Memory is the part of the computer where your program and data are while
they are being used by the CPU.
Contrast this to a hard disk or floppy, where
the program is sitting on the disk and is not being used. (Of course with
operating systems like Linux, parts of both the program and the data can be
stored on the disk, even as the program is running.) There are two types of
memory that are most common talked about: RAM
In most memory today, an extra bit is added for each byte. This is a parity
bit. Parity is a simple way of detecting errors within a memory chip
(among other things). If an odd number of bits is set, the parity
bit will be
set to make the total number of bits set an even number (most memory uses even
parity). For example, if three bits are set, the parity
bit will also be set to
make the total bits set four.
When data is written, the number of set bits
is calculated and the parity
bit is set accordingly. When the data is read, the
parity bit is also read. If the total number of bits set is even, all is well.
However, if an odd number of data bits is set and the parity
bit is not set, or
if an even number of data bits is set and the parity
bit is set, a parity error
When a parity
error occurs in memory, the state of the
system is uncertain. To prevent any further problems, the parity
generates a Non-maskable Interrupt (NMI), and the CPU
immediately jumps to
special codes called NMI
When Linux is interrupted with
as the result of a parity
error, it too realizes things are not good, and
the system panics. The panic
causes the system to stop everything and shut down.
Certain machines support ECC
(Error Correcting Code) RAM,
which corrects parity
problems before it kills your system.