than disk memory. Regardless of the technology used to store the binary data, all memory has common attributes and terminology that are discussed in this chapter.
10.1.1 Memory Map Model The information stored in memory is called the data. When information is placed into memory, it is called a write. When information is retrieved from memory, it is called a read. In order to access data in memory, an address is used. While data can be accessed as individual bits, in order to reduce the number of address locations needed, data is typically grouped into N-bit words. If a memory system has N ¼ 8, this means that 8 bits of data are stored at each address. The number of address locations is described using the variable M. The overall size of the memory is typically stated by saying “MxN.” For example, if we had a 168 memory system, that means that there are 16 address locations, each capable of storing a byte of data. This memory would have a capacity of 168 ¼ 128 bits. Since the address is implemented as a binary code, the number of lines in the address bus (n) will dictate the number of address locations that the memory system will have (M ¼ 2n). Figure 10.1 shows a graphical depiction of how data resides in memory. This type of graphic is called a memory map model.
# Springer Nature Switzerland AG 2019 B. J. LaMeres, Introduction to Logic Circuits & Logic Design with VHDL, https://doi.org/10.1007/978-3-030-12489-2_10
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Chapter 10: Memory
Fig. 10.1 Memory map model
10.1.2 Volatile Versus Non-volatile Memory Memory is classified into two categories depending on whether it can store information when power is removed or not. The term non-volatile is used to describe memory that holds information when the power is removed, while the term volatile is used to describe memory that loses its information when po
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