Instruction Set Architecture

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Instruction Set Architecture Posted: 02 Feb 2019 04:00 PM PST The (main) memory can be modeled as an array of millions of adjacent cells, each capable of storing a binary digit (bit), having value of 1 or 0. These cells are organized in the form of groups of fixed number, say n, of cells that can be dealt with as an atomic entity. An entity consisting of 8 bits is called a byte. In many systems, the entity consisting of n bits that can be stored and retrieved in and out of the memory using one basic memory operation is called a word (the smallest addressable entity). Typical size of a word ranges from 16 to 64 bits. It is, however, customary to express the size of the memory in terms of bytes. For example, the size of a typical memory of a personal computer is 256 MB, that is, 256*2^20 = 2^28 bytes. In order to be able to move a word in and out of the memory, a distinct address has to be assigned to each word. This address will be used to determine the location in the memory in which a given word is to be stored. This is called a memory write operation. Similarly, the address will be used to determine the memory location from which a word is to be retrieved from the memory. This is called a memory read operation. Three basic steps are needed in order for the CPU to perform a write operation into a specified memory location: The word to be stored into the memory location is first loaded by the CPU into a specified register, called the memory data register (MDR). The address of the location into which the word is to be stored is loaded by the CPU into a specified register, called the memory address register (MAR). A signal, called write, is issued by the CPU indicating that the word stored in the MDR is to be stored in the memory location whose address in loaded in the MAR. ADDRESSING MODES Information involved in any operation performed by the CPU needs to be addressed. In computer terminology, such information is called the operand. Therefore, any instruction issued by the processor must carry at least two types of information. These are the operation to be performed, encoded in what is called the op-code field, and the address information of the operand on which the operation is to be performed, encoded in what is called the address field. Instructions can be classified based on the number of operands as: three-address, two-address, one-and-half-address, one-address, and zero-address. The different ways in which operands can be addressed are called the addressing modes. Addressing modes differ in the way the address information of operands is specified. The simplest addressing mode is to include the operand itself in the instruction, that is, no address information is needed. This is called immediate addressing. A more involved addressing mode is to compute the address of the operand by adding a constant value to the content of a register. This is called indexed addressing. Between these two addressing modes there exist a number of other addressing modes including absolute addressing, direct addressing, and indirect addressing.

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