arch: x86_64: Add real-mode bootstrap stub

arch/x86_64/src/intel64/intel64_head.S

@@ -107,6 +107,38 @@ header_start:
 #endif
 header_end:
 
+	.code16
+	.section ".realmode", "ax"
+
+    .type   __reset_entry, @function
+__reset_entry:
+    // Load a GDT for protected mode
+    movl $loader_gdt_ptr, %ebx
+	lgdtl (%ebx)
+
+    // enable protected mode in CR0
+	mov %cr0,%eax
+	or $X86_CR0_PE,%al
+	mov %eax,%cr0
+
+    // Long jump into protected mode
+    // Hardcode the address
+	ljmpl $0x8,$0x400000
+
+    // Loader GDT and GDTR
+	.align(16)
+	.global loader_gdt
+loader_gdt:
+	.quad	0
+	.quad	0x00cf9a000000ffff
+	.quad	0x00cf92000000ffff
+
+loader_gdt_ptr:
+	.short	loader_gdt_ptr - loader_gdt - 1
+	.long	loader_gdt
+
+    .size	__reset_entry, . - __reset_entry
+
 
 /****************************************************************************
  * .text
@@ -124,6 +156,11 @@ header_end:
  *
  ****************************************************************************/
 
+start32_0:
+    mov $0x10, %ax
+    mov %ax, %ss
+    mov %ax, %ds
+
     .type   __pmode_entry, @function
 __pmode_entry:
 start32:

boards/x86_64/intel64/qemu-intel64/scripts/qemu.ld

@@ -22,12 +22,20 @@ OUTPUT_ARCH(i386:x86-64)
 ENTRY(__pmode_entry)
 SECTIONS
 {
+
+    . = 0x0;
+
+    .realmode : {
+        . = ALIGN(8);
+        KEEP(*(.multiboot))
+        *(.realmode)
+    }
+
     . = 0x1M;
     _kernel_physical_start = .;
 
     .loader.text : {
         . = ALIGN(8);
-        KEEP(*(.multiboot))
         *(.loader.text)
     }