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Lecture_5_powerpoint

Without a memory no information can be stored or retrieved in a computer. Computer memory has experienced a similar increase along a different dimension. 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, 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.

13-Memory Hierarchy

Memory in a conventional digital computer is organized in a hierarchy as illustrated in Figure 7. 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.

Cache and main memory are built using solid-state semiconductor material. It is customary to call the fast memory level the primary memory. The solid-state memory is followed by larger, less expensive, and far slower magnetic memories that consist typically of the (hard) disk and the tape. It is customary to call the slower level the secondary memory. The objective behind designing a memory hierarchy is to have a memory system that performs as if it consists entirely of the fastest unit and whose cost is dominated by the cost of the slowest unit.

External storage is normally the place where programs and data are stored when the power is turned off. This type of memory used for large-term memory storage than internal memory. The most common external storage devices are tape and disk devices.

External memory operates at much slower speed than internal memory and it stores programs and data that are not currently being used by the CPU. External storage is also known as secondary storage or auxiliary storage.

External storage devices can be classified either mechanical or magnetic:-

1.     Mechanical storage devices: are punched paper card and punched paper tape. Both of these are less popular now than in the past.

2.     Magnetic storage devices: This device store information as magnetic spots on oxide surfaces. It used now in the personal computer systems. Because the magnetic spots do not need a constant supply of power to “ refresh “ themselves ( as most RAM chips do ), external storage units provides nonvolatile storage. Also, external storage is cheaper per unit of storage than the internal memory.

The Three most common forms of magnetic media used on personal computer systems are Magnetic tapes, floppy disks, and hard disks. All of these can be erased and rerecord again and again.

As new information is written, it automatically covers-up whatever was there before. To avoid accident erasure, both cassette tapes and floppy disks can be write-protected. Tape recorders and floppy disk drives will not write on write-protected media. Most hard disks cannot be write protected.

The tape or disk drive reads and writes on tapes and disks by moving them past a read/write head. The head reads their magnetized surfaces, converting the information into electrical impulses that it sends to the computer and vice versa.