The Memory Interface

Introducation

In a memory read operation the CPU loads the address onto the address bus. Most cases these lines are fed to a decoder which selects the proper memory location. The CPU then sends a read control signal. The data is stored in that location is transferred to the processor via the data lines.

In the memory write operation after the address is loaded the CPU sends the write control signal followed by the data to the requested memory location.

The memory can be classified in various ways i.e. based on the location, power consumption, way of data storage etc



The memory at the basic level can be classified as
1. Processor Memory (Register Array)
2. Internal on-chip Memory
3. Primary Memory
4. Cache Memory
5. Secondary Memory

Processor Memory (Register Array)

Most processors have some registers associated with the arithmetic logic units. They store the operands and the result of an instruction. The data transfer rates are much faster without needing any additional clock cycles. The number of registers varies from processor to processor. The more is the number the faster is the instruction execution. But the complexity of the architecture puts a limit on the amount of the processor memory.

Internal on-chip Memory

In some processors there may be a block of memory location. They are treated as the same way as the external memory. However it is very fast.

Primary Memory

This is the one which sits just out side the CPU. It can also stay in the same chip as of CPU. These memories can be static or dynamic.

Cache Memory

This is situated in between the processor and the primary memory. This serves as a buffer to the immediate instructions or data which the processor anticipates. There can be more than one levels of cache memory.

Secondary Memory

These are generally treated as Input/Output devices. They are much cheaper mass storage and slower devices connected through some input/output interface circuits. They are generally magnetic or optical memories such as Hard Disk and CDROM devices.
The memory can also be divided into Volatile and Non-volatile memory.

* Volatile Memory

The contents are erased when the power is switched off. Semiconductor Random Access Memories fall into this category.

* Non-volatile Memory

The contents are intact even of the power is switched off. Magnetic Memories (Hard Disks), Optical Disks (CDROMs), Read Only Memories (ROM) fall under this category.





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