Flash Lock Out
Original Publication Date: 1995-Jan-01
Included in the Prior Art Database: 2005-Mar-29
Curry, SE: AUTHOR [+2]
Disclosed is a means to prevent unauthorized code from changing the contents of Flash memory. At power-up, software in the Flash updates the Flash memory if needed, then locks Flash using a hardware-supplied register bit.
Flash Lock Out
a means to prevent unauthorized code from changing
the contents of Flash memory. At power-up, software in the Flash
updates the Flash memory if needed, then locks Flash using a
hardware-supplied register bit.
systems use Flash memory to store the boot code, some
system vital product data, and power-on self test. Flash memory is
writable; this is desirable so that field upgrades to Flash may be
made without physically changing the part. However, Flash is
exposed, and errant code could overwrite the memory and destroy the
system's ability to boot up and run.
This exposure is eliminated on PowerPC systems as
o The Flash comes up in an open (writable) state.
o During power-on, software in the Flash checks to see if a Flash
update is required. A key sequence or special record on the
media may indicate that an upgrade is called for.
o If an upgrade is needed, then the upgrade boot occurs and Flash
o The PowerPC system hardware supplies a register bit which powers
on to a reset condition. When this register bit is written to
it cannot be reset except with a power-off-and-on.
o After a Flash upgrade, or after it is determined that no Flash
upgrade cycle is needed, the software sets the lockout register
bit, thus locking the Flash.
The system is now safe from any accidental...