Boot to C in 64-bit long mode

kernel/core/kmain.c

@@ -0,0 +1,28 @@
+/* Nadir kernel entry point: first C code to run.
+ *
+ * Called once from the stage-2 entry stub after the CPU is in 64-bit long
+ * mode with a valid stack, zeroed BSS and identity-mapped memory. Runs at
+ * Ring 0 (CPL 0), interrupts disabled, no IDT yet: any fault triple-faults,
+ * so this function proves the whole boot chain simply by running.
+ * It never returns; on return the caller halts the CPU.
+ */
+
+#include "console.h"
+
+/* A value that only comes out right if the CPU truly executes 64-bit
+ * arithmetic: every nibble survives the shifts above 32 bits. */
+#define PROOF_VALUE 0x0123456789ABCDEFULL
+
+void kmain(void)
+{
+    console_clear();
+    console_puts("Hello World from Nadir Kernel !\n");
+    console_puts("Running C code in 64-bit long mode (Ring 0).\n");
+    console_puts("64-bit proof value: ");
+    console_puthex64(PROOF_VALUE);
+    console_puts("\nHalting.\n");
+
+    for (;;) {
+        __asm__ volatile("hlt");
+    }
+}