pnr Posted January 20, 2018 Author Share Posted January 20, 2018 I am not sure if I understood your assumptions correctly - is your working assumption that some 99105 chips are actually 99000 chips without any support for macro code? I thought that we've been lucky with some 99105 chips behaving as 99110, but that a 99105 would still be a 99105 as a minimum. In short my assumption is that all of the 99000, 99105 and 99110 actually have macro code support, but only differ in what is inside the macro ROM. So far we have yet to find a true 99105 chip with a blank ROM: so far we have three 99105's that have 99110 ROMs inside, one that most likely has a 99000 ROM inside, and two that are as yet unidentified (one of which is your second chip - which also seems to have the 99000 ROM, the other is JH's chip - which I suspect has a 99110 ROM). The longer answer is that the 99xxx was designed in the closing days of manual mask production, with masks being taped out by hand on huge mylar sheets. In that context it does not make sense to have multiple designs, other than ROM contents. Some two years ago I thought that there perhaps never was a 99000 and that the TI990/10A actually used a 99105 chip. This idea has now reversed: perhaps there never was a 99105 ROM, but all are 99000 or 99110 ROMs. Maybe in the early years 99105's included chips that were fully functional except for an error in the ROM: this would have improved yields and thus reduced cost. On the other hand, the ROM covers only ~10% of the chip surface area, so functional chips with only an error in the ROM would be relatively rare among defects. As time progressed the yield must have gone up, perhaps as high as 80-90% in the second half of the 80's. If TI persisted with having no specific 99105 ROM that would mean that a very large share of 99105's currently floating around have functional 99000 or 99110 ROMs inside (and by 1985 it would have made no sense for TI to put new money into the 99xxx, at that point they were just producing them to recoup the investment). That theory seems to fit with what we observe. Based on findings so far I would say that the odds are 50-50 or perhaps 60-40 in favour of finding a chip with a 99110 ROM. 1 Quote Link to comment Share on other sites More sharing options...
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