Memory Classification
Memory can be classified into two groups: prime(or main) memory and storage memory .The RIWM and ROM discussed in the last section are examples of prime memory; this is the memory the microcomputer uses in executing and storing programs. This memory should be able to respond fast enough to keep up with the execution speed of the microprocessor. Therefore. it should be random-access memory, meaning that the microprocessor should be able to access information from any register with the same speed (independent of its place in the chip).
Storage memory includes examples such as magnetic disks and tapes (see Figure 13). This memory is used to store programs and results after the completion of program execution. Information stored in’ these memories is nonvolatile, meaning information remains intact even if the system is turned off. Generally, these memory devices are not a part of any system; they are made part of the system only when stored programs need to be accessed. The microprocessor cannot execute or directly process programs stored in these devices: programs must be copied into the prime memory first. Therefore, the size of the prime memory (e.g., 64K or 128K) determines how large a program the system can process. The size of the storage memory is unlimited; when one disk or tape is full. Another can be used.
Figure 13 shows two subdivisions of storage memory: secondary storage and backup storage. The secondary storage is similar to what you put on your shelf in your study, and the backup is similar to what you store in your attic. Storage memory includes such devices as disks, magnetic tapes, magnetic. bubble memory, and charged-coupled devices (CCD), The primary features of all these
FIGURE 13
Memory Classification
devices are high capacity, low cost, and slow access. A disk is similar to a record; the access to the stored information in the disk is semi-random. The remaining devices shown in Figure 13 are serial: if information is stored in the middle of the tape, it can be accessed only after running half the tape. We will discuss some of these memory storage devices again in Chapter 7. In this chapter, we will focus on various types of prime memory.
Figure 13 shows that the prime memory is divided into two main groups:
Read/Write Memory (R/WM) and Read-Only Memory (ROM), and each group includes several different types of memory.
R/WM (READIWRITE MEMORY)
As the name suggests, the microprocessor can write into or read from this memory, and it is popularly known as Random-Access Memory (RAM). It is used primarily for information that is likely to be altered, such as writing programs or receiving data. This memory is volatile, meaning that when the power is turned off, all its contents are destroyed.
Two types of R/W memories-static and dynamic-are available . Static memory is made of flip-flops ,and it stores the bit as a voltage. Dynamic memory is made MOS transistor gates, and it stores the bit as a charge . the advantage is of the dynamic memory are that it has higher density. Lower power consumption, and is cheaper than the static memory. The disadvantage is that the charge(bit information). therefore, stored information needs to be read and written again every few milliseconds. This is called refreshing the memory ,and it requires extra circuitry, which adds to the cost of the system. It is generally economical to use dynamic memory when the system memory size is larger than 16K; for smaller systems, the static memory is appropriate.
ROM (READ-ONLY MEMORY)
The ROM is a nonvolatile memory; it retains stored information even if the power is turned off. This memory is used for programs and data that need not be altered because, as the name suggests, the information can be read only so that once a bit pattern is stored, it is permanent or at least semi permanent. The permanent group includes two types of memory: masked ROM and PROM, and the semi permanent group also includes two types of memory: EPROM and EE-PROM as shown in Figure 13.
MASKED ROM
In this ROM, a bit pattern is permanently recorded by the masking and metallization process. which memory manufacturers are generally equipped to do. It is an expensive and specialized process, but economical for large production quantities.
PROM (PROGRAMMABLE READ-ONLY MEMORY)
This memory has nichrome or polysilicon wires arranged in a matrix; these wires can be functionally viewed as diodes or fuses. This memory can be programmed by the user with a special PROM programmer that selectively burns the fuses according to the bit pattern to be stored. The process is known a "burning the PROM," and the information stored is permanent.
EPROM (ERASABLE PROGRAMMABLE READ-ONLY MEMORY)
This memory stores a bit by charging the floating gate of a field-effect transistor (FET). Information is stored by using an EPROM programmer .which applies high voltages to charge the gate. All the information can be erased by exposing the chip to ultraviolet light through its quartz window, and the chip can be reprogrammed. Because the chip can be reused many times, this memory is ideally suited for product development, experimental projects, and college laboratories.
EE-PROM (ELECTRICALLY ERASABLE PROM)
This memory is functionally similar to EPROM, except that information can be altered by using electrical signals at the register level rather than erasing all the information. This has an advantage in field and remote control applications. In microprocessor systems, software update is a common occurrence. If EE-PROMs are used in the systems, they can be updated from a central computer by using a remote link via telephone lines. Similarly, in a process control in which timing information has to be changed, it can be done by sending electrical signals from a central place. This memory also includes a chip-erase mode whereby the entire chip can be erased in 10 ms as opposed to 15 to 20 minutes for an EPROM.
RECENT ADVANCES IN MEMORY TECHNOLOGY
Memory technology has advanced considerably in recent years. In addition to static and dynamic R/W memory, there are now more options available in memory devices. Recent examples include Zero Power RAM from MOSTEK, Non-Volatile RAM from Intel, and Integrated RAM from several manufacturers.
The Zero Power RAM is a complementary metal-oxide semiconductor (CMOS) Read/Write memory with battery backup built internally. It includes lithium cells and voltage-sensing circuitry. When the external power supply voltage falls below + 3 Y, the power switching circuitry connects the lithium battery; thus, this memory provides the advantages of both R/W and Read-Only Memory.
The Non- Volatile RAM is a high speed static R/W Memory array backed lip, bit for bit, by an EE-PROM array for nonvolatile storage. When the power is about to go off, the contents of R/W memory are quickly stored in the EE-PROM by activating a STORE signal on the memory chip, and the stored data can be read into the R/W memory segment when the power is turned on again. This memory chip combines the flexibility of static R/W memory with the novolatility of EE-PROM.
The Integrated RAM (iRAM) is a dynamic memory with the refreshed circuitry built on the chip. For the user. it is similar to the static R/W memory. The user can derive the advantages of the dynamic memory without having to build the external refreshing circuitry. At present, this memory is economical for a system with medium-sized memory (between 8K and 64K).