Hardware Reference
In-Depth Information
havedisappearedwhenthepowerwaspreviouslyturnedoff,andtheprocessorwouldsub-
sequently find no instructions to run the next time the power was turned on. By placing a
ROM chip at this address, a system startup program can be permanently loaded into the
ROM and will be available every time the system is turned on.
“
Memory
”).
Normally,thesystemROMstartsataddressE0000horF0000h,whichis128KBor64KB
prior to the end of the first megabyte. Because the ROM chip usually is up to 128KB in
size, the ROM programs are allowed to occupy the entire last 128KB of the first mega-
byte, including the critical FFFF0h startup instruction address, which is located 16 bytes
from the end of the BIOS space. Some motherboard ROM chips are larger, up to 256KB
or 512KB in size. The additional code in these is configured to act as a video card ROM
(addressesC0000h-C7FFFh)onmotherboardswithbuilt-invideoandmightevencontain
additional ROM drivers configured anywhere from C8000h to DFFFFh to support addi-
tional onboard devices, such as RAID or network adapters.
Figure5.2
showsamapofthefirstmegabyteofmemoryinaPC;noticetheuppermemory
areas reserved for adapter card and motherboard ROM BIOS at the end of the first mega-
byte.
Figure 5.2
PC memory map showing ROM BIOS.
