Memory is pervasive in digital products. Consider, for example, the personal computer (PC). It has 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 may also contain one or more levels of cache. A typical PC is depicted in the block diagram of Figure . It is basically a memory hierarchy connected by several buses and adapters and controlled by a CPU. . | CHAPTER 10 Memory Test INTRODUCTION Memory is pervasive in digital products. Consider for example the personal computer PC . It has 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 may also contain one or more levels of cache. A typical PC is depicted in the block diagram of Figure . 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 In the Alpha 21164 80 of the million transistors are used for three on-chip caches but the remaining 20 of the transistors occupy a majority of the physical die The various storage devices in Figure employ different kinds of circuits for storing and retrieving data and different .
