Merge branch 'master' of https://github.com/fxlin/p1-kernel

src/exp4b/src/entry.S

@@ -146,7 +146,7 @@ el1_irq:
 ret_from_fork:
 	bl	schedule_tail
 	mov	x0, x20
-	blr	x19 		//should never return
+	blr	x19 		// goes to a task's body. should never return. what if it returns?
 
 .globl err_hang
 err_hang: b err_hang

src/exp4b/src/fork.c

@@ -17,7 +17,7 @@ int copy_process(unsigned long fn, unsigned long arg)
 
 	p->cpu_context.x19 = fn;
 	p->cpu_context.x20 = arg;
-	p->cpu_context.pc = (unsigned long)ret_from_fork;
+	p->cpu_context.pc = (unsigned long)ret_from_fork;	// entry.S
 	p->cpu_context.sp = (unsigned long)p + THREAD_SIZE;
 	int pid = nr_tasks++;
 	task[pid] = p;	

src/exp5/src/kernel.c

@@ -59,6 +59,9 @@ void kernel_process(){
 	if (err < 0) {
 		printf("Error while moving process to user mode\n\r");
 	} 
+	// this func is called from ret_from_fork (entry.S). after returning, it goes back to 
+	// ret_from_fork and does kernel_exit there. hence, pt_regs populated by move_to_user_mode()
+	// will take effect. 
 }
 
 void kernel_main(void)

src/exp5/src/sched.S

@@ -5,21 +5,25 @@ cpu_switch_to:
 	mov	x10, #THREAD_CPU_CONTEXT
 	add	x8, x0, x10
 	mov	x9, sp
+
+	// below: all callee-saved registers are stored in the order, in which they are defined in `cpu_context` structure
 	stp	x19, x20, [x8], #16		// store callee-saved registers
 	stp	x21, x22, [x8], #16
 	stp	x23, x24, [x8], #16
 	stp	x25, x26, [x8], #16
 	stp	x27, x28, [x8], #16
 	stp	x29, x9, [x8], #16
-	str	x30, [x8]
-	add	x8, x1, x10
+	str	x30, [x8]	// save LR (x30) to cpu_context.pc, pointing to where this function is called from
+
+	add	x8, x1, x10 // calculate the address of the next task's `cpu_contex`
+
 	ldp	x19, x20, [x8], #16		// restore callee-saved registers
 	ldp	x21, x22, [x8], #16
 	ldp	x23, x24, [x8], #16
 	ldp	x25, x26, [x8], #16
 	ldp	x27, x28, [x8], #16
 	ldp	x29, x9, [x8], #16
-	ldr	x30, [x8]
+	ldr	x30, [x8]	// load cpu_context.pc to x30 == LR
 	mov	sp, x9
 	ret