In the old days if two devices shared an irq the

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Unformatted text preview: l-in-One Exam Guide / Meyers & Jernigan / 170133-8 / Chapter 8 CompTIA A+Certification All-in-One Exam Guide 308 Port I/O Base Address IRQ COM1 03F8 4 COM2 02F8 3 COM3 03E8 4 COM4 02E8 3 LPT1 0378 7 LPT2 Table 8-1 COM and LPT Assignments 0278 5 Notice that the four COM ports share two IRQs. In the old days, if two devices shared an IRQ, the system instantly locked up. The lack of available IRQs in early systems led IBM to double up the IRQs for the serial devices, creating one of the few exceptions to the rule that no two devices could share IRQs. You could share an IRQ between two devices, but only if one of the devices would never actually access the IRQ. You’d see this with a dedicated fax/modem card, for example, which has a single phone line connected to a single card that has two different functions. The CPU needed distinct sets of I/O addresses for fax commands and modem commands, but as there was only the one modem doing both jobs, it needed only a single IRQ. Direct Memory Access CPUs do a lot of work. They run the BIOS, operating system, and applications. CPUs handle interrupts and I/O addresses. CPUs also deal with one other item: data. CPUs constantly move data between devices and RAM. CPUs move files from the hard drive to RAM. They move print jobs from RAM to laser printers, and they move images from scanners to RAM, just to name a very few examples of this RAM-to-device-andback process. Moving all this data is obviously necessary, but it is a simple task—the CPU has better things to do with its power and time. Moreover, with all of the caches and such on today’s CPUs, the system spends most of its time waiting around doing nothing while the CPU handles some internal calculation. Add these facts together and the question arises: Why not make devices that access memory directly, without involving the CPU (Figure 8-23)? The process of accessing memory without using the CPU is called direct memory access (DMA). DMA is very common and is excellent for creating background sounds in games and...
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