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Diffstat (limited to 'src/bootloader/gdt.asm')
| -rw-r--r-- | src/bootloader/gdt.asm | 70 |
1 files changed, 70 insertions, 0 deletions
diff --git a/src/bootloader/gdt.asm b/src/bootloader/gdt.asm new file mode 100644 index 0000000..d506cdf --- /dev/null +++ b/src/bootloader/gdt.asm @@ -0,0 +1,70 @@ +protected_gdt: +.gdt_start: ; we need to start with a null gdt +dd 0 +dd 0 + +.gdt_code: +dw 0xffff +dw 0x0000 +db 0x00 +db 10011010b +db 11001111b +db 0x0000 + + +.gdt_data: +dw 0xffff +dw 0x0000 +db 0x00 +db 10010010b +db 11001111b +db 0x0000 + +.gdt_end: + +.descriptor: + dw .gdt_end - .gdt_start - 1 +dq .gdt_start + +PROTECTED_CODE_SEGMENT equ .gdt_code - .gdt_start +PROTECTED_DATA_SEGMENT equ .gdt_data - .gdt_start + + +long_gdt: +.gdt_start: +;and now we set up a temporary GDT creating a 1:1 mapping +dw 0xffff +dw 0 +db 0 ;this is the invalid GDT +db 0 +db 1 +db 0 + +;now for the code GDT: +.gdt_code: +dw 0 ; segment limit 15:00 (I don't think this matters in 64 bit mode!) +dw 0 ; base address 15:00 +db 0 ; base address 23:16 +db 10011010b ;1st flags and type. The first four bits (1010) are type, and the last are flags. See https://www.cs.bham.ac.uk/~exr/lectures/opsys/10_11/lectures/os-dev.pdf +db 10101111b ;1111 is segment limit continued. 0: available, 0: 64 bit (change?), 1: 32 bit segment, 1: granularity (shifts 3 hex didgets to get all of memory) +db 0 + +; the data GDT is the exact same, and it overlaps. We don't care, we'll be booting into long mode right after then programming in C +; TODO: clarify once you figure out what you're doing +.gdt_data: +dw 0 +dw 0 +db 0 +db 10010010b +db 00000000b +db 0 + +.gdt_end: ; later calculates offset in defs below + + +.descriptor: + dw .gdt_end - .gdt_start - 1 +dq .gdt_start + +LONG_CODE_SEGMENT equ .gdt_code - .gdt_start +LONG_DATA_SEGMENT equ .gdt_data - .gdt_start |