Commented everything

apps/tests.c

@@ -333,8 +333,8 @@ int test_init(void)
 	mailbox_int32_init(&mbox);
 
 	create_kernel_task(NULL, measure_ctx_switch, NULL, NORMAL_PRIO);
-	create_kernel_task(NULL, foo, "Hello from foo1", NORMAL_PRIO);
-	create_kernel_task(NULL, foo, "Hello from foo2", NORMAL_PRIO);
+	//create_kernel_task(NULL, foo, "Hello from foo1", NORMAL_PRIO);
+	//create_kernel_task(NULL, foo, "Hello from foo2", NORMAL_PRIO);
 	//create_kernel_task(NULL, join_test, NULL, NORMAL_PRIO);
 	//create_kernel_task(NULL, producer, , NORMAL_PRIO);
 	//create_kernel_task(NULL, consumer, NULL, NORMAL_PRIO);

arch/x86/kernel/entry.asm

@@ -507,22 +507,32 @@ hack:
      jmp 0x00 : 0xDEADBEAF     
      ret
 
+; This procedure is used by scheduler() to switch tasks.
+; It is the software-equivalent to the hw-procedure switch_task from above.
+; Call it in C with the following arguments: 
+; sw_switch_context(&old_tasks_stack_pointer, &new_tasks_stack_pointer)
 global sw_switch_context
 sw_switch_context:
-    ;pushf
-    push DWORD 0x8
-    push DWORD [esp+4]
-    push DWORD 0
-    push DWORD 0xc0edbabe
-    pusha
+				; The stack layout looks like this:
+				; [new stack pointer]
+				; [old stack pointer]
+    ;pushf                      ; [this procedure's return address] overwritten by: EFLAGS (*1)
+    push DWORD 0x8              ; CS
+    push DWORD [esp+4]          ; EIP
+    push DWORD 0                ; Interrupt number
+    push DWORD 0xc0edbabe       ; Error code
+    pusha                       ; Registers...
+    				; ---- This will be popped off by iret later.
+
     pushf
     pop eax
-    mov [esp+48], eax
+    mov [esp+48], eax           ; Move EFLAGS to position (*1) by overwriting
+                                ; the return address of sw_switch_context()
 
     mov ecx, [esp+52]
-    mov [ecx], esp
+    mov [ecx], esp              ; Save stack position in old task structure
     mov ecx, [esp+56]
-    mov esp, [ecx]
+    mov esp, [ecx]              ; Load new stack
 
 sw_rollback:
     popa

arch/x86/kernel/gdt.c

@@ -149,10 +149,29 @@ int create_default_frame(task_t* task, internal_entry_point_t ep, void* arg)
 #ifdef SW_TASK_SWITCH
 	memset(kstacks[id], 0xCD, KERNEL_STACK_SIZE);
 
+	/* The difference between setting up a task for SW-task-switching
+	 * and not for HW-task-switching is setting up a stack and not a TSS.
+	 * This is the stack which will be activated and popped off for iret later.
+	 */
 	stack = kstacks[id] +KERNEL_STACK_SIZE -sizeof(uint32_t);
+
+	/* The next three things on the stack are a marker for debugging purposes, ... */
 	*stack-- = 0xDEADBEEF;
+	/* the first-function-to-be-called's arguments, ... */
 	*stack-- = arg;
+	/* and the "caller" we shall return to.
+	 * This procedure cleans the task after exit. */
 	*stack = leave_kernel_task;
+
+	/* Next bunch on the stack is the initial register state. 
+	 * The stack must look like the stack of a task which was
+	 * scheduled away previously. */
+
+	/* short_state_size was introduced because the convenient "struct state"
+	 * is used for filling the stack with initial values. But the problem is that
+	 * "iret" will not remove the last two entries from the stack, since we're
+	 * "returning" from kernel space to kernel space. Therefore it is shortened
+	 * by its last two entries. */
 	stack -= short_state_size;
 
 	stptr = stack;
@@ -161,12 +180,14 @@ int create_default_frame(task_t* task, internal_entry_point_t ep, void* arg)
 	stptr->int_no = 0xB16B00B5;
 	stptr->error =  0xC03DB4B3;
 
+	/* The instruction pointer shall be set on the first function to be called
+	 * after IRETing */
 	stptr->eip = ep;
 	stptr->cs = cs;
 	stptr->eflags = 0x1002;
-	//stptr->ss = ds;
-	//stptr->useresp = kstacks[id] +KERNEL_STACK_SIZE - 3*sizeof(uint32_t);
 
+	/* Set the task's stack pointer entry to the stack we have crafted right now. 
+	 * This is the pointer which will be used by sw_switch_task(old_task, new_task) later.*/
 	task->stack = stack;
 #else
 	/* reset buffers */

kernel/tasks.c

@@ -1382,11 +1382,10 @@ get_task_out:
 		//	orig_task->id, curr_task->id, (uint32_t)curr_task->prio, CORE_ID);
 #ifndef SW_TASK_SWITCH
 		switch_task(curr_task->id);
-#else
-		write_cr3(virt_to_phys((size_t)curr_task->pgd));
 #endif
 		finish_task_switch(0);		
 #ifdef SW_TASK_SWITCH
+		write_cr3(virt_to_phys((size_t)curr_task->pgd));
 		sw_switch_context(&orig_task->stack, &curr_task->stack);
 #endif
 	}