panic: Go back to real mode instead of hanging with interrupts disabled so BIOS can handle events like ctrl-alt-delete

stage2/lib/blib.c

@@ -5,6 +5,7 @@
 #include <lib/blib.h>
 #include <lib/print.h>
 #include <lib/trace.h>
+#include <lib/real.h>
 
 uint8_t boot_drive;
 int     boot_partition = -1;
@@ -78,9 +79,7 @@ __attribute__((noreturn)) void panic(const char *fmt, ...) {
     print("\n");
     print_stacktrace(NULL);
 
-    for (;;) {
-        asm volatile ("hlt" ::: "memory");
-    }
+    rm_hcf();
 }
 
 int digit_to_int(char c) {

stage2/lib/real.asm

@@ -1,5 +1,35 @@
 section .realmode
 
+global rm_hcf
+rm_hcf:
+    ; Jump to real mode
+    jmp 0x08:.bits16
+  .bits16:
+    bits 16
+    mov ax, 0x10
+    mov ds, ax
+    mov es, ax
+    mov fs, ax
+    mov gs, ax
+    mov ss, ax
+    mov eax, cr0
+    btr ax, 0
+    mov cr0, eax
+    jmp 0x00:.cszero
+  .cszero:
+    xor ax, ax
+    mov ds, ax
+    mov es, ax
+    mov fs, ax
+    mov gs, ax
+    mov ss, ax
+
+    sti
+  .hang:
+    hlt
+    jmp .hang
+    bits 32
+
 global rm_int
 rm_int:
     ; Self-modifying code: int $int_no

stage2/lib/real.h

@@ -28,4 +28,6 @@ struct rm_regs {
 
 void rm_int(uint8_t int_no, struct rm_regs *out_regs, struct rm_regs *in_regs);
 
+__attribute__((noreturn)) void rm_hcf(void);
+
 #endif