feat: add initial process, thread and scheduler

kernel/src/process/asm_helper.s

@@ -2,15 +2,16 @@
     .global switch_context
 
 switch_context:
-//    ldr r1, [r0, #16]
-//    msr cpsr, r1
+    ; Set the CPSR register.
+    ldr r1, [r0, #16]
+    msr cpsr, r1
+    ; Load the stack pointer.
     ldr sp, [r0, #52]
-
+    ; Push all other registers on the stack.
     ldr r1, [r0, #60]
     push {r1}
     ldr r1, [r0, #56]
     push {r1}
-
     ldr r1, [r0, #48]
     push {r1}
     ldr r1, [r0, #44]
@@ -38,7 +39,12 @@ switch_context:
     ldr r1, [r0, #0]
     push {r1}
 
-//    msr CPSR_c, #0x10
+    ; TODO: Enter user space
+    ; cps #16
+    ; Or:
+    ; msr CPSR_c, #0x10
 
+    ; Load all register states.
+    ; Loading the PC register effectively jumps into the user program
     pop {r0-r12}
     pop {r14-pc}