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Unformatted text preview: sector address, reads the contents of the sector, and transfers the contents directly to main memory, without any intervention from the CPU (Figure 6.12(b)). This process where a device performs a read or write bus transaction on its own, without any involvement of the CPU, is known as direct memory access (DMA). The transfer of data is known as a DMA transfer. After the DMA transfer is complete and the contents of the disk sector are safely stored in main memory, the disk controller notiﬁes the CPU by sending an interrupt signal to the CPU (Figure 6.12(c)). The basic idea is that an interrupt signals an external pin on the CPU chip. This causes the CPU to stop what it is currently working on and to jump to an operating system routine. The routine records the fact that the I/O has ﬁnished and then returns control to the point where the CPU was interrupted.
Aside: Anatomy of a commercial disk. Disk manufacturers publish a lot of high-level technical information on their Web pages. For examp...
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