04-Multitasking: Fix processor mode in first activate

As mention in issue #27, `task_init()` is added before
task created.
This is reference to 06-Preemptive, and also, it's not
the time yet to disclose the systick interrupt so use
`svc` to switch to handler mode after reset.

In previous way, we call the `activate` in both thread
and handler mode, and use `if(first)` to distinguish them.
This always bother the newbie to mini-arm-os to figure out
what `activate` is going on.

This commit makes `activate` only effect in handler mode
as I use `task_init()` to ""init"" reset mode from thread mode
to handler mode.

Author: lecopzer

04-Multitasking/asm.h

@@ -2,6 +2,7 @@
 #define __ASM_H_
 
 unsigned int *activate(unsigned int *stack);
+void task_init_env(unsigned int *stack);
 void syscall(void);
 
 #endif

04-Multitasking/context_switch.S

@@ -1,5 +1,10 @@
 .syntax unified
 
+.macro save_kernel_state
+	mrs ip, psr
+	push {r4, r5, r6, r7, r8, r9, r10, r11, ip, lr}
+.endm
+
 .type svc_handler, %function
 .global svc_handler
 svc_handler:
@@ -15,13 +20,23 @@ svc_handler:
 
 .global activate
 activate:
-	/* save kernel state */
-	mrs ip, psr
-	push {r4, r5, r6, r7, r8, r9, r10, r11, ip, lr}
+	save_kernel_state
 
 	/* load user state */
 	ldmia r0!, {r4, r5, r6, r7, r8, r9, r10, r11, lr}
 	msr psp, r0
 
 	/* jump to user task */
 	bx lr
+
+.global task_init_env
+task_init_env:
+	save_kernel_state
+
+	/* switch to process stack */
+	msr psp, r0
+	mov r0, #3
+	msr control, r0
+	isb
+	bl syscall
+	bx lr

04-Multitasking/os.c

@@ -63,6 +63,15 @@ unsigned int *create_task(unsigned int *stack, void (*start)(void))
 	return stack;
 }
 
+/* After reset, processor is at thread and privileged mode.
+ * Switch to handler mode to ensure an appropriate exception return.
+ */
+void task_init(void)
+{
+	unsigned int empty[32];
+	task_init_env(empty + 32);
+}
+
 void task1_func(void)
 {
 	print_str("task1: Created!\n");
@@ -95,6 +104,7 @@ int main(void)
 	size_t current_task;
 
 	usart_init();
+	task_init();
 
 	print_str("OS: Starting...\n");
 	print_str("OS: First create task 1\n");