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Daisy Chain Priority Interrupt: To find the priority of devices that send the interrupt request, we adopt this method. The daisy chaining method of establishing priority consists of a serial connection of all devices that request an interrupt. The device with the highest priority is placed in the first position, followed by lower priority devices up to the device with the lowest priority, which is placed last in the chain. This method of connection between three devices and the CPU is shown in figure given below.
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Types of Interrupts: There are three major types of interrupts that cause a break in the normal execution of a program. They can be classified as: Read more »

Input-Output and Interrupt: A computer can serve no useful purpose unless it communicates with the external environment. Instructions and data stored in memory must come from some input device. Computational results must be transmitted to the user through some output device. Commercial computers include many types of input and output devices. To demonstrate the most basic requirements for input ad output communication, we will use as an illustration a terminal unit with keyboard and printer. Read more »

Instruction set completeness: Before investigating the operations performed by the instructions, let us discuss the type of instructions that must be included in a computer. A computer should have a set of instructions so that the user can construct machine language programs to evaluate any function that is known to be computable. The set of instructions are said to be complete if the computer includes a sufficient number of instructions in each of the following categories: Read more »

Subroutine call and return: A subroutine is a self-contained sequence of instructions that performs a given computational task. During the execution of a program, a subroutine may be called to perform its function many times at various points in the main program. Each time a subroutine is called, a branch is executed to the beginning of the subroutine to start execution its set of instructions. After the subroutine has been executed, a branch is made back to the main program. Read more »

Instruction Format: The simplest way to organize a computer is to have one processor register and an instruction code format with two parts. The first part specifies the operation to be performed and the second specifies an address. The memory address tells the control where to find an operand in memory. This operand is read from memory and used as the data to be operated on together with the data stored in the processor register. Read more »

The basic computer has three instruction code formats, as shown in figure. Each format has 16-bits. The operation code (opcode) part of the instruction contains three bits and the meaning of the remaining 13 bits depends on the operation code encountered. A memory reference instruction uses 12 bits to specify an address and one bit to specify the addressing mode I. I is equal to 0 for direct address and to 1 for indirect address. Read more »

Computer Registers: Computer instructions are normally stored in consecutive memory locations and are executed sequentially one at a time. The control reads an instruction from a specific address in memory and executes it. It then continues by reading the next instruction in sequence and executes it and so on. This type of instruction sequencing needs a counter to calculate the address of the next instruction after execution of the current instruction is completed. Read more »

Indirect Addressing: It is sometimes convenient to use the address bits of an instruction code not as an address but as the actual operand. When the second part of an instruction code specifies an operand, the instruction is said to have an immediate operand. When the second part specifies the address of an operand, the instruction is said to have a direct address. This is in contrast to a third possibility called indirect address, where the bits in the second part of the instruction designate an address of a memory word in which the address of the operand is found. One bit of the instruction code can be used to distinguish between a direct and indirect address. Read more »

A program is a set of instructions that specify the operations, operands and the sequence by which processing has occurred. The data processing task may be altered by specifying a new program with different instructions or specifying the same instructions with different data. Read more »