Hardware Reference
In-Depth Information
that enables the PC to boot; otherwise, the processor would have no program in memory
to execute when it was powered on.
The main ROM BIOS is contained in a ROM chip on the motherboard, but there are
also adapter cards with ROMs on them. ROMs on adapter cards contain auxiliary BIOS
routines and drivers needed by the particular card, especially for those cards that must be
active early in the boot process, such as video cards. Cards that don't need drivers active
at boot time typically don't have a ROM because those drivers can be loaded from the
hard disk later in the boot process.
Most systems today use a type of ROM called electrically erasable programmable ROM
(EEPROM) , which is a form of flash memory. Flash is a truly nonvolatile memory that is
rewritable, enabling users to easily update the ROM or firmware in their motherboards or
any other components (video cards, SCSI cards, peripherals, and so on).
For more information on BIOS upgrades, see Upgrading the BIOS ,” p. 265 ( Chapter
5 , BIOS ”).
DRAM
DynamicRAM(DRAM)isthetypeofmemorychipusedformostofthemainmemoryin
a modern PC. The main advantages of DRAM are that it is very dense, meaning you can
pack a lot of bits into a small chip, and it is inexpensive, which makes purchasing large
amounts of memory affordable.
The memory cells in a DRAM chip are tiny capacitors that retain a charge to indicate a
bit. The problem with DRAM is that it is dynamic—that is, its contents can be changed.
With every keystroke or every mouse swipe, the contents of RAM change. And the entire
contents ofRAMcanbewipedoutbyasystemcrash.Also,because ofthedesign,itmust
be constantly refreshed; otherwise, the electrical charges in the individual memory capa-
citors drain and the data is lost. Refresh occurs when the system memory controller takes
a tiny break and accesses all the rows of data in the memory chips. The standard refresh
timeis15ms(milliseconds),whichmeansthatevery15ms,alltherowsinthememoryare
automatically read to refresh the data.
See Chipsets ,” p. 169 ( Chapter 4 , Motherboards and Buses ”).
Unfortunately,refreshingthememorytakesprocessortimeawayfromothertasksbecause
eachrefreshcycletakesseveralCPUcyclestocomplete.Inoldersystems,therefreshcyc-
ling could take up to 10% or more of the total CPU time, but with modern systems run-
ning in the multigigahertz range, refresh overhead is now on the order of a fraction of a
percent or less of the total CPU time. Some systems allow you to alter the refresh timing
 
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