1. ; Paging.asm
2. ; Copyright (C) 1997, Jean L. Gareau
3. ;
4. ; This program demonstrates how to enable paging in protected mode.
5. ; A flat memory model and a simplified segment
definition are used.
6. ;
7. ; This program has been assembled with MASM 6.11:
8. ;	C:\>ML ProtMode32.asm
9. ;
10. ; When linked, it must have a base address of BASE (F0000000h in this example), 
11. ; which is where this code is to be mapped in its address space.
12.
13. BASE	EQU	0F0000000h	; Base address (virtual)
14.
15. .386P	; Use 386+ privileged instructions
16.
17. ;ıııııııııııııııııııııııııııııııııııııı-;
18. ; Macros (to use 32-bit instructions while in real mode)		   ;
19.
 ;ıııııııııııııııııııııııııııııııııııııı-;
20.
21. LGDT32	MACRO	Addr		; 32-bit LGDT Macro in 16-bit
22.		DB	66h		; 32-bit operand override
23.		DB	8Dh		; lea (e)bx,Addr
24.		DB	1Eh
25.		DD	Addr
26.		DB	0Fh		; lgdt fword ptr [bx]
27.		DB	01h
28.		DB	17h
29. ENDM
30.
31. FJMP32	MACRO	Selector,Offset	; 32-bit Far Jump Macro in 16-bit
32.		DB	66h		; 32-bit operand override
33.		DB	0EAh		; far jump
34.		DD	Offset		; 32-bit offset
35.		DW	Selector		; 16-bit selector
36. ENDM
37.
38.		PUBLIC	_EntryPoint	; The linker needs it.
39.

40. _TEXT	SEGMENT PARA USE32 PUBLIC ıCODE'
41.		ASSUME	CS:_TEXT
42.
43. ;ıııııııııııııııııııııııııııııııııııııı-;
44. ; Page Directory and Page Table.			    		;
45. ;ıııııııııııııııııııııııııııııııııııııı-;
46.
47.	 	ORG	3000h		; => Depends on code location. 
<
=
48.
49. PD:		
50.		dd	1024 DUP (0)	; Page Directory: all entries at 0.
51. PT:		
52.		dd	1024 DUP (0)	; Page Table  : all entries at 0.
53.
54. ;ıııııııııııııııııııııııııııııııııııııı-;
55. ; Entry Point. The CPU is executing in 16-bit real mode.
   		;
56. ;ıııııııııııııııııııııııııııııııııııııı-;
57.
58.		ORG	5000h		; => Depends on code location. 
<
=
59.
60. _EntryPoint:
61.
62.		LGDT32	GdtDesc - BASE	; Load GDT descriptor
63.
64.		mov	eax,cr0		; Get control register 0
65.		or	ax,1		; Set PE bit (bit #0) in (e)ax
66.		mov	cr0,eax		; Activate protected mode!
67.		jmp	$+2		; Flush the instruction queue.
68.
69. ; The CPU is now executing in 16-bit protected mode. Make a far jump in order to
70. ; load CS with a selector to a 32-bit executable code
 descriptor.
71.
72.		FJMP32	08h,Start32 - BASE	; Jump to Start32 (below)
73. 
74. ; This point is never reached. Data follow.
75.
76. GdtDesc:				; GDT descriptor
77.		dw	GDT_SIZE - 1	; GDT limit
78. 		dd	Gdt		; GDT base address (below)
79. 
80. ;ıııııııııııııııııııııııııııııııııııııı-;
81. ; The CPU is now executing in 32-bit protected mode.			   ;
82. ;ıııııııııııııııııııııııııııııııııııııı-;
83.
84. Start32:
85.
86. ; Initialize all segment registers to 10h (entry #2 in the GDT)
87.
88.		mov	ax,10h		; entry #2
 in GDT
89. 		mov	ds,ax		; ds = 10h
90. 		mov	es,ax		; es = 10h
91.		mov	fs,ax		; fs = 10h
92. 		mov	gs,ax		; gs = 10h
93. 		mov	ss,ax		; ss = 10h
94. 
95. ; Set the top of stack to allow stack operations.
96.
97. 		mov	esp,8000h		; arbitrary top of stack
98.
99. ; Store the PT address into PDE 0 and 960.
100.
101.		mov	eax,offset Pd - BASE	; eax  = &PD
102.		mov	ebx,offset Pt - BASE + 3		; ebx  = &PT | 3
103.		mov	[eax],ebx		; PD[0] = &PT
104.
105.		mov	eax,offset Pd - BASE + 960 * 4	; eax = &PDE[960]
106.
		mov	[eax],ebx		; PD[960] = &PT
107. 
108. ; Initialize the PT to cover the first 4 MB of physical memory.
109.
110. 		mov	edi,offset Pt - BASE	; edi = &PT
111.		mov	eax,3		; Address 0, bit p & r/w set
112.		mov	ecx,1024		; 1024 entries
113. InitPt:
114.		stosd			; Write one entry
115.		add	eax,1000h		; Next page address
116.		loop	InitPt		; Loop
117. 
118. ; Turn on paging by:
119. ;	 1) setting the PD address into CR3 and
120.
121.		mov	eax,offset Pd - BASE	; eax = &PD
122. 		mov	cr3,eax		; cr3 = &PD

