Bare Metal Programming

[MasterOktagon]

I would like to use RVVM for some bare metal programming in Assembly. However there is no tutorial on this (ex. where is the address space for the Display located). This is probably already specified somewhere but I did not find it. Can you direct me to the required specs do put some colorful pixels on my virtual display (and do other stuff) in bare metal?

Thanks in advance

[LekKit]

You can run RVVM in verbose mode by passing -verbose in CLI mode, or putting verbose into rvvm.cfg if you're using that. That will show you, for example, locations of NS-16550A comptible UART, Framebuffer, and the power management syscon:

INFO: Attached MMIO device at 0x10000000, type "ns16550a"
INFO: Attached MMIO device at 0x18000000, type "simple-framebuffer"
INFO: Attached MMIO device at 0x00100000, type "syscon"

Note however, that those location are not exactly hardcoded, they are picked at runtime (With a well-defined base address) and there may be e.g. multiple framebuffers/uarts (Multiple arguments may be passed, -res 1280x720 -res 800x600 will create two displays of varying sizes), or none at all (-nogui).

Now the real way an OS detects them is via FDT (Flattened Device Tree blob). It's a chunk of binary data describing all those devices, amount of RAM, present CPUs, and it's accessible to both M-mode firmware and passed down to S-mode kernel via physical memory pointer in register a1. You can look up how to parse it, this is how almost all current RISC-V HW works too.

[MasterOktagon]

Thank you. I got it to load my compiled binary, but I can't seem to set a pixel. Is there a way to debug at which address rvvm is right now?

[LekKit]

You should be able to debug the guest by passing -gdbstub to RVVM, and connecting GDB debugger to it:

$ gdb

(gdb) set architecture riscv
(gdb) load ./path/to/your/baremetal_binary # Loads debug symbols (Optional) 
(gdb) layout split
(gdb) target remote localhost:1234

Note however the -gdbstub does not wait for debugger connection as of yet, and starts running right away, so if your baremetal program crashed immediately you might not see it - I am yet working on proper pause mechanism before GDB debugger is connected.