Hardware: Convenient debugging

[sy2002]

This is a spin-off of #66 . This topic needs to be thought through holistically:

• Hardware debug module using the interrupt system
• Monitor enhanements
• The ideas of Michael, as described in this comment and copy pasted below, including the VGA overlay:

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I agree, we need something like this. I tried debugging the latest cpu_test.asm on hardware, and that was quite tedious.

Regarding ncurses I would love to have support for that. It would make porting existing C-programs to QNICE almost trivial in many cases. I do think this is the best long-term solution, i.e. to implement a ncurses library.

Regarding debugging in hardware, I've in previous projects used an VGA overlay, i.e. a virtual layer on top of the current VGA screen that displays all relevant state information of the processor. This would all be implemented in pure VHDL code, and could be enabled/disabled by one of the switches. It would be "simple" to e.g. disassemble ten lines around the current PC, to implement single-step, to set breakpoints, etc.

Something like that can be done in various stages, where the first stage is just to display the current value of R0-R15, the second stage is to disassemble the current instruction, etc. Controlling the execution could be done using the push-buttons.

We still have a way to go before V1.6 is finished, and then we have to decide on, what we would like to include in V1.7. Only then can we give a very rough estimate of when V1.7 might be finished.