1 /* 2 * linux/boot/head.s 3 * 4 * Copyright (C) 1991, 1992 Linus Torvalds 5 */ 6
7 #if (MAX_MEGABYTES != 16) && (MAX_MEGABYTES != 32)
8 #error "MAX_MEGABYTES must be 16 or 32"
9 #endif 10
11 /* 12 * head.s contains the 32-bit startup code. 13 * 14 * NOTE!!! Startup happens at absolute address 0x00000000, which is also where 15 * the page directory will exist. The startup code will be overwritten by 16 * the page directory. 17 */ 18 .text
19 .globl _idt,_gdt,_swapper_pg_dir,_tmp_floppy_area,_floppy_track_buffer
20 /* 21 * swapper_pg_dir is the main page directory, address 0x00000000 22 */ 23 _swapper_pg_dir:
24 startup_32:
25 cld
26 movl $0x10,%eax
27 mov %ax,%ds
28 mov %ax,%es
29 mov %ax,%fs
30 mov %ax,%gs
31 lss _stack_start,%esp
32 call setup_idt
33 xorl %eax,%eax
34 1: incl %eax # check that A20 really IS enabled
35 movl %eax,0x000000 # loop forever if it isn't
36 cmpl %eax,0x100000
37 je 1b
38 /* check if it is 486 or 386. */ 39 movl %esp,%edi # save stack pointer
40 andl $0xfffffffc,%esp # align stack to avoid AC fault
41 pushfl # push EFLAGS
42 popl %eax # get EFLAGS
43 movl %eax,%ecx # save original EFLAGS
44 xorl $0x40000,%eax # flip AC bit in EFLAGS
45 pushl %eax # copy to EFLAGS
46 popfl # set EFLAGS
47 pushfl # get new EFLAGS
48 popl %eax # put it in eax
49 xorl %ecx,%eax # check if AC bit is changed. zero is 486.
50 jz 1f # 486
51 pushl %ecx # restore original EFLAGS
52 popfl
53 movl %edi,%esp # restore esp
54 movl %cr0,%eax # 386
55 andl $0x80000011,%eax # Save PG,PE,ET
56 orl $2,%eax # set MP
57 jmp 2f
58 /* 59 * NOTE! 486 should set bit 16, to check for write-protect in supervisor 60 * mode. Then it would be unnecessary with the "verify_area()"-calls. 61 * 486 users probably want to set the NE (#5) bit also, so as to use 62 * int 16 for math errors. 63 */ 64 1: pushl %ecx # restore original EFLAGS
65 popfl
66 movl %edi,%esp # restore esp
67 movl %cr0,%eax # 486
68 andl $0x80000011,%eax # Save PG,PE,ET
69 orl $0x10022,%eax # set NE and MP
70 2: movl %eax,%cr0
71 call check_x87
72 jmp after_page_tables
73
74 /* 75 * We depend on ET to be correct. This checks for 287/387. 76 */ 77 check_x87:
78 fninit
79 fstsw %ax
80 cmpb $0,%al
81 je 1f
82 movl %cr0,%eax /* no coprocessor: have to set bits */ 83 xorl $6,%eax /* reset MP, set EM */ 84 movl %eax,%cr0
85 ret
86 .align 2
87 1: .byte 0xDB,0xE4 /* fsetpm for 287, ignored by 387 */ 88 ret
89
90 /* 91 * setup_idt 92 * 93 * sets up a idt with 256 entries pointing to 94 * ignore_int, interrupt gates. It doesn't actually load 95 * idt - that can be done only after paging has been enabled 96 * and the kernel moved to 0xC0000000. Interrupts 97 * are enabled elsewhere, when we can be relatively 98 * sure everything is ok. This routine will be over- 99 * written by the page tables. 100 */ 101 setup_idt:
102 lea ignore_int,%edx
103 movl $0x00080000,%eax
104 movw %dx,%ax /* selector = 0x0008 = cs */ 105 movw $0x8E00,%dx /* interrupt gate - dpl=0, present */ 106
107 lea _idt,%edi
108 mov $256,%ecx
109 rp_sidt:
110 movl %eax,(%edi)
111 movl %edx,4(%edi)
112 addl $8,%edi
113 dec %ecx
114 jne rp_sidt
115 ret
116
