diff options
Diffstat (limited to 'linux/boot/head.s')
| -rw-r--r-- | linux/boot/head.s | 175 |
1 files changed, 175 insertions, 0 deletions
diff --git a/linux/boot/head.s b/linux/boot/head.s new file mode 100644 index 0000000..c008ba8 --- /dev/null +++ b/linux/boot/head.s @@ -0,0 +1,175 @@ +/* + * head.s contains the 32-bit startup code. + * + * NOTE!!! Startup happens at absolute address 0x00000000, which is also where + * the page directory will exist. The startup code will be overwritten by + * the page directory. + */ +.text +.globl _idt,_gdt,_pg_dir +_pg_dir: +startup_32: + movl $0x10,%eax + mov %ax,%ds + mov %ax,%es + mov %ax,%fs + mov %ax,%gs + lss _stack_start,%esp + call setup_idt + call setup_gdt + movl $0x10,%eax # reload all the segment registers + mov %ax,%ds # after changing gdt. CS was already + mov %ax,%es # reloaded in 'setup_gdt' + mov %ax,%fs + mov %ax,%gs + lss _stack_start,%esp + xorl %eax,%eax +1: incl %eax # check that A20 really IS enabled + movl %eax,0x000000 + cmpl %eax,0x100000 + je 1b + movl %cr0,%eax # check math chip + andl $0x80000011,%eax # Save PG,ET,PE + testl $0x10,%eax + jne 1f # ET is set - 387 is present + orl $4,%eax # else set emulate bit +1: movl %eax,%cr0 + jmp after_page_tables + +/* + * setup_idt + * + * sets up a idt with 256 entries pointing to + * ignore_int, interrupt gates. It then loads + * idt. Everything that wants to install itself + * in the idt-table may do so themselves. Interrupts + * are enabled elsewhere, when we can be relatively + * sure everything is ok. This routine will be over- + * written by the page tables. + */ +setup_idt: + lea ignore_int,%edx + movl $0x00080000,%eax + movw %dx,%ax /* selector = 0x0008 = cs */ + movw $0x8E00,%dx /* interrupt gate - dpl=0, present */ + + lea _idt,%edi + mov $256,%ecx +rp_sidt: + movl %eax,(%edi) + movl %edx,4(%edi) + addl $8,%edi + dec %ecx + jne rp_sidt + lidt idt_descr + ret + +/* + * setup_gdt + * + * This routines sets up a new gdt and loads it. + * Only two entries are currently built, the same + * ones that were built in init.s. The routine + * is VERY complicated at two whole lines, so this + * rather long comment is certainly needed :-). + * This routine will beoverwritten by the page tables. + */ +setup_gdt: + lgdt gdt_descr + ret + +.org 0x1000 +pg0: + +.org 0x2000 +pg1: + +.org 0x3000 +pg2: # This is not used yet, but if you + # want to expand past 8 Mb, you'll have + # to use it. + +.org 0x4000 +after_page_tables: + pushl $0 # These are the parameters to main :-) + pushl $0 + pushl $0 + pushl $L6 # return address for main, if it decides to. + pushl $_main + jmp setup_paging +L6: + jmp L6 # main should never return here, but + # just in case, we know what happens. + +/* This is the default interrupt "handler" :-) */ +.align 2 +ignore_int: + incb 0xb8000+160 # put something on the screen + movb $2,0xb8000+161 # so that we know something + iret # happened + + +/* + * Setup_paging + * + * This routine sets up paging by setting the page bit + * in cr0. The page tables are set up, identity-mapping + * the first 8MB. The pager assumes that no illegal + * addresses are produced (ie >4Mb on a 4Mb machine). + * + * NOTE! Although all physical memory should be identity + * mapped by this routine, only the kernel page functions + * use the >1Mb addresses directly. All "normal" functions + * use just the lower 1Mb, or the local data space, which + * will be mapped to some other place - mm keeps track of + * that. + * + * For those with more memory than 8 Mb - tough luck. I've + * not got it, why should you :-) The source is here. Change + * it. (Seriously - it shouldn't be too difficult. Mostly + * change some constants etc. I left it at 8Mb, as my machine + * even cannot be extended past that (ok, but it was cheap :-) + * I've tried to show which constants to change by having + * some kind of marker at them (search for "8Mb"), but I + * won't guarantee that's all :-( ) + */ +.align 2 +setup_paging: + movl $1024*3,%ecx + xorl %eax,%eax + xorl %edi,%edi /* pg_dir is at 0x000 */ + cld;rep;stosl + movl $pg0+7,_pg_dir /* set present bit/user r/w */ + movl $pg1+7,_pg_dir+4 /* --------- " " --------- */ + movl $pg1+4092,%edi + movl $0x7ff007,%eax /* 8Mb - 4096 + 7 (r/w user,p) */ + std +1: stosl /* fill pages backwards - more efficient :-) */ + subl $0x1000,%eax + jge 1b + xorl %eax,%eax /* pg_dir is at 0x0000 */ + movl %eax,%cr3 /* cr3 - page directory start */ + movl %cr0,%eax + orl $0x80000000,%eax + movl %eax,%cr0 /* set paging (PG) bit */ + ret /* this also flushes prefetch-queue */ + +.align 2 +.word 0 +idt_descr: + .word 256*8-1 # idt contains 256 entries + .long _idt +.align 2 +.word 0 +gdt_descr: + .word 256*8-1 # so does gdt (not that that's any + .long _gdt # magic number, but it works for me :^) + + .align 3 +_idt: .fill 256,8,0 # idt is uninitialized + +_gdt: .quad 0x0000000000000000 /* NULL descriptor */ + .quad 0x00c09a00000007ff /* 8Mb */ + .quad 0x00c09200000007ff /* 8Mb */ + .quad 0x0000000000000000 /* TEMPORARY - don't use */ + .fill 252,8,0 /* space for LDT's and TSS's etc */ |