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54 / 330 page  <%27 2GTKRJGTCNU 7UGT /CPWCN UM008101-0601 Direct Memory Access DMACs are used when one or more of the following situations or require- ments are present: • CPU has too much I/O and cannot perform other tasks properly • Data transfer must be faster than the CPU can perform • Transfer response tune (startup) must be faster than the CPU can provide Small and low-performance systems generally run without DMA. Medium-performance systems can also be designed without DMA if the CPU can handle transfers fast enough and still perform other operations. When systems require fast transfers or fast response, DMACs are strong candidates for performance enhancement. Not only do DMACs transfer faster than most CPUs, but the response time is better. Response times can even be improved using the techniques described above for CPU response. The following examples are cases in which DMA is usually the best choice: • Disk and diskette controllers • Scanning operations, such as CRT I/O • Data acquisition • Memory-to-memory transfers • Memory searches • Backup storage (I/O-to-I/O) • Parallel bus systems such as the IEEE 488 • Fiber optic links • Block transfers in networking, multiprocessing, or multiprogramming The trade-off for speed is that the CPU typically remains idle and lacks full or partial control of the system bus while the DMA is operating. This condition can affect total system throughput, and can also affect such things as memory refresh and other interrupts. |
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