Untitled_Kernel/src/gdt.s

global gdtr

section .gdt_sect
    gdt:
        ;Null descriptor
        dd 0x00000000
        dd 0x00000000
        ;Kernel code segment.
        ;Limit: in 4kib pages. 0xFFFFF * 4K = full address space.
        dw 0xFFFF
        ;Base: Start at 0. We want the whole thing.
        dw 0
        ;ALSO BASE: bits 16-23. All zeroes, still.
        db 0
        ;Access byte. Defines flags for access permissions to the segment. This segment is:
        ;RX, and code/data segment 
        db 0b10011010
        ;Next two segments are nibbles so I put them together (cant db only a nibble at once).
        ;Upper limit bits (right hand nibble) is all ones to fill out the full 4gib in pages
        ;Flags (left hand nibble) are set to say that the limit is meant to be read as pages, and we're working in 32bit.
        db 0b11001111
        ;Final upper base bits. Still zero lol.
        db 0 
        ;Done! Now we move onto our next table entries, they are back to back.
        ;Kernel Data Segment
        dw 0xFFFF
        dw 0
        db 0
        db 0b10010010
        db 0b11001111
        db 0
        ;User Code Segment
        dw 0xFFFF
        dw 0
        db 0
        db 0b11111010
        db 0b11001111
        db 0
        ;User Data Segment
        dw 0xFFFF
        dw 0
        db 0
        db 0b11110010
        db 0b11001111
        db 0
        ;Task State Segment
        ;For a lot of this it's going to be zeroes so we can do it dynamically later (such as finding &tss).
        ;Really, we just want to set the access bytes correctly.
        dd 0
        db 0
        db 0b10001001
        dw 0
        gdt_end:
    gdtr:
        ;size of the gdt
        dw gdt_end - gdt - 1
        ;location of the gdt
        dd gdt - 0xC0000000