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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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- Spring '10