Organization of Multilevel Memory System:
The high-speed registers in CPU are used as the temporary storage for instructions and data. They usually form a general purpose register file for storing data as it is processed. A capacity of 32 data words is typical for a register file and each register can be accessed within a single clock cycle, that is in a few nanoseconds.
Most computer now have another level of IC memory called the Cache Memory which is positioned logically between register files and the main memory. Its capacity is less than main memory but access time is much lesser, that is this types of memories are much more faster than the main memory. This is because some or all of it can reside within a single IC or CPU. These are highly expensive but also provide high-performance. Today’s systems provide up to 8 MB of cache memory.
Main or Primary Memory:
This is large and fairly fast external memory which stores active data and programs. Storage location in this memory is directly addressed by the CPU’s load and store instructions. Though the technology of this memory is same as that of register file, access time is larger in this case because of its large capacity and as it is physically separated from the CPU. Now a days about 4 GB maximum of this is available though up to 8 GB can be used in a single system. Access times of five or more clock cycles are usual.
This types of memories are giant in capacity but comparatively very slow than all the other types of memory. This stores large data files, programs and files that will not be required continuously by the CPU. It also acts as an overflow memory when the capacity of the main memory exceeded. The information stays online of this memory but accessed via input/output operations provided programs. Examples are magnetic disks, optical disks etc where the mechanism which is applied is very slow. Access time is measured in milliseconds and up to 1 terabyte (1024 GB) is available.