Register Memory (Basic Knowledge)

Register Memory

Registers are memories located within the Central Processing Unit (CPU).   They are few in number (there are rarely more than 64 registers) and also small in size, typically a register is less than 64 bits; 32-bit and more recently 64-bit are common in desktops.

The contents of a register can be “read” or “written” very quickly however, often an order of magnitude faster than main memory and several orders of magnitude faster than disk memory. 

Different kinds of register are found within the CPU.  General Purpose Registers are available for general use by the programmer.  Unless the context implies otherwise we’ll use the term "register" to refer to a General Purpose Register within the CPU.  Most modern CPU’s have between 16 and 64 general purpose registers.  Special Purpose Registers have specific uses and are either non-programmable and internal to the CPU or accessed with special instructions by the programmer.  Examples of such registers that we will encounter later in the course include: the Program Counter register (PC), the Instruction Register (IR), the ALU Input & Output registers, the Condition Code (Status/Flags) register, the Stack Pointer register (SP). The size (the number of bits in the register) of the these registers varies according to register type. The Word Size of an architecture is often (but not always!) defined by the size of the general purpose registers.

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[1]      e.g. less than a nanosecond (10-9 sec)

[2]      Occasionally called Working Registers

[3]      Used for performing calculations, moving and manipulating data etc.