sanchayanmaity/xv6-cs450
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rsc 2006-09-08 15:31:23 +00:00
parent 3689c7f5ce
commit b02e13f0f6
1 changed files with 9 additions and 13 deletions
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22
bootasm.S
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@ -54,16 +54,15 @@ seta20.2:
movb $0xdf,%al # Enable
outb %al,$0x60 # A20
# Switch from real to protected mode
# The descriptors in our GDT allow all physical memory to be accessed.
# Furthermore, the descriptors have base addresses of 0, so that the
# segment translation is a NOP, ie. virtual addresses are identical to
# their physical addresses. With this setup, immediately after
# enabling protected mode it will still appear to this code
# that it is running directly on physical memory with no translation.
# This initial NOP-translation setup is required by the processor
# to ensure that the transition to protected mode occurs smoothly.
# Switch from real to protected mode
# The descriptors in our GDT allow all physical memory to be accessed.
# Furthermore, the descriptors have base addresses of 0, so that the
# segment translation is a NOP, ie. virtual addresses are identical to
# their physical addresses. With this setup, immediately after
# enabling protected mode it will still appear to this code
# that it is running directly on physical memory with no translation.
# This initial NOP-translation setup is required by the processor
# to ensure that the transition to protected mode occurs smoothly.
real_to_prot:
cli # Mandatory since we dont set up an IDT
lgdt gdtdesc # load GDT -- mandatory in protected mode
@ -85,19 +84,16 @@ protcseg:
movw %ax, %fs # -> FS
movw %ax, %gs # -> GS
movw %ax, %ss # -> SS: Stack Segment
call cmain # finish the boot load from C.
# cmain() should not return
spin:
jmp spin # ..but in case it does, spin
.p2align 2 # force 4 byte alignment
gdt:
SEG_NULLASM # null seg
SEG_ASM(STA_X|STA_R, 0x0, 0xffffffff) # code seg
SEG_ASM(STA_W, 0x0, 0xffffffff) # data seg
gdtdesc:
.word 0x17 # sizeof(gdt) - 1
.long gdt # address gdt