first commit

kernel/entry.S

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+        # 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.
+        # 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
+        addi a1, a1, 1
+        mul a0, a0, a1
+        add sp, sp, a0
+        # jump to start() in start.c
+        call start
+spin:
+        j spin