On Tuesday, July 19, 2005 @ 6:25 AM, John Perry wrote:
mlist@safenet-inc.com wrote:
Anders Johansson [mailto:andjoh@rydsbo.net] wondered:
For many years, computers used non-volatile internal memory. It was called "core" memory.
That was news to me, as well. Any core memory that I ever saw used tiny ferrite cores, wound with a couple of turns of really fine wire... thousands of them.
Well, "core" originally referred to the donut-shaped cores. It was also applied (for a while) to plated-wire magnetic memory, bubble memory, and even semiconductor memory. In this sense, the idea was that the internal memory was the "core" of the memory system, and the drum, disk, and tape drives were auxiliary "non-core" memory. This was 25-30 years ago, when semiconductor memory first started replacing magnetic cores as the mainstream internal memory for computers.
In other words, little electro-magnets, and when the power went off, they reverted to their nominal state.
No, they didn't. Magnetic cores were set by applying a current sufficient to magetize them permanently in one direction, and read by applying the same current in the opposite direction to reverse the (permanent) magnetization. This reversal of magnetization induced a voltage in a sense wire that could then be read by the processor. If the core was originally magnetized in the "1" direction, a voltage was induced into the sense wire; if it was already in the "0" direction, no change in magnetization occured, and no voltage was induced.
The cores were stable in both directions, with or without power. That's why a rewrite pulse after each read pulse was needed to restore the original direction. On many computers, the write circuitry was not set up to avoid a spurious write pulse when power was lost, so having core did not really take advantage of the nonvolatility of the cores. "Nonvolatility" was an expensive option on many mainframes, and consisted simply of implementing special power-upset-proof write circuits.
John Perry
Interesting. I recall the part about the doughnuts being magnetized, but I didn't know that reading them reversed the polarity. It's pretty incredible how far things have progressed and how fast it's changing now. I think the pace of change is what has most companies outsourcing now. You send your new IT people in for training and within a short period of time, those skills are becoming passé. So, you either send everyone in for re-training or just give up, outsource everything, and let someone else worry about it. I've been doing some retraining of my own on my own dime here for a while. I'm just wondering if I'll have to check the help wanted ads in Bangalore when it's time to try to get back into the market. Greg Wallace P. S.: Yeah, way off topic, but the guy that started the thread has his answer and is moving on, so at least we're not holding him up.