1 /*
2 * linux/boot/head.s
3 *
4 * (C) 1991 Linus Torvalds
5 */
6
7 /*
8 * head.s contains the 32-bit startup code.
9 *
10 * NOTE!!! Startup happens at absolute address 0x00000000, which is also where
11 * the page directory will exist. The startup code will be overwritten by
12 * the page directory.
13 */
14 .text
15 .globl _idt,_gdt,_pg_dir,_tmp_floppy_area
16 _pg_dir:
17 startup_32:
18 movl $0x10,%eax
19 mov %ax,%ds
20 mov %ax,%es
21 mov %ax,%fs
22 mov %ax,%gs
23 lss _stack_start,%esp
24 call setup_idt
25 call setup_gdt
26 movl $0x10,%eax # reload all the segment registers
27 mov %ax,%ds # after changing gdt. CS was already
28 mov %ax,%es # reloaded in 'setup_gdt'
29 mov %ax,%fs
30 mov %ax,%gs
31 lss _stack_start,%esp
32 xorl %eax,%eax
33 1: incl %eax # check that A20 really IS enabled
34 movl %eax,0x000000 # loop forever if it isn't
35 cmpl %eax,0x100000
36 je 1b
37 /*
38 * NOTE! 486 should set bit 16, to check for write-protect in supervisor
39 * mode. Then it would be unnecessary with the "verify_area()"-calls.
40 * 486 users probably want to set the NE (#5) bit also, so as to use
41 * int 16 for math errors.
42 */
43 movl %cr0,%eax # check math chip
44 andl $0x80000011,%eax # Save PG,PE,ET
45 /* "orl $0x10020,%eax" here for 486 might be good */
46 orl $2,%eax # set MP
47 movl %eax,%cr0
48 call check_x87
49 jmp after_page_tables
50
51 /*
52 * We depend on ET to be correct. This checks for 287/387.
53 */
54 check_x87:
55 fninit
56 fstsw %ax
57 cmpb $0,%al
58 je 1f /* no coprocessor: have to set bits */
59 movl %cr0,%eax
60 xorl $6,%eax /* reset MP, set EM */
61 movl %eax,%cr0
62 ret
63 .align 2
64 1: .byte 0xDB,0xE4 /* fsetpm for 287, ignored by 387 */
65 ret
66
67 /*
68 * setup_idt
69 *
70 * sets up a idt with 256 entries pointing to
71 * ignore_int, interrupt gates. It then loads
72 * idt. Everything that wants to install itself
73 * in the idt-table may do so themselves. Interrupts
74 * are enabled elsewhere, when we can be relatively
75 * sure everything is ok. This routine will be over-
76 * written by the page tables.
77 */
78 setup_idt:
79 lea ignore_int,%edx
80 movl $0x00080000,%eax
81 movw %dx,%ax /* selector = 0x0008 = cs */
82 movw $0x8E00,%dx /* interrupt gate - dpl=0, present */
83
84 lea _idt,%edi
85 mov $256,%ecx
86 rp_sidt:
87 movl %eax,(%edi)
88 movl %edx,4(%edi)
89 addl $8,%edi
90 dec %ecx
91 jne rp_sidt
92 lidt idt_descr
93 ret
94
95 /*
96 * setup_gdt
97 *
98 * This routines sets up a new gdt and loads it.
99 * Only two entries are currently built, the same
100 * ones that were built in init.s. The routine
101 * is VERY complicated at two whole lines, so this
102 * rather long comment is certainly needed :-).
103 * This routine will beoverwritten by the page tables.
104 */
105 setup_gdt:
106 lgdt gdt_descr
107 ret
108
109 /*
110 * I put the kernel page tables right after the page directory,
111 * using 4 of them to span 16 Mb of physical memory. People with
112 * more than 16MB will have to expand this.
113 */
114 .org 0x1000
115 pg0:
116
117 .org 0x2000
118 pg1:
119
120 .org 0x3000
121 pg2:
122
123 .org 0x4000
124 pg3:
125
126 .org 0x5000
127 /*
128 * tmp_floppy_area is used by the floppy-driver when DMA cannot
129 * reach to a buffer-block. It needs to be aligned, so that it isn't
130 * on a 64kB border.
