ABSTRACT
The instruction set repertoire depicts the architecture of a computer. Besides opcode, there are two major issues in designing an instruction: first, how many addresses are placed in an instruction, and second, where to find each operand. Thus, an instruction contains an opcode along with an addressing mode and/or addresses. Interrupt is the fourth major programming concept implemented in third generation computers. Microcomputers usually support a relative address in a conditional branch instruction. On a microcomputer, the central processing unit switches the program counter and status register referred to as partial context switching. An external interrupt is a signal generated from an outside device, for example, real-time clock, another computer, an instrument, etc. If the external signal is important, it should have a higher priority as demonstrated in the microcomputer design. Such a system supports an external interrupt named nonmaskable interrupt which has a higher priority than any of the I/O interrupts.
