1 files changed, 34 insertions, 0 deletions
diff --git a/start.c b/start.c
new file mode 100644
index 0000000..5167ab4
--- /dev/null
+++ b/start.c
@@ -0,0 +1,34 @@
+#include "types.h"
+#include "memlayout.h"
+#include "riscv.h"
+#include "defs.h"
+
+void main();
+
+// entry.S uses this as the initial stack.
+char stack0[4096];
+
+// entry.S jumps here in machine mode on stack0.
+void
+mstart()
+{
+  // set M Previous Privilege mode to Supervisor, for mret.
+  unsigned long x = r_mstatus();
+  x &= ~MSTATUS_MPP_MASK;
+  x |= MSTATUS_MPP_S;
+  w_mstatus(x);
+
+  // set M Exception Program Counter to main, for mret.
+  // requires gcc -mcmodel=medany
+  w_mepc((uint64)main);
+
+  // disable paging for now.
+  w_satp(0);
+
+  // delegate all interrupts and exceptions to supervisor mode.
+  w_medeleg(0xffff);
+  w_mideleg(0xffff);
+  
+  // jump to main in supervisor mode.
+  asm("mret");
+}