A register machine is an easily accessed place on a computer that can only be accessed by the processor. Most registers are made up of some special hardware components, and can only be read or write-write, but generally consist of a relatively small amount of storage. As you can imagine, the more components a register has, the harder it is to access the information stored within. As the computer begins to get slower, so does the ability to access registry files. This slows down performance, as the computer takes longer to process commands to and from the registry. You may also notice that your computer gets hung up more often when starting up or closing down.
The CPU, or central processing unit, is the brain of the computer. It controls and coordinates the operations of all of the computer’s processes. All instructions are carried out by the CPU, but only those instructions which are directly executed by the CPU are actually seen by the internal registers. Instructions which are indirectly executed in the registers, via a cache, are not seen by the CPU; these are known as indirect instructions. In a single CPU, there are typically about 16 different registers.
The main purpose of the CPU register is to store instructions which are then executed by the computer memory. Instruction execution occurs whenever the computer is started up. Instructions stored in the register are then read by the rest of the CPU and executed in turn by the computer memory. Instructions that are not directly executed are passed to the ROM, or random access memory, for execution by the CPU. Instructions that are directly executed are passed to the PC for execution by the individual processors.
The PCA is the most general register type. The PCA consists of 16 bits which describe the assembly/load/store mode of the instruction. The PCA is used mainly when the target machine has a large address space and/or if the instruction needs to be divided into several segments. The PCA is used as the central information unit (MIU) of the instruction execution system (IA).
The PC register is used primarily for storing machine instructions and constant values. In a multi-user program, the PCA is used to store the value that indicates how many times a particular instruction should be executed. In a system with a program counter, the PCA is used to store the program counter so that every instruction in the program can be easily evaluated. In addition, the PCA is used to store the memory address of every processor register for the CPU to determine the next instruction to be executed.
The PC register performs two important functions. First, it provides the execution status of the instruction before the next instruction is executed and second, it stores the memory address of every instruction and program counter register for the CPU to evaluate the next instruction. A PC register is therefore a very important register for every processor.