Number and character operands, as well as instructions, are stored in the memory of a computer. The memory consists of many millions of storage cells, each of which can store a bit of information having the value 0 or 1. Because a single bit represents a very small amount of information, bits are seldom handled individually. The usual approach is to deal with them in groups of fixed size. For this purpose, the memory is organized so that a group of n bits can be stored or retrieved in a single, basic operation. Each group of n bits is referred to as a word of information, and n is called the word length. The memory of a computer can be schematically represented as a collection of words as shown in figure (a).Modern computers have word lengths that typically range from 16 to 64 bits.
If the word length of a computer is 32 bits, a single word can store a 32-bit 2’s complement number or four ASCII characters, each occupying 8 bits. A unit of 8 bits is called a byte .Accessing the memory to store or retrieve a single item of information, either a word or a byte, requires distinct names or addresses for each item location. It is customary to use numbers from 0 through 2K -1, for some suitable values of k, as the addresses of successive locations in the memory. The 2k addresses constitute the address space of the computer, and the memory can have up to 2k addressable locations. 24-bitaddress generates an address space of 224(16,777,216) locations. A 32-bit address creates an address space of 232or 4G (4 giga) locations. MEMORY OPERATIONS:-
Both program instructions and data operands are stored in the memory. To execute an instruction, the processor control circuits must cause the word (or words) containing the instruction to be transferred from the memory to the processor. Operands and results must also be moved between the memory and the processor. Thus, two basic operations involving the memory are needed, namely, Load (or Read or Fetch) and Store (or Write).The load operation transfers a copy of the contents of a specific memory location to the processor. The memory contents remain unchanged. To start a Load operation, the processor sends the address of the desired location to the memory and requests that its contents be read. The memory reads the data stored at that address and sends them to the processor.
The store operation transfers an item of information from the processor to a specific memory location, destroying the former contents of that location. The processor sends the address of the desired location to the memory, together with the data to be written into that location .An information item of either one word or one byte can be transferred between the processor and the memory in a single operation. Actually this transfer in between the CPU register & main memory. INSTRUCTIONS AND INSTRUCTION SEQUENCING:-
A computer must have instructions capable of performing four types of operations.
•Data transfers between the memory and the processor registers
•Arithmetic and logic operations on data
•Program sequencing and control