Can you explain this c code to me? It is part of xv6.
// Create PTEs for linear addresses starting at la that refer to
// physical addresses starting at pa. la and size might not
// be page-aligned.
static int
mappages(pde\_t \*pgdir, void \*la, uint size, uint pa, int perm)
{
 char \*a, \*last;
 pte\_t \*pte;
 
 a = PGROUNDDOWN(la);
 last = PGROUNDDOWN(la + size - 1);
 for(;;){
 pte = walkpgdir(pgdir, a, 1);
 if(pte == 0)
 return -1;
 if(\*pte & PTE\_P)
 panic("remap");
 \*pte = pa | perm | PTE\_P;
 if(a == last)
 break;
 a += PGSIZE;
 pa += PGSIZE;
 }
 return 0;
}
This code is part of the xv6 operating system, which is a microkernel-based operating system. This code creates page tables for linear addresses starting at the given 'la' parameter and physical addresses starting at the given 'pa' parameter. The 'size' parameter specifies the size of the region being mapped. The 'perm' parameter specifies the protection flags for the mapped region. The function returns 0 on success and -1 on failure.
Can you explain this c code to me? It is part of xv6.
// Create PTEs for linear addresses starting at la that refer to
// physical addresses starting at pa. la and size might not
// be page-aligned.
static int
mappages(pde\_t \*pgdir, void \*la, uint size, uint pa, int perm)
{
 char \*a, \*last;
 pte\_t \*pte;
 
 a = PGROUNDDOWN(la);
 last = PGROUNDDOWN(la + size - 1);
 for(;;){
 pte = walkpgdir(pgdir, a, 1);
 if(pte == 0)
 return -1;
 if(\*pte & PTE\_P)
 panic("remap");
 \*pte = pa | perm | PTE\_P;
 if(a == last)
 break;
 a += PGSIZE;
 pa += PGSIZE;
 }
 return 0;
}

This function maps linear addresses starting at `la` to physical addresses starting at `pa` in the page table `pgdir`. The mapping is done for a continuous range of linear addresses with size `size`.

Here's a step-by-step breakdown:

1. `PGROUNDDOWN(la)` rounds down the linear address `la` to the nearest multiple of `PGSIZE` (4096 bytes). This ensures that the linear address is page-aligned.

2. `PGROUNDDOWN(la + size - 1)` does the same for the last linear address in the range plus one (to include the last page).

3. The function enters an infinite loop, which iterates over each page in the range.

4. `walkpgdir(pgdir, a, 1)` is a helper function that walks through the page table `pgdir` starting at the page table entry for the page address `a`. It returns a pointer to the page table entry if it exists, and `NULL` otherwise.

5. If the page table entry already points to a physical page (`*pte & PTE_P` is non-zero), the function panics with a message "remap" (this is a bug in the code, as it should not happen).

6. If the page table entry does not point to a physical page