with addresses 640K to 1M being reserved for device data buffers in IBM PC

With addresses 640k to 1m being reserved for device

This preview shows page 80 - 82 out of 140 pages.

with addresses 640K to 1M being reserved for device data buffers in IBM PC compatibles, in addition to I/O ports 0 through 64K. [Page 226] How do these schemes work? In all cases, when the CPU wants to read a word, either from memory or from an I/O port, it puts the address it needs on the address lines of the bus and then asserts a READ signal on a bus control line. A second signal line is used to tell whether I/O space or memory space is needed. If it is memory space, the memory responds to the request. If it is I/O space, the I/O device responds to the request. If there is only memory space [as in Fig. 3-3(b)], every memory module and every I/O device
Image of page 80
compares the address lines to the range of addresses that it services. If the address falls in its range, it responds to the request. Since no address is ever assigned to both memory and an I/O device, there is no ambiguity and no conflict. 3.1.4. Interrupts Usually, controller registers have one or more status bits that can be tested to determine if an output operation is complete or if new data is available from an input device. A CPU can execute a loop, testing a status bit each time until a device is ready to accept or provide new data. This is called polling or busy waiting. We saw this concept in Sec. 2.2.3 as a possible method to deal with critical sections, and in that context it was dismissed as something to be avoided in most circumstances. In the realm of I/O, where you might have to wait a very long time for the outside world to accept or produce data, polling is not acceptable except for very small dedicated systems not running multiple processes. [Page 227] In addition to status bits, many controllers use interrupts to tell the CPU when they are ready to have their registers read or written. We saw how interrupts are handled by the CPU in Sec. 2.1.6. In the context of I/O, all you need to know is that most interface devices provide an output which is logically the same as the "operation complete" or "data ready" status bit of a register, but which is meant to be used to drive one of the IRQ (Interrupt ReQuest) lines of the system bus. Thus when an interrupt-enabled operation completes, it interrupts the CPU and starts the interrupt handler running. This piece of code informs the operating system that I/O is complete. The operating system may then check the status bits to verify that all went well, and either harvest the resulting data or initiate a retry. The number of inputs to the interrupt controller may be limited; Pentium-class PCs have only 15 available for I/O devices. Some controllers are hard-wired onto the system parentboard, for example, the disk and keyboard controllers of an IBM PC. On older systems, the IRQ used by the device was set by a switch or jumper associated with the controller. If a user bought a new plug-in board, he had to manually set the IRQ to avoid conflicts with existing IRQs. Few users could do this correctly, which led the industry to develop Plug 'n Play, in which the BIOS can automatically assign IRQs to devices at boot time to avoid conflicts.
Image of page 81
Image of page 82

You've reached the end of your free preview.

Want to read all 140 pages?

  • One '20

What students are saying

  • Left Quote Icon

    As a current student on this bumpy collegiate pathway, I stumbled upon Course Hero, where I can find study resources for nearly all my courses, get online help from tutors 24/7, and even share my old projects, papers, and lecture notes with other students.

    Student Picture

    Kiran Temple University Fox School of Business ‘17, Course Hero Intern

  • Left Quote Icon

    I cannot even describe how much Course Hero helped me this summer. It’s truly become something I can always rely on and help me. In the end, I was not only able to survive summer classes, but I was able to thrive thanks to Course Hero.

    Student Picture

    Dana University of Pennsylvania ‘17, Course Hero Intern

  • Left Quote Icon

    The ability to access any university’s resources through Course Hero proved invaluable in my case. I was behind on Tulane coursework and actually used UCLA’s materials to help me move forward and get everything together on time.

    Student Picture

    Jill Tulane University ‘16, Course Hero Intern

Stuck? We have tutors online 24/7 who can help you get unstuck.
A+ icon
Ask Expert Tutors You can ask You can ask ( soon) You can ask (will expire )
Answers in as fast as 15 minutes
A+ icon
Ask Expert Tutors