mirror of
https://github.com/cfenollosa/os-tutorial.git
synced 2024-10-27 20:34:19 +00:00
36 lines
1.0 KiB
NASM
36 lines
1.0 KiB
NASM
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gdt_start: ; don't remove the labels, they're needed to compute sizes and jumps
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; the GDT starts with a null 8-byte
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dd 0x0 ; 4 byte
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dd 0x0 ; 4 byte
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; GDT for code segment. base = 0x00000000, length = 0xfffff
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; for flags, refer to os-dev.pdf document, page 36
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gdt_code:
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dw 0xffff ; segment length, bits 0-15
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dw 0x0 ; segment base, bits 0-15
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db 0x0 ; segment base, bits 16-23
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db 10011010b ; flags (8 bits)
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db 11001111b ; flags (4 bits) + segment length, bits 16-19
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db 0x0 ; segment base, bits 24-31
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; GDT for data segment. base and length identical to code segment
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; some flags changed, again, refer to os-dev.pdf
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gdt_data:
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dw 0xffff
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dw 0x0
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db 0x0
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db 10010010b
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db 11001111b
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db 0x0
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gdt_end:
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; GDT descriptor
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gdt_descriptor:
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dw gdt_end - gdt_start - 1 ; size (16 bit), always one less of its true size
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dd gdt_start ; address (32 bit)
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; define some constants for later use
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CODE_SEG equ gdt_code - gdt_start
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DATA_SEG equ gdt_data - gdt_start
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