comment nits

kernel/entry.S

@@ -1,21 +1,21 @@
-	# qemu -kernel loads the kernel at 0x80000000
-        # and causes each CPU to jump there.
+        # qemu -kernel loads the kernel at 0x80000000
+        # and causes each hart (i.e. CPU) to jump there.
         # kernel.ld causes the following code to
         # be placed at 0x80000000.
 .section .text
 .global _entry
 _entry:
-	# set up a stack for C.
+        # set up a stack for C.
         # stack0 is declared in start.c,
         # with a 4096-byte stack per CPU.
         # sp = stack0 + (hartid * 4096)
         la sp, stack0
         li a0, 1024*4
-	csrr a1, mhartid
+        csrr a1, mhartid
         addi a1, a1, 1
         mul a0, a0, a1
         add sp, sp, a0
-	# jump to start() in start.c
+        # jump to start() in start.c
         call start
 spin:
         j spin

kernel/kernelvec.S

@@ -1,17 +1,19 @@
-	#
+        #
         # interrupts and exceptions while in supervisor
         # mode come here.
         #
-        # push all registers, call kerneltrap(), restore, return.
+        # the current stack is a kernel stack.
+        # push all registers, call kerneltrap().
+        # when kerneltrap() returns, restore registers, return.
         #
 .globl kerneltrap
 .globl kernelvec
 .align 4
 kernelvec:
-        // make room to save registers.
+        # make room to save registers.
         addi sp, sp, -256
 
-        // save the registers.
+        # save the registers.
         sd ra, 0(sp)
         sd sp, 8(sp)
         sd gp, 16(sp)
@@ -44,14 +46,14 @@ kernelvec:
         sd t5, 232(sp)
         sd t6, 240(sp)
 
-	// call the C trap handler in trap.c
+        # call the C trap handler in trap.c
         call kerneltrap
 
-        // restore registers.
+        # restore registers.
         ld ra, 0(sp)
         ld sp, 8(sp)
         ld gp, 16(sp)
-        // not this, in case we moved CPUs: ld tp, 24(sp)
+        # not tp (contains hartid), in case we moved CPUs
         ld t0, 32(sp)
         ld t1, 40(sp)
         ld t2, 48(sp)
@@ -82,7 +84,7 @@ kernelvec:
 
         addi sp, sp, 256
 
-        // return to whatever we were doing in the kernel.
+        # return to whatever we were doing in the kernel.
         sret
 
         #
@@ -109,8 +111,9 @@ timervec:
         add a3, a3, a2
         sd a3, 0(a1)
 
-        # raise a supervisor software interrupt.
-	li a1, 2
+        # arrange for a supervisor software interrupt
+        # after this handler returns.
+        li a1, 2
         csrw sip, a1
 
         ld a3, 16(a0)

kernel/riscv.h

@@ -295,7 +295,7 @@ r_sp()
   return x;
 }
 
-// read and write tp, the thread pointer, which holds
+// read and write tp, the thread pointer, which xv6 uses to hold
 // this core's hartid (core number), the index into cpus[].
 static inline uint64
 r_tp()
@@ -342,7 +342,7 @@ typedef uint64 *pagetable_t; // 512 PTEs
 #define PTE_R (1L << 1)
 #define PTE_W (1L << 2)
 #define PTE_X (1L << 3)
-#define PTE_U (1L << 4) // 1 -> user can access
+#define PTE_U (1L << 4) // user can access
 
 // shift a physical address to the right place for a PTE.
 #define PA2PTE(pa) ((((uint64)pa) >> 12) << 10)

kernel/start.c

@@ -54,8 +54,9 @@ start()
   asm volatile("mret");
 }
 
-// set up to receive timer interrupts in machine mode,
-// which arrive at timervec in kernelvec.S,
+// arrange to receive timer interrupts.
+// they will arrive in machine mode at
+// at timervec in kernelvec.S,
 // which turns them into software interrupts for
 // devintr() in trap.c.
 void