61 lines
1.5 KiB
Nim
61 lines
1.5 KiB
Nim
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type
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GDTEntry{.packed.} = object
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limit_low: uint16
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base_low: uint16
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base_middle: uint8
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access: uint8
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granularity: uint8
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base_high: uint8
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GDTPointer{.packed.} = ptr object
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limit: uint16 # Size of GDT
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base: uint32 # GDT memory location
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GDTEntries = ptr array[0..2, GDTEntry]
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var
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gdt: GDTEntries
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gp: GDTPointer
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let gpAddr = cast[uint32](addr(gp))
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{.push stackTrace:off.}
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proc gdtFlush(){.asmNoStackFrame, inline.} =
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asm """
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lgdt [`gpAddr`]
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mov %ax, 0x10
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mov %ds, %ax
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mov %es, %ax
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mov %fs, %ax
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mov %gs, %ax
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mov %ss, %ax
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"""
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{.pop.}
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proc gdtSetGate(num: int, base: uint16, limit: uint32, access: uint8, granularity: uint8) =
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# Setup the descriptor base address
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gdt[num].base_low = (base and 0xFFFF)
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gdt[num].base_middle = (base shr 16) and 0xFF
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gdt[num].base_high = (base shr 24) and 0xFF
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# Setup the descriptor limits
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gdt[num].limit_low = (limit and 0xFFFF)
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gdt[num].granularity = ((limit shr 16) and 0x0F)
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# Finally, set up the granularity and access flags
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gdt[num].granularity = gdt[num].granularity or (granularity and 0xF0)
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gdt[num].access = access
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proc gdtInstall*() =
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gp.limit = uint16(sizeof(GDTEntry) * 3) - 1;
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gp.base = cast[uint32](addr(gdt));
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# NULL descriptor
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gdtSetGate(0, 0, 0, 0, 0);
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# The second entry is our Code Segment
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gdtSetGate(1, 0'u16, 0xFFFFFFFF'u32, 0x9A'u8, 0xCF'u8);
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# The third entry is our Data Segment
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gdtSetGate(2, 0'u16, 0xFFFFFFFF'u32, 0x92'u8, 0xCF'u8);
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gdtFlush();
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