What is a register and how is it used?

Answer 1

In a computer a register is a device that stores a single unit of information. Some examples:

• accumulator register - stores a word of numerical data between arithmetic operations
• index register - stores data used in memory addressing operations
• status register - stores information from operations that will be used in branching decisions (e.g. sign of arithmetic result, invalid arithmetic result, operation success/failure)
• control/command register - stores mode and/or setup information for various operations
• program status word register - combination status/control register containing all information relevant to managing the current program/process/thread (e.g. program counter, status flags, mode bits), the operating system only needs to save the current contents of this register and load it with new contents to switch tasks (on computers with this register the switch can usually be done in one instruction)
• timer register - a register that counts at a constant rate, used to measure time intervals or trigger periodic interrupts
• instruction register - stores the current instruction the computer is executing so it can be decoded
• program counter register - stores the address of the next instruction to execute, as each instruction is fetched this register counts and is loaded when a branch instruction changes this default sequence
• stack pointer register - stores the address of the top item on a Last In First Out stack stored in RAM, automatically incremented and decremented as stack contents change
• etc.

Usually registers are implemented in dedicated hardware that is not part of RAM, but on some architectures (usually to reduce costs) some registers may be implemented in "reserved" RAM locations. On a few architectures (e.g. UNIVAC 1100) even though the registers are implemented in dedicated hardware that is not part of RAM, they "overlay" the low end of the RAM allocated to the current program making them accessible either by their register name/number or their memory address (and making those RAM locations inaccessible to that program).

We typically think of registers as being accessible to the program for software purposes, but some registers (e.g. program counter register, instruction register) are not as they have dedicated functions in the hardware that make the operation of the software possible. Other registers provide communication between hardware and software and thus may be either read only or write only registers and may change system operation if changed inappropriately.