Scheduler with working threads.

Currently only one process but it is a start.

zion/scheduler/context_switch.s

@@ -0,0 +1,41 @@
+.global context_switch
+context_switch:
+  push %rax
+  push %rcx
+  push %rdx
+  push %rbx
+  push %rbp
+  push %rsi
+  push %rdi
+  push %r8
+  push %r9
+  push %r10
+  push %r11
+  push %r12
+  push %r13
+  push %r14
+  push %r15
+  mov %cr3, %rax
+  push %rax
+
+  mov %rsp, (%rdi) # Save rsp to the prev task.
+  mov (%rsi), %rsp # Load the next task's rsp.
+
+  pop %rax
+  mov %rax, %cr3
+  pop %r15
+  pop %r14
+  pop %r13
+  pop %r12
+  pop %r11
+  pop %r10
+  pop %r9
+  pop %r8
+  pop %rdi
+  pop %rsi
+  pop %rbp
+  pop %rbx
+  pop %rdx
+  pop %rcx
+  pop %rax
+  retq