Hi all, I think the way to go to replicate a PDP-11 (as somebody said before me in this thread), is to try to replicate the 11/20, as it is the simplest model. I think it would be a nice project to try to replicate a PDP-11 with easily obtainable parts, as nowadays its getting difficult to find one (or at least i'm not getting lucky). If the 11/20 gets done, somebody can use it as a base in order to try to build an 11/34 or a more complex machine.
So I have started to try to achieve this as a "side project" for me. Right now, I'm trying to replicate the a "simple" 11/20-like CPU in logisim, trying to follow the
wikipedia architecture page (not trying to clone it from the maintenance manuals on bitsavers). I'm not trying to make a perfect replica right now, as it is just a proof of concept, but I'm trying to follow wikipedia's specification. If I suceed, I would try to "fine tune" the design in order to get a proper "clone" of a 11/20. Eventually from that I could try to assemble it with real hardware (but I'm designing it with that purpose in sight)
The ideas i have with this project are:
- Easy to find parts (74LSXX series, the "rarest" parts i'm planning to use are 74181, 74182 and 74170), no proms or EPROMS (I maybe need to use one for the microcode)
- Modular, 2-3 PCB, data path, control and console (to keep the cost low, as i think that making two medium size boards is cheaper than a really big one)
- Trying to use as less parts as I can, always that it doesn't affect the performance a lot.
Right now I'm designing the data path board (i still have to make the rotate and the flags logic) and the aproximated IC count is 28 (of course this number will only go up). If I end my logisim design and if it looks that i'm going to go serious with this, i post an update to get some feedback about the design.