主机操作系统不再需要考虑块到文件的映射，存储设备本身会提供这种映射，因此主机可以在文件级进行操作。
Rather than the host operating system needing to worry about block-to-file mapping, the storage device itself provides this mapping, allowing the host to operate at the file level.
只有当时钟线为低的时候，主机才可以改变数据线(也就是将数据写入到数据线)。数据将在时钟为高电平的时候被设备读龋。
The host changes the data line only when the Clock line is low, and data is read by the device when Clock is high.
US B协议层描述了USB主机与usb设备交互时的语法和协议，从中定义了字段、包、事务和传送的结构，以及由字段到包、由包到事务、由事务到传送所组成的组织层次关系。
USB protocol layer describes the grammar and protocol used in the communication of USB host with USB devices, and presents the USB hierarchy consisting of fields, packets, transactions and transfers.
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