A register machine is an easily accessible place on a computer that the CPU can directly access for storing instructions. Usually, registers consist of some special hardware components and can only be read-write, but most registers are fast-access memory devices available to the CPU. When a computer is in operation, it loads the necessary instructions into the CPU registers and executes them as the computer is working.
In case a register is non-volatile, meaning that its contents are not immediately changed by the CPU when it needs to read or write it, then it becomes an integral data register. These types of registers are used as an intermediary between the memory and the CPU. A typical chip having an internal register set consists of a multiplexer–a kind of logic engine which conducts operations in execution engines–and a register book, which list all the instructions executed by the internal register. These two kinds of registers make it possible to execute several instructions at a time, thereby allowing the CPU to execute multiple instructions at a time when the computer is running.
A processor register has special instructions stored in it that tell the processor how to interpret a certain instruction. The processor register also stores conditional instructions and the result of previous instructions. If a particular instruction fails, the processor register can indicate to the CPU the reason why this has happened.
In the early days of microcomputers, registers were implemented as simple numbers, but over the years they have become much more complex. As the need for computers increased, so did the range of possible registers. In fact, some computers have many different registers, some for storing different kinds of information, like system calls, interruptions to the main CPU, and so on. Early systems used a single general-purpose register to store all the different registers; however, today there are actually two registers open at any given time: one large register and one smaller register. The larger register is called EDX, while the smaller register is called RDR. The contents of both registers are different: the larger register contains all the values that are accessed throughout the whole program, while the smaller register only contains the specific values that are used for specific instructions.
In order to see the complete definition of the various processor registers, you will need to understand a little bit about how every instruction is executed. An instruction is an instruction that is carried out by the CPU (or your computer) and its effect is either being read or executed. Instructions are different for every kind of instruction, and for each type of CPU register. When you look at your CPU register to see what instruction it corresponds to, you will see that the operation is specified either by a constant expression (an expression that evaluates to a true or false value) or by a series of constant expressions (an expression that produces a series of results, each of which is dependent on one or more previous results).
Let’s look at an example. The accumulator in your PC is set to start at the address of zero (zero memory), to accumulate data registers into your PC RAM. As instructions are executed, the PC accumulator is updating one by one as instructions are executed. When you stop your program, the accumulator will be updated (the result will be the result that you see at the PC screen) and the instruction that caused the accumulator to be updated will be forgotten, freeing up the memory for instructions to execute. The PC will continue to process your commands until you come to the PC RAM and then it will continue the execution.