> > > > To reinforce what others are saying disks are different from RAM. They > > are NOT just 0s or 1s. There are residual images. Think of them as > > being partly analog rather than truly digital. 1 and 0 on a disk are > > really 1ish or 0ish. > > This flies in the face of everything I think I know about digital. > It's either on or off, plus or minus, north or south, zero or one. A > miniscule grain of silicon is charged or not. No in between. >
In software, things are 1 or 0. In hardware, everything is really analog, and its viewed as 1 or 0 as an abstraction to make everything else feasible. Everything is ultimately made of transistors, which are just as analog as vacuum tubes (except that they are smaller, cheaper, faster, more durable, and more energy efficient.) Nothing is ever '1' or '0' in an actual circuit: its between 2V and 5V or its between 0V to 0.8V (using TTL levels as an example.) The engineers need to make sure that the inputs stay within those ranges, because otherwise the digital abstraction really falls apart, and the output depends entirely on the internal construction of the device. > When a disk file is "erased", the disk space allocated for it is > declared empty in the disk's table of contents, or file map, and that > section of the disk is available for writing new stuff. yes. > Once new stuff > is written is that space, the old information that had been there is > gone, forever. no. You're blurring the software and the hardware levels. At the hardware level, you have a disk with a lot of magnets that have a specific orientation and a specific field strength. At the software level, you have disk formats, and file allocation tables, and you "delete" a file by simply marking the space as unused. You're correct that "deleting" a file doesn't really remove it until the de-allocated space is re-used by something else. but even after those physical locations have been re-written to, the old data can STILL be re-read, because the physical bits on the disk are analog. Wiping the disk in the way that we've been discussing systematically overwrites every bit on the drive with a new value. But even after this, the old value can still be determined by reading it with a more precise device, which is why the process must be repeated several times. > In digital there are no palimpsests of which I am aware. thats a reasonably good analogy for whats going on. --Mike _______________________________________________ pen-l mailing list [email protected] https://lists.csuchico.edu/mailman/listinfo/pen-l
