improved comments

Signed-off-by: ablechen <17895010372@163.com>
Signed-off-by: Wang Chen <wangchen20@iscas.ac.cn>

code/os/03-contextswitch/entry.S

@@ -89,12 +89,16 @@
 .align 4
 switch_to:
 	csrrw	t6, mscratch, t6	# swap t6 and mscratch
-	beqz	t6, 1f			# Notice: previous task may be NULL
+	beqz	t6, 1f			# Notice: the first time switch_to() is
+	                                # called, mscratch is initialized as zero
+					# (in sched_init()), which makes t6 zero,
+					# and that's the special case we have to
+					# handle with t6
 	reg_save t6			# save context of prev task
 
 	# Save the actual t6 register, which we swapped into
 	# mscratch
-	mv	t5, t6		# t5 points to the context of current task
+	mv	t5, t6		# t5 points to the context of next task
 	csrr	t6, mscratch	# read t6 back from mscratch
 	sw	t6, 120(t5)	# save t6 with t5 as base
 

code/os/04-multitask/entry.S

@@ -89,12 +89,16 @@
 .align 4
 switch_to:
 	csrrw	t6, mscratch, t6	# swap t6 and mscratch
-	beqz	t6, 1f			# Notice: previous task may be NULL
+	beqz	t6, 1f			# Notice: the first time switch_to() is
+	                                # called, mscratch is initialized as zero
+					# (in sched_init()), which makes t6 zero,
+					# and that's the special case we have to
+					# handle with t6
 	reg_save t6			# save context of prev task
 
 	# Save the actual t6 register, which we swapped into
 	# mscratch
-	mv	t5, t6		# t5 points to the context of current task
+	mv	t5, t6		# t5 points to the context of next task
 	csrr	t6, mscratch	# read t6 back from mscratch
 	sw	t6, 120(t5)	# save t6 with t5 as base
 

code/os/04-multitask/sched.c

@@ -43,7 +43,7 @@ void schedule()
 /*
  * DESCRIPTION
  * 	Create a task.
- * 	- start_routin: task routune entry
+ * 	- start_routin: task routine entry
  * RETURN VALUE
  * 	0: success
  * 	-1: if error occured

code/os/05-traps/entry.S

@@ -122,12 +122,16 @@ trap_vector:
 .align 4
 switch_to:
 	csrrw	t6, mscratch, t6	# swap t6 and mscratch
-	beqz	t6, 1f			# Notice: previous task may be NULL
+	beqz	t6, 1f			# Notice: the first time switch_to() is
+	                                # called, mscratch is initialized as zero
+					# (in sched_init()), which makes t6 zero,
+					# and that's the special case we have to
+					# handle with t6
 	reg_save t6			# save context of prev task
 
 	# Save the actual t6 register, which we swapped into
 	# mscratch
-	mv	t5, t6		# t5 points to the context of current task
+	mv	t5, t6		# t5 points to the context of next task
 	csrr	t6, mscratch	# read t6 back from mscratch
 	sw	t6, 120(t5)	# save t6 with t5 as base
 

code/os/05-traps/sched.c

@@ -38,7 +38,7 @@ void schedule()
 /*
  * DESCRIPTION
  * 	Create a task.
- * 	- start_routin: task routune entry
+ * 	- start_routin: task routine entry
  * RETURN VALUE
  * 	0: success
  * 	-1: if error occured

code/os/06-interrupts/entry.S

@@ -122,12 +122,16 @@ trap_vector:
 .align 4
 switch_to:
 	csrrw	t6, mscratch, t6	# swap t6 and mscratch
-	beqz	t6, 1f			# Notice: previous task may be NULL
+	beqz	t6, 1f			# Notice: the first time switch_to() is
+	                                # called, mscratch is initialized as zero
+					# (in sched_init()), which makes t6 zero,
+					# and that's the special case we have to
+					# handle with t6
 	reg_save t6			# save context of prev task
 
 	# Save the actual t6 register, which we swapped into
 	# mscratch
-	mv	t5, t6		# t5 points to the context of current task
+	mv	t5, t6		# t5 points to the context of next task
 	csrr	t6, mscratch	# read t6 back from mscratch
 	sw	t6, 120(t5)	# save t6 with t5 as base
 

code/os/06-interrupts/sched.c

@@ -38,7 +38,7 @@ void schedule()
 /*
  * DESCRIPTION
  * 	Create a task.
- * 	- start_routin: task routune entry
+ * 	- start_routin: task routine entry
  * RETURN VALUE
  * 	0: success
  * 	-1: if error occured

code/os/07-hwtimer/entry.S

@@ -122,12 +122,16 @@ trap_vector:
 .align 4
 switch_to:
 	csrrw	t6, mscratch, t6	# swap t6 and mscratch
-	beqz	t6, 1f			# Notice: previous task may be NULL
+	beqz	t6, 1f			# Notice: the first time switch_to() is
+	                                # called, mscratch is initialized as zero
+					# (in sched_init()), which makes t6 zero,
+					# and that's the special case we have to
+					# handle with t6
 	reg_save t6			# save context of prev task
 
 	# Save the actual t6 register, which we swapped into
 	# mscratch
-	mv	t5, t6		# t5 points to the context of current task
+	mv	t5, t6		# t5 points to the context of next task
 	csrr	t6, mscratch	# read t6 back from mscratch
 	sw	t6, 120(t5)	# save t6 with t5 as base
 

code/os/07-hwtimer/sched.c

@@ -38,7 +38,7 @@ void schedule()
 /*
  * DESCRIPTION
  * 	Create a task.
- * 	- start_routin: task routune entry
+ * 	- start_routin: task routine entry
  * RETURN VALUE
  * 	0: success
  * 	-1: if error occured

code/os/08-preemptive/sched.c

@@ -41,7 +41,7 @@ void schedule()
 /*
  * DESCRIPTION
  * 	Create a task.
- * 	- start_routin: task routune entry
+ * 	- start_routin: task routine entry
  * RETURN VALUE
  * 	0: success
  * 	-1: if error occured

code/os/09-lock/sched.c

@@ -41,7 +41,7 @@ void schedule()
 /*
  * DESCRIPTION
  * 	Create a task.
- * 	- start_routin: task routune entry
+ * 	- start_routin: task routine entry
  * RETURN VALUE
  * 	0: success
  * 	-1: if error occured

code/os/10-swtimer/sched.c

@@ -41,7 +41,7 @@ void schedule()
 /*
  * DESCRIPTION
  * 	Create a task.
- * 	- start_routin: task routune entry
+ * 	- start_routin: task routine entry
  * RETURN VALUE
  * 	0: success
  * 	-1: if error occured

code/os/11-syscall/sched.c

@@ -41,7 +41,7 @@ void schedule()
 /*
  * DESCRIPTION
  * 	Create a task.
- * 	- start_routin: task routune entry
+ * 	- start_routin: task routine entry
  * RETURN VALUE
  * 	0: success
  * 	-1: if error occured