Main memory and mass storage

1 Bits and Their Storage Inside today’s computers information is encoded as patterns of 0s and 1s. These digits are called bits (short for binary digits). Although you may be inclined to associate bits with numeric values, they are really only symbols whose meaning depends on the application at hand. Sometimes patterns of bits are used to represent

1 Bits and Their Storage numeric values; sometimes they represent characters in an alphabet and punctuation marks; sometimes they represent images; and sometimes they represent sounds.

2 Main Memory Main Memory For the purpose of storing data, a computer contains a large collection of circuits (such as flip-flops), each capable of storing a single bit. This bit reservoir is known as the machine’s main memory.

2 Main Memory Memory Organization A computer’s main memory is organized in manageable units called cells, with a typical cell size being eight bits. (A string of eight bits is called a byte. Thus, a typical memory cell has a capacity of one byte.) Small computers used in such household devices as microwave ovens may have main memories consisting of only a few hundred cells, whereas large computers may have billions of cells in their main memories.

Primary storage (main memory) a manageable unit( cell) size is eight bits. 1KB=210B=1024B 1MB= 210KB= 1024KB 1GB= 210MB= 1024MB 1TB= 210GB= 1024GB BYTE 7 6 5 4 3 2 1 0 Bit

2 Main Memory To identify individual cells in a computer’s main memory, each cell is assigned a unique “name”, called its addresses. The system is analogous to the technique of identifying houses in a city by addresses. In the case of memory cells, however, the addresses used are entirely numeric. To be more precise, we envision(想象) all the cells being placed in a single row and numbered in this order starting with the value zero.

2 Main Memory Such an addressing system not only gives us a way of uniquely identifying each cell but also associates an order to the cells (Figure 1.8), giving us phrases such as “ the next cell ” or “the previous cell.”

Figure 1.8 Memory cells arranged by address

3 Mass Storage Due to the volatility and limited size of a computer’s main memory, most computers have additional memory devices called mass storage (or secondary storage) systems, including magnetic disks, CDs, DVDs, magnetic tapes, and flash drives. The advantages of mass storage systems over main memory include less volatility, large storage capacities, low cost, and in many cases, the ability to remove the storage medium from the machine for archival purposes.

3 Mass Storage A major disadvantages of mass storage systems is that they typically require mechanical motion and therefore require significantly more time to store and retrieve data than a machine’s main memory, where all activities are performed electronically. Figure 1.9 a disk storage system

Figure 1.9 A magnetic disk storage system

3 Mass Storage Summary bit and its storage main memory mass storage