Memories - Introduction - Part 1

Memories are pervasive in digital computing. Consider, the personal computer which has a main memory, video memory, translation ROMs, shadow ROMs, scratchpad memory, hard disk, floppy disk, CDROM, and various other kinds of storage distributed throughout. In addition, the die that contains the microprocessor also contains one or more levels of cache.

A typical PC is depicted in the block diagram below. It is basically a memory hierarchy connected by several buses and adapters and controlled by a CPU. The purpose for much of the hierarchy is to combine two or more storage systems with divergent capacities, speeds, and costs such that the combined system has almost the speed of the smaller, faster, more expensive memory at almost the cost, speed, and storage capacity of the larger, slower, less expensive memory. Clearly, not all storage devices are part of this hierarchy.

The CDROM may be used to deliver programs and/or data to an end user, and video memory is dedicated to the display console. The central processing unit (CPU) accesses many of these auxiliary memory devices through a peripheral component interconnect (PCI) bus, which regulates the flow of data through the system. Unlike the random logic that has been considered up to this point, memory storage devices are characterized by a high degree of regularity. For example, a semiconductor memory is organized as an array of cells, while storage on a hard drive is organized into cylinders. This regularity of semiconductor memories permits much greater packing of transistors on die. For example, in the PowerPC MPC750, memory accounts for 85% of the transistors but only 44% of the die area. In the Alpha 21164, 80% of the 9.6 million transistors are used for three on-chip caches, but the remaining 20% of the transistors occupy a majority of the physical die area. The various storage devices in the Figure above employ different kinds of circuits for storing and retrieving data, and different kinds of media for retaining data, hence they have unique failure mechanisms, that require different test strategies. These memories may also employ varying levels of redundancy to detect and/or correct errors during operation.

To be continued...

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