123. 
124. ;	2) setting CR0's PG bit.
125.
126.		mov	eax,cr0
127.		or	eax,80000000h	; Set PG bit
128.		mov	cr0,eax		; Paging is on!
129.		jmp	$+2		; Flush the instruction queue.
130.
131. ; Let's now jump to F000xxxx.
132.
133.		push	offset PagingMode	; Keep full address (F000xxxxh)
134.		ret			; Jump at Paging Mode (below)
135.
136. PagingMode:
137.
138. ;ıııııııııııııııııııııııııııııııııııııı-;
139. ; -> Paging is now enabled, executing code at F000xxxxh 
<
ı    		;
140.
 ;ıııııııııııııııııııııııııııııııııııııı-;
141. 
142. ; Other initialization instructions come here.
143. ;		...
144. 
145. ; This point is never reached. Data follow.
146.
147. ;ıııııııııııııııııııııııııııııııııııııı-;
148. ; GDT							   ;	
149. ;ıııııııııııııııııııııııııııııııııııııı-;
150. 
151. ; Global Descriptor Table (GDT) (faster accessed if aligned on 4).
152.
153.		ALIGN	4
154. 
155. Gdt:
 
156.
157. ; GDT[0]: Null entry, never used.
158.   
159. 		dd	0
160. 		dd	0
161. 
162. ; GDT[1]: Executable, read-only code, base address of 0,
 limit of FFFFFh, 
163. ;	 granularity bit (G) set (making the limit 4GB)
164. 
165. 		dw	0FFFFh		; Limit[15..0]
166.		dw	0000h		; Base[15..0]
167. 		db	00h		; Base[23..16]
168. 		db	10011010b		; P(1) DPL(00) S(1) 1 C(0) R(1) A(0)
169. 		db	11001111b		; G(1) D(1) 0 0 Limit[19..16]
170. 		db	00h		; Base[31..24]
171. 		
172. ; GDT[2]: Writable data segment, covering the save address space than GDT[1].
173. 
174. 		dw	0FFFFh		; Limit[15..0]
175. 		dw	0000h		; Base[15..0]
176. 		db	00h		; Base[23..16]
177. 		db
	10010010b		; P(1) DPL(00) S(1) 0 E(0) W(1) A(0)
178. 		db	11001111b		; G(1) B(1) 0 0 Limit[19..16]
179. 		db	00h		; Base[31..24]
180. 
181. GDT_SIZE	EQU	$ - offset Gdt	; Size, in bytes
182. 
183. _TEXT		ENDS
184.		END