加入尚未完成的保護模式切換功能

1 files changed, 133 insertions, 0 deletions

diff --git a/pmgdt.s b/pmgdt.s
new file mode 100644
index 0000000..4978871
--- /dev/null
+++ b/pmgdt.s
@@ -0,0 +1,133 @@
+global _main
+global _pm_bootstrap
+global gdt_start
+global gdt_null
+global gdt_code
+global gdt_data
+global gdt_user_code
+global gdt_user_data
+global gdt_end
+global gdt_descriptor
+
+[bits 16]
+
+_main:
+_pm_bootstrap:
+	mov		dx, msg_title
+	call	putstr
+	mov		dx, msg_ldgdt
+	call	putstr
+
+	cli
+	lgdt	[gdt_descriptor]
+
+	in		al, 0x92
+	or		al, 2
+	out		0x92, al
+
+	mov		eax, cr0
+	or		eax, 0x1
+	mov		cr0, eax
+
+	jmp		dword CODE_SEGMENT:pmkernel
+
+msg_title: db 13, 10, 'YOTOS protected mode loader', 13, 10, 0
+msg_ldgdt: db 'Loading Global Descriptor Table (GDT)... ', 0
+
+putstr:				; dx=argument
+	mov		cx, bx	; we must preserve bx
+putstr_start:
+	mov		bx, dx	; [dx] is not a effective address
+	mov		al, byte [bx]
+	test	al, al
+	jz		putstr_end
+	mov		ah, 0x0e
+	mov		bx, 0x0007
+	int		0x10
+	inc		dx
+	jmp		putstr_start
+putstr_end:
+	mov		bx, cx
+	ret
+
+
+
+; GDT entry
+
+
+gdt_start:
+
+gdt_null:	; null descriptor
+	dd	0x00
+	dd	0x00
+
+gdt_code:	; code segment descriptor
+	dw	0xffff		; segment limit (bits 0-15)
+	dw	0x0			; base (bits 0-15)
+	db	0x0			; base (bits 16-23)
+
+	db	10011010b	; (present) 1,
+					; (privilege) 00,
+					; (descriptor type) 1,
+					; type = 
+					; (code) 1, 
+					; (conforming) 0, 
+					; (readable) 1,
+					; (accessed) 0
+
+	db	11001111b	; (granularity) 1,
+					; (32-bit default) 1,
+					; (64-bit segment) 0, 
+					; (AVL) 0,
+					; segment limit (bits 16-19)
+	
+	db	0x0			; base (bits 24-31)
+
+gdt_data:	; data segment descriptor
+	dw	0xffff		; segment limit (bits 0-15)
+	dw	0x0			; base (bits 0-15)
+	db	0x0			; base (bits 16-23)
+
+	db	10010010b	; (present) 1,
+					; (privilege) 00,
+					; (descriptor type) 1,
+					; type = 
+					; (code) 0, 
+					; (expand down) 0, 
+					; (writable) 1,
+					; (accessed) 0
+
+	db	11001111b	; (granularity) 1,
+					; (32-bit default) 1,
+					; (64-bit segment) 0, 
+					; (AVL) 0,
+					; segment limit (bits 16-19)
+	
+	db	0x0			; base (bits 24-31)
+
+gdt_user_code:	; user mode code segment descriptor
+	dw	0xffff		
+	dw	0x0			
+	db	0x0			
+	db	11111010b	
+	db	11001111b	
+	db	0x0			
+
+gdt_user_data:	; user mode data segment descriptor
+	dw	0xfff
+	dw	0x0
+	db	0x0
+	db	11110010b
+	db	11001111b  
+	db	0x0  
+
+gdt_end:
+
+gdt_descriptor:
+	dw	gdt_end - gdt_start - 1
+	dd	load_offset + gdt_start
+
+CODE_SEGMENT	equ		gdt_code - gdt_start
+DATA_SEGMENT	equ		gdt_data - gdt_start
+
+