78 lines
2.2 KiB
Nim
78 lines
2.2 KiB
Nim
import serial
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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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flags_and_limit_mid: uint8
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base_high: uint8
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GDTPointer{.packed.} = object
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limit: uint16 # Size of GDT
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base: uint32 # GDT memory location
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AccessByte = uint8
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Flags = uint8
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GDTEntries{.packed.} = array[0..2, GDTEntry]
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GDT{.packed.} = object
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descriptor: GDTPointer
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entries: GDTEntries
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var
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address: uint32
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gdt: GDT
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proc gdtFlush(){.inline,asmNoStackFrame.} =
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asm """
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lgdt (%%eax)
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ret
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::"a"(`address`)
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"""
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proc gdtSetGate(num: int, base: uint16, limit: uint32, access: AccessByte, flags: Flags) =
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# Setup the descriptor base address
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gdt.entries[num].base_low = uint16(base and 0xFFFF)
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gdt.entries[num].base_middle = uint8((base shr 16) and 0xFF)
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gdt.entries[num].base_high = uint8((base shr 24) and 0xFF)
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# Setup the descriptor limits
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gdt.entries[num].limit_low = uint16(limit and 0xFFFF)
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gdt.entries[num].flags_and_limit_mid = uint8(((limit shr 16) and 0x0F) or flags)
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gdt.entries[num].access = access
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proc createAccessByte(priv: range[0..3], directionConform: bool, readWrite: bool): AccessByte =
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result = 0b10011000'u8 # Present and executable bit
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result = result or (uint8(priv) shl 5)
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if directionConform:
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result = result or 0b00000100'u8
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if readWrite:
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result = result or 0b00000010'u8
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proc createFlags(granularity: bool, size: bool): Flags =
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if granularity:
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result = result or 0b10000000'u8
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if size:
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result = result or 0b01000000'u8
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proc gdtInstall*() =
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gdt.descriptor.limit = uint16(sizeof(gdt.entries)) - 1
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gdt.descriptor.base = cast[uint32](gdt.entries.addr)
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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, createFlags(true, true));
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# The third entry is our Data Segment
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gdtSetGate(2, 0'u16, 0xFFFFFFFF'u32, 0x92, createFlags(true, true));
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#gdtSetGate(3, 0'u16, 0xFFFFFFFF'u32, 0xFA, createFlags(true, true));
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#gdtSetGate(4, 0'u16, 0xFFFFFFFF'u32, 0xF2, createFlags(true, true));
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serial.writeLine("Flushing GDT.")
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address = cast[uint32](gdt.addr)
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serial.write("GDT Address:")
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serial.writeLine(address)
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gdtFlush()
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