This past week, outside of our group meetings, I looked into what would become our requirements for the processor in our handheld. Previously, all we really had nailed down was that we wanted something Arm clocked at at least 16MHz. We can now specify some more detail on that.
Based on our projects overall goals of providing a safe development environment, we decided to create a small software kernel for our console. This means our processor must provide the tools to implement such a kernel, namely memory protection. Arm processors do offer memory protection, but as an optional component, so we’ll need to ensure that if we use an Arm processor it offers memory protection. We must also ensure that any processor we use allows us to externally send interrupts to it. This will become important when managing our graphics and sound output. Finally, we’ll need a timing register to allow our kernel to support some basic timing facilities. Currently, I think our best bet is to simply expose a configurable timing interrupt to the user. Such an interrupt would hand off to some user provided function in a specified interval and allow the user to perform some time sensitive operations. Doing so would allow the user to track the passage of time and perform time based actions like multi-threaded code, should they wish to. Due to timing constraints, providing the user with a full threading library is outside the scope of our project.
Additionally, I also was able to get more firm requirements on the memory of our console. The GBA offers 256KB of slow ram + 32KB of fast ram, with the latter often used to cache full arm instructions as it was the only ram in the console to use a 32-bit bus. Since our project is aiming to provide a similar experience to consoles like the GBA, we must provide at least 512KB of ram. Since we’ll be including a kernel with our console, we should provide more than that, likely closer to a megabyte. In any case, I doubt this requirement will be difficult to meet. The more interesting one is the long term storage requirement. GBA cartridges ranged in size from 4MB to 32MB. We’ll need to ensure that our console offers long term storage in this range, and preferably said storage will not be unbearably slow. One option could be to ensure that we find an FPGA which offers at least this much flash storage.
At this moment, we’re right on schedule. In this next week, we will finish off our proposal presentation and research the hardware we will use for our project.
The basic GBA hardware information was obtained here:
https://www.copetti.org/writings/consoles/game-boy-advance/
The cartridge size information was obtained from here:
https://projectpokemon.org/home/forums/topic/22987-what-are-all-possible-rom-sizes-for-clean-dumps-gb-gbc-gba-nds/