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From the operating system's point of view, we have a large pool of memory from some starting address to some ending address. We can use our linker script (the virt.lds file) to provide us symbols representing the address of the starting and ending points.
I have taught operating systems at my university, so I will link my notes from that course here regarding the virt I/O protocol. This protocol has changed over the years, but the one that QEMU implements is the legacy MMIO interface.
System calls are a way for unprivileged, user applications to request services from the kernel. In the RISC-V architecture, we invoke the call using the ecall instruction. This will cause the CPU to halt what it's doing, elevate privilege modes, and then jump to whatever function handler is stored in the mtvec…
I have taught operating systems at my university, so I will link my notes from that course here regarding processes.