Wow, 2 years since I posted!  I have not stopped working on this but sometimes there is very little time spent on it.

However I will report on some of the happenings.  

Just before my last blog post I had started thinking about  mass storage.   An SD card seemed to be the thing to use.  I found that the smallest micro SD card that I had would very easily store 4 platters worth of the RK05  - in fact about 1000...  So no problem with size. 

I constructed an adapter card from an SD to microSD adapter and connected it to the FPGA board.

I first thought about using buffer memories to store data that goes and comes from the SD card.  Eventually I decided to go with the data break operation that DEC implemented.  We now call that DMA!  The state machine was enhanced to allow data break.

The next problem was getting or writing the HDL to control the SD card.  After a search, I settled on using code that Rob Doyle had posted on the opencores site.  I had previously looked at using the PDP8 code from that same package but was unable to make it fit into any of the FPGAs I had access to.  The code is written in VHDL.  I have been using Verilog.  

The open source tool chain I have been using is yosys.  It expects verilog as an input.  I looked at translation tools and found nothing that worked.  I then hand translated the VHDL to SystemVerilog.  From there I used the tool sv2v that takes systemverilog as an input and outputs verilog.  This tool is highly recommended.  If I were starting over I would probably write the whole thing in systemverilog then convert to verilog before feeding it to yosys.

I also used iverilog with the g2012 flag that signifies that the source is system verilog.  Two other tools are worth mentioning here:  verilator and verible-verilog.  Verilator  is primarily a simulator but it also does an excellent job of linting the source.  Verible-verilog-format  does a nice job of formatting the code.

I was able to boot OS8 in simulation up to the "." dot prompt.  However when trying to do the same thing on hardware it failed.   I had read on the cave pearl project that some sd cards take large amounts of current during boot up.  So I added a 5 volt to 3.3 volt regulator.  I then used that to source the current for the LEDs and to provide the power and ground for the sd card.  This was still a failure.

I used a program called dumprk05 to try and dump the sd card through the pdp8.  I was able to get the first sector to read and dump properly.  However the second sector was corrupted.  When the second sector is being read, the first sector is being output on the serial port.

So my current ideas are that I still have a power supply problem, signal integrity problems, or a bad instruction execution..  This whole thing is running from a USB port of a computer, it is still entirely possible that the power supply is drooping.  The second idea is that I have signal integrity problems.  I have no test equipment to trouble shoot this.

It is possible that I have an instruction  that is misbehaving on hardware.  I previously found an instruction that was misbehaving on both the simulator and hardware but the DEC diagnostics did not find.   I also found that accessing memory within one clock cycle would simulate correctly but would not work on hardware.

I have made changes to the state_machine to reduce the number of places where I repeat code just in case I have missed some, this will be the subject of another blog post in the near future.  This also did not result in the SD card working properly.

So I am going to leave the code in the git repository and explore using spiflash, probably as a read only file system.