High Speed Register Storage

Programmers who write in low-level "assembly languages" will be aware of these data and address registers in the CPU. Assembly language code defines details such as how data should be moved to specific data registers and how addresses are to be calculated and saved temporarily in address registers.

Generally, programmers working with high level languages don’t have to be concerned about such details; but, when necessary, a programmer can find out how the CPU registers are used in their code.

In addition to the main data and address registers, the CPU contains many other registers. The ALU will contain numerous registers for holding temporary values that are generated as arithmetic operations are performed. The timing and control component contains a number of registers that hold control information.

The Program Counter (PC) holds the address of the memory location containing the next instruction to be executed. The Instruction Register (IR) holds the bit pattern that represents the current instruction; different parts of the bit pattern are used to encode the "operation code" and the address of any data required. Most CPUs have a "flags" register. The individual bits in this register record various status data. Typically, one bit is used to indicate whether the CPU is executing code from an ordinary program or code that forms part of the controlling Operating Systems (OS) program. (The OS code has privileges; it can do things, which ordinary programs can not do, like change settings of peripheral device controllers. When the OS-mode bit is not set, these privileged instructions can not be executed.) Commonly, another group of bits in the flags register will be used to record the result of comparison instructions performed by the ALU. One bit in the flags register would be set if the comparator circuits found two values to be equal; a different bit would be set if the first of the two values was greater than the second.