(^302) PC Hardware: A Beginner’s Guide
Memory Addresses
InadditiontoorinplaceofI/Oaddresses,manydevicesrequireablockofmemoryinthe
upper memory area for their own use. This block of memory, referred to in conjunction
withthesystemresourcesusedbythedevice,isprimarilyformappingadeviceBIOSinto
memory or as a temporary holding area for data it is using or both. Memory address
blocks are assigned during the system boot process.
These memory blocks are not system resources in the sense of IRQs, I/O addresses,
and DMA channels, but Windows lists them along with the system resources on the
Computer Properties window (see Figure 13-14). Like the system resources, memory ad-
dressescancreateproblemsorconflictsshouldtwodevicesoverlaptheirmemoryblocks.
As illustrated in Figure 13-13, the devices that commonly use the memory address
blocks are Plug-and-Play device BIOS, bus architectures, CPU to bus bridges, and other
chipset and add-in card bus-related device controllers—in other words, devices that re-
quire their own device BIOS running in memory. A SCSI host adapter is another common
device that uses a dedicated memory address for its own BIOS. Network cards that fea-
ture Wake-on-LAN technology that allow a PC to be booted across a network also use a
memory address block to hold its boot BIOS.Direct Memory Access (DMA)
Direct memory access (DMA) is a technology that provides non-PCI bus adapters and de-
vices with the ability to access memory directly to move data in and out of RAM without
theneedforassistancefromtheCPU.Normally,theCPUcontrolsallactivitiesonthebus,
but on most newer systems, the DMA controller, which is a device that is integrated into
the motherboard, is permitted to move data into and out of RAM while the CPU takes
care of other tasks. ISA expansion cards and slots and IDE/ATA bus devices have access
to the DMA channels on the system. The PCI and AGP buses do not support DMA.Logical Device I/O Address IRQ
COM1 3F8 – 3FFh 4
COM2 2F8 – 2FFh 3
COM3 3E8 – 3Efh 4
COM4 2E8 – 2Efh 3
LPT1 378 – 37Fh 7
LPT2 278 – 27Fh 5Table 13-4. Logical Device Names for Serial and Parallel Ports