The registers in a processor unit can be enclosed within a small memory unit.When included in a processor unit,a small memory is sometimes called scratchpad memory.The use of a small memory is a cheaper alternative to connecting processor registers through a bus system.The difference between the two systems is the manner in which information is selected for transfer into the ALU.In a bus system,the information transfer is selected by the multiplexers that form the buses.A single register in a group of registers organised as a small memory unit must be selected by means of an address to the memory unit. A memory register can function as any other processor registers as long as its only function is to hold binary information to be processed in the ALU.
A scratchpad memory should be distinguished from a main memory of the computer.A small memory in a processor unit is merely an alternative to connecting a number of processor registers through a common transfer path.The information stored in the scratchpad memory would normally come from main memory by means of instructions in the program.
Consider,a processor unit that employs eight registers of 16 bits each . The registers can be enclosed within a small memory of 8 words of 16 bits each or an 8*16 RAM.The eight memory words can be designated R0 through R7,corresponding to address 0 through 7,and constitute the registers for the processor.
