Chapter 11: Expansion Cards^241
I/O Addresses
After a device requests an action from the CPU using an IRQ, the CPU responds to the
requesting device with a signal that indicates that either the task is completed or it could-
n’t be done, or the CPU has some data or value it needs to pass to the device. The CPU
can’t send the data to the device over the IRQ line, so a small amount of memory is set
aside for each device to receive responses from the CPU. Sort of like one-way message
boxes. Each of these boxes has an assigned address that represents where it is in memory
and its size. This message box is more formally called the I/O address (or base memory
address, I/O port, or port address). The address of the I/O area assigned to each device is
represented as a hexadecimal address range in memory. Table 11-2 lists a few of the I/O
addresses assigned on a PC. I haven’t listed all 65,000+ addresses that are available to be
assigned, only a few that deal with expansion cards.Direct Memory Access
Direct memory access (DMA) allows a device to communicate directly with the PC’s
system memory without the assistance or intervention of the CPU. In a normal PIO
(programmable input/output) data transfer (the normal kind of data transfer), the CPU
controls the movement of the data into RAM. A DMA transfer moves data directly from
its source to RAM.IRQ Default Assignment
4 COM1, COM3
5 Sound card
6 Floppy Disk Controller (FDC)
7 LPT1
8 CMOS Clock
9 Reserved link to IRQs 0–7
10 Available
11 Available
12 Available
13 Math coprocessor
14 Hard disk controller
15 AvailableTable 11-1. Default IRQ Assignments on a PC(continued)