131 */
132 _tmp_floppy_area:
133 .fill 1024,1,0
134
135 after_page_tables:
136 pushl $0 # These are the parameters to main :-)
137 pushl $0
138 pushl $0
139 pushl $L6 # return address for main, if it decides to.
140 pushl ___FCKpd___0
main
141 jmp setup_paging
142 L6:
143 jmp L6 # main should never return here, but
144 # just in case, we know what happens.
145
146 /* This is the default interrupt "handler" 
*/
147 int_msg:
148 .asciz "Unknown interrupt/n/r"
149 .align 2
150 ignore_int:
151 pushl %eax
152 pushl %ecx
153 pushl %edx
154 push %ds
155 push %es
156 push %fs
157 movl $0x10,%eax
158 mov %ax,%ds
159 mov %ax,%es
160 mov %ax,%fs
161 pushl $int_msg
162 call _printk
163 popl %eax
164 pop %fs
165 pop %es
166 pop %ds
167 popl %edx
168 popl %ecx
169 popl %eax
170 iret
171
172
173 /*
174 * Setup_paging
175 *
176 * This routine sets up paging by setting the page bit
177 * in cr0. The page tables are set up, identity-mapping
178 * the first 16MB. The pager assumes that no illegal
179 * addresses are produced (ie >4Mb on a 4Mb machine).
180 *
181 * NOTE! Although all physical memory should be identity
182 * mapped by this routine, only the kernel page functions
183 * use the >1Mb addresses directly. All "normal" functions
184 * use just the lower 1Mb, or the local data space, which
185 * will be mapped to some other place - mm keeps track of
186 * that.
187 *
188 * For those with more memory than 16 Mb - tough luck. I've
189 * not got it, why should you The source is here. Change
190 * it. (Seriously - it shouldn't be too difficult. Mostly
191 * change some constants etc. I left it at 16Mb, as my machine
192 * even cannot be extended past that (ok, but it was cheap
193 * I've tried to show which constants to change by having
194 * some kind of marker at them (search for "16Mb"), but I
195 * won't guarantee that's all 
)
196 */
197 .align 2
198 setup_paging:
199 movl $1024*5,%ecx /* 5 pages - pg_dir+4 page tables */
200 xorl %eax,%eax
201 xorl %edi,%edi /* pg_dir is at 0x000 */
202 cld;rep;stosl
203 movl $pg0+7,_pg_dir /* set present bit/user r/w */
204 movl $pg1+7,_pg_dir+4 /* --------- " " --------- */
205 movl $pg2+7,_pg_dir+8 /* --------- " " --------- */
206 movl $pg3+7,_pg_dir+12 /* --------- " " --------- */
207 movl $pg3+4092,%edi
208 movl $0xfff007,%eax /* 16Mb - 4096 + 7 (r/w user,p) */
209 std
210 1: stosl /* fill pages backwards - more efficient */
211 subl $0x1000,%eax
212 jge 1b
213 xorl %eax,%eax /* pg_dir is at 0x0000 */
214 movl %eax,%cr3 /* cr3 - page directory start */
215 movl %cr0,%eax
216 orl $0x80000000,%eax
217 movl %eax,%cr0 /* set paging (PG) bit */
218 ret /* this also flushes prefetch-queue */
219
220 .align 2
221 .word 0
222 idt_descr:
223 .word 256*8-1 # idt contains 256 entries
224 .long _idt
225 .align 2
226 .word 0
227 gdt_descr:
228 .word 256*8-1 # so does gdt (not that that's any
229 .long _gdt # magic number, but it works for me :^)
230
231 .align 3
232 _idt: .fill 256,8,0 # idt is uninitialized
233
234 _gdt: .quad 0x0000000000000000 /* NULL descriptor */
235 .quad 0x00c09a0000000fff /* 16Mb */
236 .quad 0x00c0920000000fff /* 16Mb */
237 .quad 0x0000000000000000 /* TEMPORARY - don't use */
238 .fill 252,8,0 /* space for LDT's and TSS's etc */