Debug modes/debug cores for developing software

[oliverrreed]

Idea

It would be kinda cool to be able to turn the GameBub into a dev target for GBA software development. Things like being able to use it as a remote GDB server (for stepping through execution, breakpoints, and looking at memory/registers) and having a non-standard serial interface to send logs to with dev builds of software could make it an alternative to software emulators.

Approach

There's a couple of different approaches to getting something to be a GDB target from what I've looked at so far.

One is to implement a GDB stub that software that would run on GameBub would link to and then expose a UART connection that the stub could read from and write to. This library could also provide basic text out for logging.

The other option is to still expose a UART connection, but make it talk to a controller in the FPGA core itself that has the ability to send and receive specific commands for controlling the core's execution and memory. For logging and such, you'd still likely need some sort of standard library that the software can link to that does non-standard GBA stuff.

I think the second option is probably easier to implement and actually use since this is an FPGA project. I also don't claim to be an expert in this and have only spent about 30 minutes reading up and trying to understand what would be needed. I was just working on some GBA software development and thought that it would be cool to see such features on the GameBub.

[elipsitz]

One way to do this would be to add JTAG support to the GBA core, and then use BSCANE2 to connect it to the FPGA’s JTAG port. Or directly to the PMOD expansion port.

This could also be used with semihosting to add logging.

If you went with the gdbstub route, it’d probably be easier to implement that on the MCU rather than the FPGA, and then expose it over USB

[oliverrreed]

Ah! I totally forgot about JTAG. Once the release hardware, firmware, and core sources are released I'll start looking at this more in-depth to see if I can't get something working. Thanks for the reply!