The two address instruction format reduces the space requirement. One address must do double duty as both operand and result. Here again each address field can specify either a processor register, or a memory word. Two-address instructions are the most common in commercial computers. DISADVANTAGE: The instructions requires too many bits to specify 3 addresses.ADVANTAGE: It results in short programs when evaluating arithmetic expressions.The three address instruction format requires a very complex design to hold the three address references. A temporary location T is used to store some intermediate result so as not to alter any of the operand location. Three address Instruction With this type of instruction, each instruction specifies two operand location and a result location. Following are the types of instructions.ġ. For example: ADDĬomputers may have instructions of several different lengths containing varying number of addresses. This operation has the effect of popping the 2 top numbers from the stack, operating the numbers and pushing the sum into the stack.
#WHY STUDY COMPUTER ORGANIZATION AND ARCHITECTURE CODE#
Stack organization:- The instruction in a stack computer consists of an operation code with no address field. General register organization:- The instruction format in this type of computer needs three register address fields. For example: ADD X, where X is the address of the operands. The instruction format in this type of computer uses one address field. Single accumulator organization:- All the operations are performed with an accumulator register. Most computers fall into one of 3 types of CPU organizations.The number of address fields in the instruction format of a computer depends on the internal organization of its registers.Computers may have instructions of several different lengths containing varying number of addresses.Operands residing in processor register are specified with a register address. Operands residing in memory are specified by their memory address. Operation specified by an instruction is executed on some data stored in the processor register or in the memory location. The bits that define the mode field of an instruction code specify a variety of alternatives for choosing the operands from the given address. The operation code field (Opcode) of an instruction is a group of bits that define various processor operations such as add, subtract, complement, shift etcetera. A Mode field that specifies the way the operand or the effective address is determined.An Address field that designates a memory address or a processor register.An Operation code field that specifies the operation to be performed.The most common fields in instruction formats are:.The bits of the instruction are divided into groups called fields.An n bit instruction that k bits in the address field and m bits in the operation code field come addressed 2k location directly and specify 2m different operation. It is the function of the control unit within the CPU to interpret each instruction code and provide the necessary control functions needed to process the instruction. It then continues by reading the next instruction in sequence and executes it until the completion of the program.Ī computer usually has a variety of Instruction Code Formats. The control reads an instruction from a specific address in memory and executes it. Computer instructions are stored in central memory locations and are executed sequentially one at a time. These ordered steps are the instructions. The collection of such ordered steps forms a ‘program’ of a computer. The computer can be used to perform a specific task, only by specifying the necessary steps to complete the task.
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