XV6 as most OSs is monolithic, preemptive and time-sharing multiplexing.

Isolation comes from the CPU which provides machine, supervisor and user modes.

- machine: just for the boot time
- supervisor: each time the kernel is active
  including when calling a syscall (with `ecall` in RISC-V)
- user: for all user code

Minix, L4 (seL4) and QNX operating systems have been mentioned as micro-kernel OSs.

Memory layout of a process:
  [text; data; user stack; heap (large) ; trapframe; trampoline]
0 ↑------------------------------------------------------------↑ 2^38-1

text: instructions
data: global variables
heap: explicitly allocated memory
trapframe: saved process registers when switching in and out of the kernel
trampoline (4 KiB): code to transition in and out of the kernel

trapframe & trampoline: explained in chapter 4

kernel `proc` structure contains:

- (kstack) kernel stack used when the process calls for syscalls
- (pagetable) pointers to physical memory pages actually used by the process,
              provided to the hardware for translation (virtual @ ←→ physical @)
- (state) UNUSED, USED, SLEEPING, RUNNABLE, RUNNING or ZOMBIE
- (trapframe) saved process registers when switching in and out of the kernel
- (pid) process identification number
- (ofile) list of opened files
- (name) process name
- (cwd) current directory
- (context) kernel registers used to enter the process
- (sz) size of process's memory
- (parent) pointer to the process's parent proc structure
- (xstate) exit status (given to its parent when it "waits" for it)
- (killed) non-zero when the process has been killed
- (chan) TODO: not currently explained
- (lock) TODO: not currently explained


RISC-V instructions

- ecall: raise hardware privilege level
  program counter change to a kernel-defined entry point which then switches to the
  process's kernel stack and executes kernel instructions for this syscall
  once done, the kernel calls sret
- sret: lower hardware privilege level

A process is the abstraction of memory and CPU for a running program,
giving it the illusion of being alone on the hardware.
A process is:

- an address space to give a running program the illusion of owning the entire memory
- a thread to give a running program the illusion of having a CPU for himself