> Date: Thu, 02 Sep 2010 20:08:01 +0200 > From: "dott.Piergiorgio" <dott.piergior...@fastwebnet.it>
> (looking to the DECTape marks issue, for example) What is "the DECtape marks issue"? The DECtape emulation in SimH does not simulate the mark track on a DECtape, so I don't understand how there could be a "DECtape marks issue". I suggested to Bob several years ago that the DECtape representation could be made more realistic (and thereby more flexible). His reasonable answer was to point out that I could do the implementation, should I choose to do so, and submit it for everyone's review; since I am not a C programmer, I have never had the time to do that. (Macro-20, sure.) I'll outline my original vision, and my present simplification, and if you (or anyone else) wants to pursue it, I'll be happy to provide insight into the working of DECtape drives and controllers. Represent the frames on a DECtape as 8-bit bytes; the timing tracks are implicit in the order of the bytes in the resulting file. Represent the mark tracks with bits 0 and 7, one the inverse of the other (as a nod to the automatic inversion of bit sense in a Read Backwards operation). The data tracks are 1 = bits 1 and 4, 2 = 2 and 5, and 3 = 3 and 6, where bits 1-2-3 are conjointly inverted with respect to bits 4-5-6. We ignore the fact that on a physical DECtape, 5 tracks' worth of data is redundantly recorded on 10 tracks (T M 1 2 3 1A 2A 3A M T). This can be simplified if we drop the inversion, and represent each frame in an 8-bit byte where the mark track is bit 0 and data tracks are 5-6-7. The controller (555, TC01, TC02, TC08, TC09, TD10, TD8E, TC11, or TC15) is responsible for assembling 3 bits of data at a time into 12, 16, 18, or 36 bit words and putting it into the CPU or memory, as appropriate. That's where all the work is. There's also the question of other brands of computer (Data General comes to mind) and their controllers, as well. Creating realistic DECtape emulations for SimH could turn into a cottage industry. But let's return to the question: WHAT "DECtape marks issue"???????? Rich _______________________________________________ Simh mailing list Simh@trailing-edge.com http://mailman.trailing-edge.com/mailman/listinfo/simh