Chapter 13: System Resources^283
devices through their mailboxes. After the CPU completes the requested task, any data or
instructions it needs to pass back to the requesting device is placed in the mailbox as-
signed to that device.Taking Control
Some devices have the ability to serve themselves for some actions and don’t need to
bother the CPU at all. This allows the CPU to continue to serve other requests and not be
interrupted or need to pass messages back to interrupting devices. Most of the actions re-
quested of the CPU by devices center around moving data in and out of memory. A device
that has the ability to directly access memory on its own without the need to interrupt the
CPU helps the whole PC operate more efficiently.The PC’s System Resources
The system resources of a PC, described in general terms above, are a set of three mecha-
nisms used by a PC’s devices and the CPU to communicate. Interrupt request (IRQ) The mechanism used by devices to request services
from the CPU. The IRQ is actually a wire on the motherboard bus over which
a signal is sent by a device to get the CPU’s attention. There are 16 IRQs on
all newer (since the PC XT) PCs. However, only 10 of them are available for
devices. The remaining 6 are reserved for system-level purposes.
Input/output (I/O) address The message box used by the CPU to pass
information to each of the devices on the PC. Every device attached to the
PC has an I/O address.
Direct memory access (DMA) A limited number of DMA channels are
available to devices that need the speed of accessing memory directly
without the assistance of the CPU.Interrupt Request (IRQ)
Peripheral devices communicate directly to the CPU through an interrupt request (IRQ).
When a device needs services that only the CPU can perform, it sets an IRQ to get the
CPU’s attention. The CPU reacts to the IRQ by interrupting its activities to service the re-
quest. There are 16 IRQs available, and they are assigned to devices that require the CPU
to handle data movement, data interpretation, error processing, and other tasks.
On the original PC design (the IBM PC and PC XT), only 8 IRQs were available. Today’s
PC has 16 IRQs that are made u pof two sets of 8 IRQs linked together by an IRQ in one set
that points to an IRQ in the other set.. Of the 16 IRQs, 5 are set aside for use by internal sys-
tem-level devices and one is used as the link between the two IRQ sets, leaving 10 available
for assignments to I/O devices. Table 13-1 lists the standard default assignments of IRQs.