Bioscall: adapt to GDT translation.

At early boot stage the kernel is in 3GB virtual memory and the
translation is done by special segments in the Global Descriptor Table.
As bioscall have to be done in real mode without virtual adress space,
the code have been adapted to enter into a proper context before, and
restore the virtual context after the BIOS call.

kernel/Arch/x86/bioscall.S

@@ -35,14 +35,68 @@ bioscall:
 	cli
 
 	/* Save the kernel IDT and GDT and segment registers. */
-	sgdt	gdt_loc
-	sidt	idt_loc
+	sgdt	gdt_backup
+	sidt	idt_backup
 	pushw	%ds
 	pushw	%es
 	pushw	%fs
 	pushw	%gs
 	pushw	%ss
-	lcall	$0x08,$0f
+
+    /* Setup new data segment registers */
+    movl    $0x10,  %eax
+    movw    %ax,    %ds
+    movw    %ax,    %es
+    movw    %ax,    %fs
+    movw    %ax,    %gs
+    movw    %ax,    %ss
+
+    /* Segments and interrupts stuff are saved, adjust pointers. */
+    movl    $0xC0000000,    %eax
+    subl    %eax,           %esp
+    subl    %eax,           %ebp
+    subl    %eax,           %ecx
+
+    /* Call with right adressing mode */
+    jmp     $0x08,  $real_code - 0xC0000000
+
+
+/*
+ * There is no more address translation, all pointers have to be physically
+ * related to memory.
+ */
+real_code:
+    /* Call real realmode code */
+	pushl	%edx
+	pushl	%ecx
+    movl    $0x2000,%eax
+	call	%eax
+	addl	$8,     %esp
+
+	/* Restore the kernel IDT and GDT and segment registers. */
+	lidt	idt_backup - 0xC0000000
+	lgdt	gdt_backup - 0xC0000000
+    jmp     $0x18,      $virtual_code
+
+
+/*
+ * Adresse are virtual again.
+ */
+virtual_code:
+    /* Restore pointers */
+    movl    $0xC0000000,    %eax
+    addl    %eax,           %esp
+    addl    %eax,           %ebp
+
+    /* Return to translated memory to 3GB */
+    movl    $0x20,  %eax
+    movw    %ax,    %ds
+    movw    %ax,    %es
+    movw    %ax,    %fs
+    movw    %ax,    %gs
+    movw    %ax,    %ss
+
+    /* Restore execution context */
 	popw	%ss
 	popw	%gs
 	popw	%fs
@@ -51,28 +105,18 @@ bioscall:
 	popfl
 	ret
 
-0:	/* Segments and interrupts stuff are saved. */
-	pushl	%edx
-	pushl	%ecx
-	call	(0x2000 + 0xC0000000)
-	addl	$8,%esp
-
-	/* Restore the kernel IDT and GDT and segment registers. */
-	lidt	idt_loc
-	lgdt	gdt_loc
-	lret
 
 /**
  * Some space to save current GDT and IDT descriptors.
  */
 	.data
 	.align  2
-idt_loc:
+idt_backup:
 	.word   0
 	.long   0
 
 	.align  2
-gdt_loc:
+gdt_backup:
 	.word   0
 	.long   0
 

kernel/Boot/crt0.S

@@ -50,13 +50,13 @@ entry:
 
     /* Load the tricky GDT */
     lgdtl   gdt_desc - 0xC0000000
-    movw    $0x10,      %ax
+    movw    $0x20,      %ax
     movw    %ax,        %ds
     movw    %ax,        %es
     movw    %ax,        %fs
     movw    %ax,        %gs
     movw    %ax,        %ss
-    jmp     $0x08,      $virtual_kernel
+    jmp     $0x18,      $virtual_kernel
 
 /*
  * Virtual kernel entry point.
@@ -101,14 +101,14 @@ virtual_gdt_desc:
 bootstrap_gdt:
     .long   0x0         /* Null gate */
     .long   0x0
-    .long   0x0000FFFF  /* Code selector with offset 0x40000000 */
-    .long   0x40CF9A00
-    .long   0x0000FFFF  /* Data selector with offset 0x40000000 */
-    .long   0x40CF9200
     .long   0x0000FFFF  /* Code selector without offset */
     .long   0x00CF9A00
     .long   0x0000FFFF  /* Data selector without offset */
     .long   0x00CF9200
+    .long   0x0000FFFF  /* Code selector with offset 0x40000000 */
+    .long   0x40CF9A00
+    .long   0x0000FFFF  /* Data selector with offset 0x40000000 */
+    .long   0x40CF9200
 bootstrap_gdt_end:
 
     .align  4