Bruce, On Tuesday 19 July 2005 06:14, Bruce Marshall wrote:
On Tuesday 19 July 2005 09:05 am, Anders Johansson wrote:
...
For many years, computers used non-volatile internal memory. It was called "core" memory.
I didn't know that, that is interesting. What was that based on? I'm guessing it wasn't based on the temporary flow of electricity, the way modern memory sticks are
Core memory usually was little 'doughnuts' of magnetic material with wires running through the core. Depending on how the current flowed, the doughnut could be magnetized in one polarity or the other.... (0 or 1)
And reading a bit erased it, necessitating a write-back after every read. The major reason the PDP-11 included modify addressing modes was to optimize the pattern of reading a value, computing a new value and writing that new value back to the same location. The Unibus memory addressing model cooperated with the instruction architecture in making this possible, with a distinct "read, but don't refresh" cycle. Also, DECtape was random access tape, in the sense you could write any block on the tape at any time and the drive would seek the the tape. This was made possible by subjecting the tape to a formatting operation that assigned distinct locations to each sector, just as formatting a disk drive does. All interesting things now mercifully consigned to museums! And yes, pretty much off-topic. Randall Schulz