Zeroing all registers in kernel entry code

kernel/entry.S

@@ -11,17 +11,55 @@
 .section .text
 .global _entry
 _entry:
+        # Clear all the registers.
+        li x1, 0x0
+        li x2, 0x0
+        li x3, 0x0
+        li x4, 0x0
+        li x5, 0x0
+        li x6, 0x0
+        li x7, 0x0
+        li x8, 0x0
+        li x9, 0x0
+        li x10, 0x0
+        li x11, 0x0
+        li x12, 0x0
+        li x13, 0x0
+        li x14, 0x0
+        li x15, 0x0
+        li x16, 0x0
+        li x17, 0x0
+        li x18, 0x0
+        li x19, 0x0
+        li x20, 0x0
+        li x21, 0x0
+        li x22, 0x0
+        li x23, 0x0
+        li x24, 0x0
+        li x25, 0x0
+        li x26, 0x0
+        li x27, 0x0
+        li x28, 0x0
+        li x29, 0x0
+        li x30, 0x0
+        li x31, 0x0
+
         # 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
+        li a0, 1024*4 # a0 = 4096
-        csrr a1, mhartid
+
-        addi a1, a1, 1
+        # Control and Status Register Read
-        mul a0, a0, a1
+        csrr a1, mhartid # a1 = unique hart (core) id
-        add sp, sp, a0
+        addi a1, a1, 1   # a1 += 1
+        mul a0, a0, a1   # a0 *= a1, aka a0 = 4096 * (hartid + 1), the base of the stack for this hart
+        add sp, sp, a0   # sp += a0, stack pointer is now properly configured
+
         # jump to start() in start.c
         call start
+
+# Infinite spin loop.
 spin:
         j spin