David King wrote:
The BIOS is also simply a piece of software, stored
in a chip on the mainboard.
In memory, a program's bits are represented by the voltages of
transistors in particular places on a DRAM chip. On a CD, by the
width of pits in the surface of the CD. In chips like BIOS and other
types of firmware that don't need power to maintain their state but
that can be re-written, how are the bits physically represented, and
how are they read out to memory?
Look up hot-carrier injection and quantum electron tunneling. Basically
such devices use over-voltage to cause electron migration within the
device. The migrated electrons result in measurable changes in the
electrical characteristics of the gate (i.e., threshold voltage). The
process is reversable but destructive, since you can't put the electrons
back exactly where they were before. This is also why EEPROMs and flash
devices have write-count lifetimes.
--
Darren Pilgrim
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