- Memory goals
- Free
- Fast
- Nonvolatile
- Portable
- Random access
- Hierarchy
CPU registers -- access in nanoseconds
CPU Cache -- access in 10 nanoseconds
Second level cache -- access in 30 nanoseconds
Main memory -- access in 100 nanoseconds
Electronic disk -- access in 100 nanoseconds
- useful to overcome address space limits
- can use cheap RAM
Magnetic disk -- access in 10,000 nanoseconds
- describe track/sector/platter/cylinder
- Can have a change media event
- normally sector write is atomic
- hard drive heads float, floppy heads ride surface
Magnetic tape -- access in 10's of seconds
- Can have a change media event
- have blocks (like sector) can have track.
- explain helix scan
- VM stored on levels down to mag disk
- Files stored on levels up to electronic disk.
- Caching Problems
- Different apps on same CPU can read different values for the same
variable
- Different CPUs can read different values for the same RAM location.
-- this must be fixed!
- After media switch, caching can cause WRONG values to be read from
floppy disk or tape.