Register-Driven Machines

A register machine is an electronically stored location on a personal computer that can be accessed by the CPU. It contains the operations that are performed on instructions from software programs. The register is generally programmed such that it performs only those operations which are relevant to the input to the register. Instructions to the register are stored in the form of code, as strings of binary numbers, which are read and executed by the CPU after being translated by the register. In simple terms, a register is a memory chip which contains operation codes that direct the execution of an instruction.


The register in a PC is generally arranged in two ways. In one way, an instruction, whose execution is controlled by the current instruction pointer, is stored in the PC register; and in another way, the instruction pointer is stored in the PC register but the currently executing instruction is not set to that address. Both ways of arrangement are equally applicable to non-volatile and volatile registers. Non-volatile registers are not easily altered, but a change in the instruction pointer or the current instruction pointer, which is an expression of the memory location of the PC register, may easily change the contents of this register.

The term “processor register” generally refers to PC registers that are accessed by the microprocessor or central processing unit. The most widely used PC register is the cpu register. Other commonly used PC registers are the pc constant address register, the debug register, the exception register, the microprocessor register, the system register, the errno register, the exception mask register, and the static immediate register. Among the PC registers, the most important are the cpu register, the memory register, the dynamic or static immediate register, and the extension registers. Besides these, the other two registers also play an important role in the computing cycle.

The cpu register stores instructions for execution into the CPU. A simple CPU register may consist of eight bits that encode the instruction. The other six bits of this register provide the instruction’s operands. Instructions of various types are composed of these bits, namely: unconditional, shift/call, conditional, loop, and branch.

Instruction pointer or IP is the instruction’s operand whose address is stored in the PC register. This register also acts as the accumulator. The two other PC registers, the memory address register and the index register, are known as the destination or accumulators. A PC instruction is a sequence of instructions whose operands are put into the appropriate PC register. Instructions are executed sequentially in the CPU.

PC address or execute instruction (XIP) and I/O registers, which store data as they become available, are called control registers. A register may be turned on or off. A register can be defined as any one of the eight binary registers including the low-order (low) RAM, high-order (high) RAM, page, stack, register, random access memory, video port, video output, input port, audio port, interlink register, and smart sector. In computer terminology, registers are regarded as parts of an architecture where different tasks are scheduled for execution.

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