Hardware and Computer Organization- P14:Today, we often take for granted the impressive array of computing machinery that surrounds us and helps us manage our daily lives. Because you are studying computer architecture and digital hardware, you no doubt have a good understanding of these machines, and you’ve probably written countless programs on your PCs and workstations. | CHAPTER Memory Revisited Caches and Virtual Memory Objectives When you are finished with this lesson you will be able to Explain the reason for caches and how caches are organized Describe how various caches are organized Design a typical cache organization Discuss relative cache performance Explain how virtual memory is organized and Describe how computer architecture supports virtual memory management. Introduction to Caches As an introduction to the topic of caches and cache-based systems let s review the types of memories that we discussed before. The major types of memories are static random access memory SRAM dynamic random access memory DRAM and nonvolatile read-only memory ROM . SRAM memory is based on the principle of the cross-coupled inverting logic gates. The output value feeds back to the input to keep the gate locked in one state or the other. SRAM memory is very fast but each memory cell required five or six transistors to implement the design so it tends to be more expensive than DRAM memory. DRAM memory stores the logical value as charge on a tiny charge-storage element called a capacitor. Since the charge can leak off the capacitor if it isn t refreshed periodically this type of memory must be continually read from at regular intervals. This is why it is called dynamic RAM rather than static RAM. The memory access cycles for DRAM is also more complicated than for static RAM because these refresh cycles must be taken into account as well. However the big advantage of DRAM memory is its density and low cost. Today you can buy a single in-line memory module or SIMM for your PC with 512 Mbytes of DRAM for 60. At those prices we can afford to put the complexity of managing the DRAM interface into specialized chips that sit between the CPU and the memory. If you re a computer hobbyist who likes to do your own PC upgrading then you ve no doubt purchased a new motherboard for your PC featuring the AMD nVidia Intel or VIA chipsets. The chipsets have become
