39 lines
1.4 KiB
NASM
39 lines
1.4 KiB
NASM
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[bits 16]
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; Sets up the GDT for the switch to 32-bit protected mode
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gdt_setup_flat:
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gdt_flat_null: ; mandatory null descriptor
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dd 0x0 ; dd means define double-word (i.e. 4 bytes)
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dd 0x0
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gdt_flat_code_section:
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; base: 0x0, limit: 0xfffff
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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 segment) 0 (AVL) 0 -> 1100b
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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 10011010b ; 1st flags, type flags
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db 11001111b ; 2nd flags, limit (bits 16 - 19)
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db 0x0 ; base (bits 24 - 31)
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gdt_flat_data_section:
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; Same as code segment except for the type flags:
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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, type flags
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db 11001111b ; 2nd flags, limit (bits 16 - 19)
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db 0x0 ; base (bits 24 - 31)
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gdt_flat_end: ; so the assembler can calculate the size of the gdt section
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gdt_flat_descriptor:
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dw gdt_flat_end - gdt_setup_flat - 1 ; size of our GDT, always less 1 of the true size
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dd gdt_setup_flat ; start address of the gdt
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i_gdt_code_segment equ gdt_flat_code_section - gdt_setup_flat
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i_gdt_data_segment equ gdt_flat_data_section - gdt_setup_flat
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