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1.1 root 1: /*
1.1.1.2 root 2: * linux/boot/head.s
3: *
1.1.1.5 ! root 4: * Copyright (C) 1991, 1992 Linus Torvalds
1.1.1.2 root 5: */
6:
7: /*
1.1 root 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
1.1.1.4 root 15: .globl _idt,_gdt,_pg_dir,_tmp_floppy_area,_floppy_track_buffer
1.1.1.5 ! root 16: /*
! 17: * pg_dir is the main page directory, address 0x00000000
! 18: */
1.1 root 19: _pg_dir:
20: startup_32:
1.1.1.4 root 21: cld
1.1 root 22: movl $0x10,%eax
23: mov %ax,%ds
24: mov %ax,%es
25: mov %ax,%fs
26: mov %ax,%gs
27: lss _stack_start,%esp
28: call setup_idt
29: call setup_gdt
30: movl $0x10,%eax # reload all the segment registers
31: mov %ax,%ds # after changing gdt. CS was already
32: mov %ax,%es # reloaded in 'setup_gdt'
33: mov %ax,%fs
34: mov %ax,%gs
35: lss _stack_start,%esp
36: xorl %eax,%eax
37: 1: incl %eax # check that A20 really IS enabled
1.1.1.2 root 38: movl %eax,0x000000 # loop forever if it isn't
1.1 root 39: cmpl %eax,0x100000
40: je 1b
1.1.1.4 root 41: /* check if it is 486 or 386. */
42: movl %esp,%edi # save stack pointer
43: andl $0xfffffffc,%esp # align stack to avoid AC fault
44: pushfl # push EFLAGS
45: popl %eax # get EFLAGS
46: movl %eax,%ecx # save original EFLAGS
47: xorl $0x40000,%eax # flip AC bit in EFLAGS
48: pushl %eax # copy to EFLAGS
49: popfl # set EFLAGS
50: pushfl # get new EFLAGS
51: popl %eax # put it in eax
52: xorl %ecx,%eax # check if AC bit is changed. zero is 486.
53: jz 1f # 486
54: pushl %ecx # restore original EFLAGS
55: popfl
56: movl %edi,%esp # restore esp
57: movl %cr0,%eax # 386
58: andl $0x80000011,%eax # Save PG,PE,ET
59: orl $2,%eax # set MP
60: jmp 2f
1.1.1.2 root 61: /*
62: * NOTE! 486 should set bit 16, to check for write-protect in supervisor
63: * mode. Then it would be unnecessary with the "verify_area()"-calls.
64: * 486 users probably want to set the NE (#5) bit also, so as to use
65: * int 16 for math errors.
66: */
1.1.1.4 root 67: 1: pushl %ecx # restore original EFLAGS
68: popfl
69: movl %edi,%esp # restore esp
70: movl %cr0,%eax # 486
1.1.1.3 root 71: andl $0x80000011,%eax # Save PG,PE,ET
1.1.1.4 root 72: orl $0x10022,%eax # set NE and MP
73: 2: movl %eax,%cr0
1.1.1.3 root 74: call check_x87
1.1 root 75: jmp after_page_tables
76:
77: /*
1.1.1.3 root 78: * We depend on ET to be correct. This checks for 287/387.
79: */
80: check_x87:
81: fninit
82: fstsw %ax
83: cmpb $0,%al
1.1.1.4 root 84: je 1f
85: movl %cr0,%eax /* no coprocessor: have to set bits */
1.1.1.3 root 86: xorl $6,%eax /* reset MP, set EM */
87: movl %eax,%cr0
88: ret
89: .align 2
90: 1: .byte 0xDB,0xE4 /* fsetpm for 287, ignored by 387 */
91: ret
92:
93: /*
1.1 root 94: * setup_idt
95: *
96: * sets up a idt with 256 entries pointing to
97: * ignore_int, interrupt gates. It then loads
98: * idt. Everything that wants to install itself
99: * in the idt-table may do so themselves. Interrupts
100: * are enabled elsewhere, when we can be relatively
101: * sure everything is ok. This routine will be over-
102: * written by the page tables.
103: */
104: setup_idt:
105: lea ignore_int,%edx
106: movl $0x00080000,%eax
107: movw %dx,%ax /* selector = 0x0008 = cs */
108: movw $0x8E00,%dx /* interrupt gate - dpl=0, present */
109:
110: lea _idt,%edi
111: mov $256,%ecx
112: rp_sidt:
113: movl %eax,(%edi)
114: movl %edx,4(%edi)
115: addl $8,%edi
116: dec %ecx
117: jne rp_sidt
118: lidt idt_descr
119: ret
120:
121: /*
122: * setup_gdt
123: *
124: * This routines sets up a new gdt and loads it.
125: * Only two entries are currently built, the same
126: * ones that were built in init.s. The routine
127: * is VERY complicated at two whole lines, so this
128: * rather long comment is certainly needed :-).
129: * This routine will beoverwritten by the page tables.
130: */
131: setup_gdt:
132: lgdt gdt_descr
133: ret
134:
1.1.1.2 root 135: /*
136: * I put the kernel page tables right after the page directory,
137: * using 4 of them to span 16 Mb of physical memory. People with
138: * more than 16MB will have to expand this.
139: */
1.1 root 140: .org 0x1000
141: pg0:
142:
143: .org 0x2000
144: pg1:
145:
146: .org 0x3000
1.1.1.2 root 147: pg2:
1.1 root 148:
149: .org 0x4000
1.1.1.2 root 150: pg3:
151:
152: .org 0x5000
153: /*
1.1.1.5 ! root 154: * empty_bad_page is a bogus page that will be used when out of memory,
! 155: * so that a process isn't accidentally killed due to a page fault when
! 156: * it is running in kernel mode..
