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Unformatted text preview: le, suppose that the disk controller is mapped to port 0xa0. Then the CPU might initiate a disk read by executing three store instructions to address 0xa: The first of these instructions sends a command word that tells the disk to initiate a read, along with other parameters such as whether to interrupt the CPU when the read is finished. (We will discuss interrupts in Section 8.1). The second instruction indicates the number of the logical block that should be read. The third instruction indicates the main memory address where the contents of the disk sector should be stored. After it issues the request, the CPU will typically do other work while the disk is performing the read. Recall that a 1 GHz processor with a 1 ns clock cycle can potentially execute 16 million instructions in the 16 ms it takes to read the disk. Simply waiting and doing nothing while the transfer is taking place would be enormously wasteful. After the disk controller receives the read command from the CPU, it translates the logical block number to a...
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