diff options
Diffstat (limited to 'mubios/arch')
| -rw-r--r-- | mubios/arch/mode_switch.S | 74 |
1 files changed, 74 insertions, 0 deletions
diff --git a/mubios/arch/mode_switch.S b/mubios/arch/mode_switch.S index df96ce4..575c78d 100644 --- a/mubios/arch/mode_switch.S +++ b/mubios/arch/mode_switch.S @@ -80,6 +80,14 @@ gdt_data: .byte 0xCF .byte 0x00 +gdt_code16: + .word 0xFFFF + .word 0x0000 + .byte 0x0F + .byte 0x9A + .byte 0x0F + .byte 0x00 + gdt_end: gdt_descriptor: @@ -105,8 +113,74 @@ protected_mode_entry: call post_main + call leave_protected_mode + protected_mode_halt: hlt jmp protected_mode_halt + + .global leave_protected_mode + .type leave_protected_mode, @function + + .extern __rom_start + .extern protected16_return + .extern real_mode_return + +leave_protected_mode: + # Keep the original 16-bit return address at 0x7FFE intact. + sub esp, 12 + + # Stack[0]: protected-mode 16-bit code offset + # Stack[2]: protected-mode 16-bit code selector + mov eax, OFFSET FLAT:protected16_return + mov ebx, OFFSET FLAT:__rom_start + sub eax, ebx + mov word ptr [esp], ax + mov word ptr [esp + 2], 0x0018 + + # Stack[4]: real-mode offset + # Stack[6]: real-mode segment + mov eax, OFFSET FLAT:real_mode_return + mov ebx, OFFSET FLAT:__rom_start + sub eax, ebx + mov word ptr [esp + 4], ax + + shr ebx, 4 + mov word ptr [esp + 6], bx + + # Enter the 16-bit protected-mode code segment. + .byte 0x66 + .byte 0xFF, 0x2C, 0x24 + + .code16 + + .section .text16.mode, "ax" + + .global protected16_return + .type protected16_return, @function + +protected16_return: + mov bx, sp + add bx, 4 + + # Leave protected mode while already using a 16-bit code segment. + mov eax, cr0 + and eax 0xFFFFFFFE + mov cr0, eax + + # Jump to the prepared real-mode CS:IP pointer. + .byte 0xFF, 0x2F + + .global real_mode_return + .type real_mode_return, @function + +real_mode_return: + xor ax, ax + mov ds, ax + mov es, ax + mov ss, ax + mov sp, 0x7FFE + + ret |