! 157: */
! 158: .globl _empty_bad_page
! 159: _empty_bad_page:
! 160:
! 161: .org 0x6000
! 162: /*
! 163: * empty_bad_page_table is similar to the above, but is used when the
! 164: * system needs a bogus page-table
! 165: */
! 166: .globl _empty_bad_page_table
! 167: _empty_bad_page_table:
! 168:
! 169: .org 0x7000
! 170: /*
1.1.1.2 root 171: * tmp_floppy_area is used by the floppy-driver when DMA cannot
172: * reach to a buffer-block. It needs to be aligned, so that it isn't
173: * on a 64kB border.
174: */
175: _tmp_floppy_area:
176: .fill 1024,1,0
1.1.1.4 root 177: /*
178: * floppy_track_buffer is used to buffer one track of floppy data: it
179: * has to be separate from the tmp_floppy area, as otherwise a single-
180: * sector read/write can mess it up. It can contain one full track of
181: * data (18*2*512 bytes).
182: */
183: _floppy_track_buffer:
184: .fill 512*2*18,1,0
1.1.1.2 root 185:
1.1 root 186: after_page_tables:
1.1.1.4 root 187: call setup_paging
1.1 root 188: pushl $0 # These are the parameters to main :-)
189: pushl $0
190: pushl $0
1.1.1.4 root 191: cld # gcc2 wants the direction flag cleared at all times
192: call _start_kernel
1.1 root 193: L6:
194: jmp L6 # main should never return here, but
195: # just in case, we know what happens.
196:
197: /* This is the default interrupt "handler" :-) */
1.1.1.2 root 198: int_msg:
199: .asciz "Unknown interrupt\n\r"
1.1 root 200: .align 2
201: ignore_int:
1.1.1.4 root 202: cld
1.1.1.2 root 203: pushl %eax
204: pushl %ecx
205: pushl %edx
206: push %ds
207: push %es
208: push %fs
209: movl $0x10,%eax
210: mov %ax,%ds
211: mov %ax,%es
212: mov %ax,%fs
213: pushl $int_msg
214: call _printk
215: popl %eax
216: pop %fs
217: pop %es
218: pop %ds
219: popl %edx
220: popl %ecx
221: popl %eax
222: iret
1.1 root 223:
224:
225: /*
226: * Setup_paging
227: *
228: * This routine sets up paging by setting the page bit
229: * in cr0. The page tables are set up, identity-mapping
1.1.1.2 root 230: * the first 16MB. The pager assumes that no illegal
1.1 root 231: * addresses are produced (ie >4Mb on a 4Mb machine).
232: *
233: * NOTE! Although all physical memory should be identity
234: * mapped by this routine, only the kernel page functions
235: * use the >1Mb addresses directly. All "normal" functions
236: * use just the lower 1Mb, or the local data space, which
237: * will be mapped to some other place - mm keeps track of
238: * that.
239: *
1.1.1.2 root 240: * For those with more memory than 16 Mb - tough luck. I've
1.1 root 241: * not got it, why should you :-) The source is here. Change
242: * it. (Seriously - it shouldn't be too difficult. Mostly
1.1.1.2 root 243: * change some constants etc. I left it at 16Mb, as my machine
1.1 root 244: * even cannot be extended past that (ok, but it was cheap :-)
245: * I've tried to show which constants to change by having
1.1.1.2 root 246: * some kind of marker at them (search for "16Mb"), but I
1.1 root 247: * won't guarantee that's all :-( )
248: */
249: .align 2
250: setup_paging:
1.1.1.2 root 251: movl $1024*5,%ecx /* 5 pages - pg_dir+4 page tables */
1.1 root 252: xorl %eax,%eax
253: xorl %edi,%edi /* pg_dir is at 0x000 */
254: cld;rep;stosl
255: movl $pg0+7,_pg_dir /* set present bit/user r/w */
256: movl $pg1+7,_pg_dir+4 /* --------- " " --------- */
1.1.1.2 root 257: movl $pg2+7,_pg_dir+8 /* --------- " " --------- */
258: movl $pg3+7,_pg_dir+12 /* --------- " " --------- */
259: movl $pg3+4092,%edi
260: movl $0xfff007,%eax /* 16Mb - 4096 + 7 (r/w user,p) */
1.1 root 261: std
262: 1: stosl /* fill pages backwards - more efficient :-) */
263: subl $0x1000,%eax
264: jge 1b
1.1.1.4 root 265: cld
1.1 root 266: xorl %eax,%eax /* pg_dir is at 0x0000 */
267: movl %eax,%cr3 /* cr3 - page directory start */
268: movl %cr0,%eax
269: orl $0x80000000,%eax
270: movl %eax,%cr0 /* set paging (PG) bit */
271: ret /* this also flushes prefetch-queue */
272:
273: .align 2
274: .word 0
275: idt_descr:
276: .word 256*8-1 # idt contains 256 entries
277: .long _idt
278: .align 2
279: .word 0
280: gdt_descr:
281: .word 256*8-1 # so does gdt (not that that's any
282: .long _gdt # magic number, but it works for me :^)
283:
284: .align 3
285: _idt: .fill 256,8,0 # idt is uninitialized
286:
287: _gdt: .quad 0x0000000000000000 /* NULL descriptor */
1.1.1.2 root 288: .quad 0x00c09a0000000fff /* 16Mb */
289: .quad 0x00c0920000000fff /* 16Mb */
1.1 root 290: .quad 0x0000000000000000 /* TEMPORARY - don't use */
291: .fill 252,8,0 /* space for LDT's and TSS's etc */
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