the bottom of the storage hierarchy. They are cost-effective for the storage of very large quantities of data when fast access time is not necessary.
The Memory System
Please note two important points here: •
The size of the memory increases as we move down the hierarchy.
•
The quantum of data that is transferred between two consecutive memory layers at a time also increases as we go from a higher to lower side. For example, from main memory to Cache transfer one or few memory words are accessed at a time, whereas in a hard disk to main memory transfer, a block of about 1 Megabyte is transferred in a single access. You will learn more about this in the later sections of the unit.
Let us now discuss various forms of memories in the memory hierarchy in more details.
1.3 RAM, ROM, DRAM, FLASH MEMORY RAM (Random Access Memory) The main memory is a random access memory. It is normally organised as words of fixed length. The length of a word is called word length. Each of these memory words has an independent address and each has the same number of bits. Normally the total numbers of words in memory are some power of 2. Some typical memory word sizes are 8 bits, 16 bits, 32 bits etc. The main memory can be both read and written into, therefore it is called read-write memory. The access time and cycle time in RAMs are constant and independent of the location accessed. How does this happen? To answer this, let us first discuss how a bit can be stored using a sequential circuit. The logic diagram of a binary cell is shown in Figure 2a:
Figure 2(a) Logic Diagram of RAM cell
The construction shown in Figure 2(a) is made up of one JK flip-flop and 3 AND gates. The two inputs to the system are one input bit and read/write signal. Input is fed in complemented form to AND gate ‘a’. The read/write signal has a value of 1 if it is a read operation. Therefore, during the read operation the AND gate ‘c’ has the read/write input as 1. Since AND gate ‘a’ and ’b’ have 0 read/write input, and if the 7