segmented memory models

Sridhar Ayengar wrote: I can't argue that. Are you stating, though, that the 8080 or 8086 design did not go through a prototype stage? I would rebut (only slightly) that I find it is getting harder to do a "prototype" design. Customers want it right the first time, there's no time or budget for a "prototype", and it cna be hard to simulate a new design to the degree needed to ensure all of the issues are resolved. Luckily, new designs ararely break a ton of new ground, and costs of raw materials are much less now, so it's easier to hedge your bets by designing in some backup plans for a worst case scenario. To keep things on topic, I find the CBM 1351 mouse design serves as a good example of my point. Long after the C64 had been introduced, it was still a good seller, and Berkely Softworks had managed to put a GUI on it in the name of GEOS. A clever chunk of code, no matter your impression of GUIs in general. While I never found the tool all that exciting from a user point of view, the developer in me marveled at the functionality they crammed into 64kB and 320x200 and 170kB disks. Still, a UI needs a mouse, and the C64 was not designed for one. If we take Tony's perspective, we know that Xerox was doing GUI and mouse-driven UI work ong before the 64 came out, so some might argue that the 64 design should have included a mouse port (heck, we coudl argue that for all PCs, but I digress). I don't take that view. Still, CBM engineers looked at their options for adding a mouse to the 64. The obvious candidates were the soft 1200 bps uart on the user port, but that would preclude telecomm apps. An expansion card option with a proper uart was another path, but multi-slot expansion cards were not the norm then, and GEOS already effectively needed the expansion slot for the RAM expansion unit (REU). The cassette port could work, but the engineers no doubt kept coming back to the joystick port, as they is the normal location for non-KB input device installation. Hmmm, 5 IO pins and 2 8-bit ADCs are all they had to work with. Other designs used the analog pins for absolute positioning, but that made pixel-level positioning impossible. Clearly, some relative positioning approach was needed, and the design should minimize the use of the digital IO, since the joystick port IO was shared with the KB (one port are the low 5 bits of the KB columns, the other is the 5 low bits of the KB rows). In order to satisfy all of the business requirements (use no other ports, > 8 bit positioning, minimize digital IO usage), they came up with a clever (in my opinion) approach. They designed an IC that kept two digital counters (X and Y) and then translated the low 6 bits of the counters on the two POT lines. Instead of trying to generate a variable resistance (that would require tuning and other such things), they took advantage of the known properties of the SID POT lines. The lines work by counting the discharge of a cap to a preset reference value over a 256 cycle period. Further, then end the cycle by clamping the POT line to ground for 256 cycles. So, the design implemented was: Watch the POT line for the line to drop to ground. Wait 256 cycles wait another 256 - (low 6 bits of internal counter)*2 bring line high for (low 6 bits of internal counter)*2 cycles repeat This process ensured the line was always high at the end of the 512 cycle period in order to see the next clamp to ground event. The *2 took care of the inherent jitter of the design (PAL 64s ran a bit slower than 1MHz, NTSC a bit above, the ASIC in the mouse ran at 1MHz exactly, so the low bit of the SID POT line would jitter. Thus, the programmer would take the low 7 bits, lsr, and then have the counter value modulo 64. A bit of additional work on the programmer's side and overflow could be handled as well. The top bit was left off because the design relied on a small resistance on the ADC line to allow the ASIC to drive the line high but still see the low clamp event. Thus, there was no way to reach above 250 or so on the scale, and the design was much simpler if just 6 effective bits were used. Now, looking at it one way, the design is awful, as it abuses so many things. On the other hand, I find it an elegant solution to the problem at hand. Many 1351 mice were sold because they were easy to understand, reliable, and easy to install/remove. Jim