TAILIEUCHUNG - Linux Device Drivers-Chapter 8 :Hardware Management

Tham khảo tài liệu 'linux device drivers-chapter 8 :hardware management', công nghệ thông tin, hệ điều hành phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Chapter 8 Hardware Management Although playing with scull and similar toys is a good introduction to the software interface of a Linux device driver implementing a real device requires hardware. The driver is the abstraction layer between software concepts and hardware circuitry as such it needs to talk with both of them. Up to now we have examined the internals of software concepts this chapter completes the picture by showing you how a driver can access I O ports and I O memory while being portable across Linux platforms. This chapter continues in the tradition of staying as independent of specific hardware as possible. However where specific examples are needed we use simple digital I O ports like the standard PC parallel port to show how the I O instructions work and normal frame-buffer video memory to show memory-mapped I O. We chose simple digital I O because it is the easiest form of input output port. Also the Centronics parallel port implements raw I O and is available in most computers data bits written to the device appear on the output pins and voltage levels on the input pins are directly accessible by the processor. In practice you have to connect LEDs to the port to actually see the results of a digital I O operation but the underlying hardware is extremely easy to use. I O Ports and I O Memory Every peripheral device is controlled by writing and reading its registers. Most of the time a device has several registers and they are accessed at consecutive addresses either in the memory address space or in the I O address space. At the hardware level there is no conceptual difference between memory regions and I O regions both of them are accessed by asserting electrical signals on the address bus and control bus . the read and wr esignals 31 and by reading from or writing to the data bus. 31 Not all computer platform use a read and a wr te signal some have different means to address external circuits. The difference is irrelevant at software level .

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