xv6-public | A disconected fork of MIT xv6 OS

Check in ulib.c file in the same repo. It is defined as

So is there a way to achieve a static physical memory layout in my UEFI-booted kernel?

No. You can try to statically allocate an area of physical memory that happens to work for one computer (the memory manager built into UEFI does have an "allocate page/s at this specific physical address" functionality), but there's no guarantee that the range of physical addresses won't be reserved by UEFI on any other computer; and it makes no difference which physical address range it is.

Instead, before your boot code enables paging it can happily use pages allocated from UEFI's memory manager and/or memory pre-allocated in your boot loader's ".bss" section without caring about what the physical addresses are; and after your boot code enables paging the physical addresses become irrelevant for almost everything (and you can use your virtual address spaces however you like, including statically allocating virtual address ranges in kernel space if you don't like security/KASLR).

Mostly, for UEFI, if it was possible/supported there'd be no benefit to statically allocated physical addresses and it wouldn't make anything simpler.

fd==1 refers to stdout, or Standard Out. It's a common feature of Unix-like Operatin Systems. The kernel knows that it's not a real file. Writes to stdout are mapped to terminal output.

switch(0) will always execute the block of code associated with the case 0: block; still, here there's no actually executed code - both cases are empty.

The point here is to make the compiler angry at compile time if the asserted expression (a) is not verified: in this case, the expanded macro will have two case 0: - the one provided explicitly, and the one that uses the result of the asserted expression (so, 0 in case it failed); this results in a switch with two identical case, which is not allowed and makes the compiler stop with an error at compile time.

This will also fail if the passed expression is not a constant evaluated at compile time (as you cannot have runtime-determined case values), which is also expected from a static_assert.