लिनक्स/linux-0.01.tar/boot/head.s
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/* * 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 */