38 lines
1.3 KiB
NASM
38 lines
1.3 KiB
NASM
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; GDT
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gdt_start:
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gdt_null:
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dd 0x0 ; the mandatory null-descriptor
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dd 0x0
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gdt_code: ; code segment of descriptor
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dw 0xFFFF ; Limit (bits 0-15)
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dw 0x0 ; Base (bits 0-15)
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db 0x0 ; Base (bits 16-23
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; 1st flags: (present)1, (privilege)00, (descriptor type)1 -> 1001b
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; type flags: (code)1, (conforming)0, (readable)1, (accessed)0 -> 1010b
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; 2nd flags: (granularity)1, (32-bit default)1, (64-bit seg)0, (AVL)0 -> 1100b
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db 10011010b ; 1st flags and type flags
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db 11001111b ; 2nd flags and Limit (bits 16-19)
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db 0x0 ; Base (bits 24-31)
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gdt_data:
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; type flags: (code)0, (expand down)0, (writable)1, (accessed)0 -> 0010b
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dw 0xFFFF ; Limit (bits 0-15)
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dw 0x0 ; Base (bits 0-15)
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db 0x0 ; Base (bits 16-23)
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db 10010010b ; 1st flags and type flags
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db 11001111b ; 2nd flags and Limit (bits 16-19)
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db 0x0 ; Base (bits 24-31)
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gdt_end: ; for calculating size of the gdt
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gdt_descriptor:
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dw gdt_end - gdt_start - 1 ; size of our GDT, always decrement true size by one
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dd gdt_start ; start address of our GDT
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;;;
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; Constants used for GDT segment offsets
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GDT_CODE_SEG: equ gdt_code - gdt_start
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GDT_DATA_SEG: equ gdt_data - gdt_start
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