Hardware Reference
In-Depth Information
SomethinkitisstrangethatthePCwouldstartexecutingBIOSinstructions16bytesfrom
the end of the ROM, but this design is intentionally built in to Intel's x86 processors. All
the ROM programmer has to do is place a JMP (jump) instruction at that address that in-
structs the processor to jump to the actual beginning of the ROM—in most cases, close to
F0000h—whichisabout64KBearlierinthememorymap.It'slikedecidingtoreadevery
book starting 16 pages from the end and then having all book publishers agree to place an
instruction there to jump back the necessary number of pages to get to page 1. By setting
the processor startup location in this way, Intel enabled the ROM to grow to be any size,
all the while keeping it at the upper end of addresses in the first megabyte of the memory
address space.
Adapter card ROMs are automatically scanned and read by the motherboard ROM during
the early part of the boot process—during the POST. The motherboard ROM scans a spe-
