So there's a lot of weird stuff about the Lisa's design, and not just the GUI, but also the hardware.
For example, the latches for various things like selecting the MMU segment can be triggered by READING from certain addresses. Sure, this was done because it's easy enough to decode a bus address and then trigger a flop without relying on the R/!W line, so that's clearly a cost saving hack.
The MMU design itself is pure genius, created by Rich Page of course. This adds an MMU to a CPU that wasn't designed for one, and thus allowing virtual memory and memory protection to work, and it was done with just some adders and some extra RAM.
But there's something really weird about the VSROM. The Video State ROM is a wonderful hack, and this isn't all that weird on the surface. It implements a state machine where rather than use up a ton of logic chips to build a state machine, a ROM was used a short cut. And you could design lots of things in this way. For example, any gate you look at, such as OR, AND, NAND, XOR, etc. can be implemented simply in ROM, or even half adders, but this isn't usually done because those gates are much cheaper than a masked ROM or PROM.
You'd take the inputs and tie them to the ROM's address lines, and your output(s) would be on the data lines, and you could do with one chip what could take dozens normally.
So, in this case the video state ROM does save on a ton of circuitry by putting all that on a single ROM, so it's quite cost effective for the 1980s. So far so good, it all makes great sense.
Given a bit of timing input on the address lines, the VSROM outputs whether or not to signal the horizontal or vertical retrace signals which define the size of the screen. and the number of vertical lines, Indeed this chip is what allows the Screen Modification kit to work, because its timing is different, and thus produces a different resolution and square pixels instead of rectangular ones.
But one thing does not make sense.
The Lisa's serial number is actually hidden in the video state ROM. And this machine is pretty much the only one that had such a feature. It would make more sense on a Mac or an Apple II where piracy would be far more rampant than on a Lisa. After all, most of the Lisa's LOS software except one or two titles was published by Apple itself. So anyone who had a Lisa, had all the Lisa software bundled with the Lisa, so really there wouldn't be much need for anti-piracy measures. At least not in that context.
But then, there's this:
https://en.wikipedia.org/wiki/Van_Eck_phreaking. This isn't really common knowledge, but it was known amongst some circles in the mid-to-late 1980s. With old CRT TVs and computer monitors it was possible to use a bit of circuitry, the right antenna and another TV. This allows the eavesdropper to view the victim's screen from a distance, and so it could have been, but wasn't, the stuff of James Bond movies. This was surely known of in the three-letter-agencies, such as the CIA and NSA, and funnily enough in the UK it was known by the TV detector van people - in the UK, you'd have to pay a tax to the BBC to watch TV, and there were vans going around looking for unlicensed TVs. (
https://en.wikipedia.org/wiki/TV_detector_van ) - when the first early computers arrived in the UK there was an anecdote about someone's computer monitor being detected as an unlicensed TV, but that they couldn't figure out which channel was being watched.
(The page mentions that optical emissions are used but it's more likely that radio signals were being used.)
But the real implication here of the VSROM is that the serial number was being pumped out one bit at a time on every frame is that someone with the proper TEMPEST equipment could identify the serial number of the Lisa from a distance (as well as get a fuzzy image of what was on the display.)
Now the real fun thing is that Van Eck didn't publish his paper until 1985, and by then the Lisa was already out, as work on it began in 1980 and it was on the market by 1983. So whomever designed the video circuitry and decided to put the serial number in the VSROM had information about TEMPEST, so they probably previously worked at one of these agencies (or had spoke with Van Eck and known of his work before deciding to add this to the Lisa)
In fact, there were such things as TEMPEST hardened computers, and you can even find an example of a Mac 512K listed on eBay incorrectly as a Lisa right now. (
https://www.ebay.com/itm/Apple-512K-Lisa-Computer-ValueTec-2900T-Tempest-Shielded-RARE/143191209815? ) Various agencies used such machines to guard against eavesdroppers.
Interestingly, the Lisa 1 and 2, had an AMD9510 floating point math coprocessor, but as John of Vintage Micros pointed out, (see:
https://www.ebay.com/itm/Apple-Lisa-2-10-XL-WD2001-Data-Encryption-Chip-with-Socket-MINT/141829462056 ) the 2/10 has the ability to add an and WD2001 encryption chip, which could do DES, but on the 2/10 there's no soldered in socket. So perhaps Apple had some relations with Uncle Sam for these. Of course, there's no currently available Lisa software that uses the coprocessor, nor the encryption chip, which is sad...
So, anyway, either the serial number on the VSROM was designed with TEMPEST in mind, or it was an insanely huge coincidence, you decide.
Has anyone played with TEMPEST detection on here? Perhaps it would be a nice electronics project to make a Lisa serial number detector and potentially see if you could read a Lisa's screen from across the street?
Edit: added some source code from the H-ROM that reads the serial number. Interesting thing is that LOS doesn't use this serial number which is written to low memory, instead, it re-reads it again after it boots up! (I suppose the logic is that you could power on your Lisa, go into service mode, and overwrite the serial number, so they re-read it again to ensure you can't do that.)
Edit2: Since the SN is presented in the vertical retrace, it might be at the top of the screen on video tape recorded off the Lisa in the same way as captions were sent. see:
https://en.wikipedia.org/wiki/Closed_captioningFor all types of NTSC programming, captions are "encoded" into line 21 of the vertical blanking interval - a part of the TV picture that sits just above the visible portion and is usually unseen. For ATSC (digital television) programming, three streams are encoded in the video: two are backward compatible "line 21" captions, and the third is a set of up to 63 additional caption streams encoded in EIA-708 format.[24]
