PDP-8 Processor

Projects you are planning or working on with the Mojo
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Posts: 4
Joined: July 5th, 2014, 10:31 pm

PDP-8 Processor

Post by pdp8 » July 5th, 2014, 10:51 pm

I've been playing with the Mojo - it's been working great. Wow, these FPGAs are really amazing compared to the PLAs I used in college. I did do Verilog in college, so I was exposed to the concepts, albeit it was around 15 years ago. I'm not an electrical engineer however, so it was really only a basic introduction. I really admire anyone picking this stuff up without a formal class. I had no idea entire processors could easily be implemented in these relatively low cost FPGAs today.

So far, I've implemented enough of the PDP-8 (a 1967 DEC minicomputer - the father of modern computing) to pass the basic DEC diagnostics that don't require more than 4K RAM or a disk. I have a front panel emulator running in pure logic via the Arduino serial interface (it emulates all the buttons and lights that were on the straight-8 front panel), and a second serial port tied to an XBee radio to simulate the teletype. I managed to figure out enough of Xilinx's documentation to implement some block RAM, which I'll probably use as cache once I move the simulated core memory from the FPGA to the SDRAM shield.

I'm not going for authentic timing (the microcoded instructions run in one clock state, which they certainly didn't do on the original) or speed, but rather just making it into the computer I wish I could have had in the early 80s. It's not doing bad performance-wise - it runs at the 50mhz clock and instructions take between 1 and 4 cycles to complete (they would take less if I dual-ported the RAM). That's significantly faster than the original (probably 50X faster on average).

I'll probably stick the code up online when I'm comfortable with it's quality (or can at least hold my nose when I post it!).

Posts: 1
Joined: September 30th, 2014, 4:43 pm

Re: PDP-8 Processor

Post by nczempin » September 30th, 2014, 4:52 pm

PDP-8, nice. I'm designing a simple CPU from scratch, using mostly ideas from an 1982 book on "computer design" (more like CPU design in today's terms), which seems to based a lot on the PDP-8.


You seem to be a lot farther ahead than I, so if I can post my stuff so can you.

I occasionally put some diagrams and/or notes of my progress on my Google+ page, for example https://plus.google.com/+NicolaiCzempin ... FWxwwDEPXX and https://plus.google.com/+NicolaiCzempin ... cPUAY8HPEA.

So, don't be shy!

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