117 /* 118 * I put the kernel page tables right after the page directory, 119 * using 4 of them to span 16 Mb of physical memory. People with 120 * more than 16MB will have to expand this. 121 * When MAX_MEGABYTES == 32, this is set up for a maximum of 32 MB 122 * (ref: 17Apr92) (redone for 0.97 kernel changes, 1Aug92, ref) 123 */ 124 .org 0x1000
125 pg0:
126
127 .org 0x2000
128 pg1:
129
130 .org 0x3000
131 pg2:
132
133 .org 0x4000
134 pg3:
135
136 .org 0x5000
137 #ifMAX_MEGABYTES == 32
138 pg4:
139
140 .org 0x6000
141 pg5:
142
143 .org 0x7000
144 pg6:
145
146 .org 0x8000
147 pg7:
148
149 .org 0x9000
150 #endif 151 /* 152 * empty_bad_page is a bogus page that will be used when out of memory, 153 * so that a process isn't accidentally killed due to a page fault when 154 * it is running in kernel mode.. 155 */ 156 .globl _empty_bad_page
157 _empty_bad_page:
158
159 #ifMAX_MEGABYTES == 32
160 .org 0xa000
161 #else 162 .org 0x6000
163 #endif 164 /* 165 * empty_bad_page_table is similar to the above, but is used when the 166 * system needs a bogus page-table 167 */ 168 .globl _empty_bad_page_table
169 _empty_bad_page_table:
170
171 #ifMAX_MEGABYTES == 32
172 .org 0xb000
173 #else 174 .org 0x7000
175 #endif 176 /* 177 * tmp_floppy_area is used by the floppy-driver when DMA cannot 178 * reach to a buffer-block. It needs to be aligned, so that it isn't 179 * on a 64kB border. 180 */ 181 _tmp_floppy_area:
182 .fill 1024,1,0
183 /* 184 * floppy_track_buffer is used to buffer one track of floppy data: it 185 * has to be separate from the tmp_floppy area, as otherwise a single- 186 * sector read/write can mess it up. It can contain one full track of 187 * data (18*2*512 bytes). 188 */ 189 _floppy_track_buffer:
190 .fill 512*2*18,1,0
191
192 after_page_tables:
193 call setup_paging
194 lgdt gdt_descr
195 lidt idt_descr
196 ljmp $0x08,$1f
197 1: movl $0x10,%eax # reload all the segment registers
198 mov %ax,%ds # after changing gdt.
199 mov %ax,%es
200 mov %ax,%fs
201 mov %ax,%gs
202 lss _stack_start,%esp
203 pushl $0 # These are the parameters to main :-)
204 pushl $0
205 pushl $0
206 cld # gcc2 wants the direction flag cleared at all times
207 call _start_kernel
208 L6:
209 jmp L6 # main should never return here, but
210 # just in case, we know what happens. 211
212 /* This is the default interrupt "handler" :-) */ 213 int_msg:
214 .asciz "Unknown interrupt\n\r"
215 .align 2
216 ignore_int:
217 cld
218 pushl %eax
219 pushl %ecx
220 pushl %edx
221 push %ds
222 push %es
223 push %fs
224 movl $0x10,%eax
225 mov %ax,%ds
226 mov %ax,%es
227 mov %ax,%fs
228 pushl $int_msg
229 call _printk
230 popl %eax
231 pop %fs
232 pop %es
233 pop %ds
234 popl %edx
235 popl %ecx
236 popl %eax
237 iret
238
239
240 /* 241 * Setup_paging 242 * 243 * This routine sets up paging by setting the page bit 244 * in cr0. The page tables are set up, identity-mapping 245 * the first 16MB. The pager assumes that no illegal 246 * addresses are produced (ie >4Mb on a 4Mb machine). 247 * 248 * NOTE! Although all physical memory should be identity 249 * mapped by this routine, only the kernel page functions 250 * use the >1Mb addresses directly. All "normal" functions 251 * use just the lower 1Mb, or the local data space, which 252 * will be mapped to some other place - mm keeps track of 253 * that. 254 * 255 * For those with more memory than 16 Mb - tough luck. I've 256 * not got it, why should you :-) The source is here. Change 257 * it. (Seriously - it shouldn't be too difficult. Mostly 258 * change some constants etc. I left it at 16Mb, as my machine 259 * even cannot be extended past that (ok, but it was cheap :-) 260 * I've tried to show which constants to change by having 261 * some kind of marker at them (search for "16Mb"), but I 262 * won't guarantee that's all :-( ) 263 * 264 * (ref: added support for up to 32mb, 17Apr92) -- Rik Faith 265 * (ref: update, 25Sept92) -- croutons@crunchy.uucp 266 */ 267 .align 2
268 setup_paging:
269 #if MAXMEGABYTES == 32
270 movl $1024*9,%ecx /* 9 pages - swapper_pg_dir+8 page tables */ 271 #else 272 movl $1024*5,%ecx /* 5 pages - swapper_pg_dir+4 page tables */ 273 #endif 274 xorl %eax,%eax
275 xorl %edi,%edi /* swapper_pg_dir is at 0x000 */ 276 cld;rep;stosl
277 /* Identity-map the kernel in low 4MB memory for ease of transition */ 278 movl $pg0+7,_swapper_pg_dir /* set present bit/user r/w */ 279 /* But the real place is at 0xC0000000 */ 280 movl $pg0+7,_swapper_pg_dir+3072 /* set present bit/user r/w */ 281 movl $pg1+7,_swapper_pg_dir+3076 /* --------- " " --------- */ 282 movl $pg2+7,_swapper_pg_dir+3080 /* --------- " " --------- */ 283 movl $pg3+7,_swapper_pg_dir+3084 /* --------- " " --------- */ 284 #ifMAX_MEGABYTES == 32
285 movl $pg4+7,_swapper_pg_dir+3088 /* --------- " " --------- */ 286 movl $pg5+7,_swapper_pg_dir+3092 /* --------- " " --------- */ 287 movl $pg6+7,_swapper_pg_dir+3096 /* --------- " " --------- */ 288 movl $pg7+7,_swapper_pg_dir+3100 /* --------- " " --------- */ 289
290 movl $pg7+4092,%edi
291 movl $0x1fff007,%eax /* 32Mb - 4096 + 7 (r/w user,p) */ 292 #else 293 movl $pg3+4092,%edi
294 movl $0x0fff007,%eax /* 16Mb - 4096 + 7 (r/w user,p) */ 295 #endif 296 std
297 1: stosl /* fill pages backwards - more efficient :-) */ 298 subl $0x1000,%eax
299 jge 1b
300 cld
301 xorl %eax,%eax /* swapper_pg_dir is at 0x0000 */ 302 movl %eax,%cr3 /* cr3 - page directory start */ 303 movl %cr0,%eax
304 orl $0x80000000,%eax
305 movl %eax,%cr0 /* set paging (PG) bit */ 306 ret /* this also flushes prefetch-queue */ 307
308 /* 309 * The interrupt descriptor table has room for 256 idt's 310 */ 311 .align 4
312 .word 0
313 idt_descr:
314 .word 256*8-1 # idt contains 256 entries
315 .long 0xc0000000+_idt
316
317 .align 4
318 _idt:
319 .fill 256,8,0 # idt is uninitialized
320
321 /* 322 * The real GDT is also 256 entries long - no real reason 323 */ 324 .align 4
325 .word 0
326 gdt_descr:
327 .word 256*8-1
328 .long 0xc0000000+_gdt
329
330 /* 331 * This gdt setup gives the kernel a 1GB address space at virtual 332 * address 0xC0000000 - space enough for expansion, I hope. 333 */ 334 .align 4
335 _gdt:
336 .quad 0x0000000000000000 /* NULL descriptor */ 337 .quad 0xc0c39a000000ffff /* 1GB at 0xC0000000 */ 338 .quad 0xc0c392000000ffff /* 1GB */ 339 .quad 0x0000000000000000 /* TEMPORARY - don't use */ 340 .fill 252,8,0 /* space for LDT's and TSS's etc */
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