generate-gdt

#!/usr/bin/env python3
#
# Global Descriptor Table generator
# https://wiki.osdev.org/Global_Descriptor_Table
#

import os

class GdtEntry:
    def __init__(self, name, base, limit, *, system, privilege_level, granularity_4k):
        self.name = name
        self.base = base
        self.limit = limit
        self.system = system
        self.privilege_level = privilege_level
        self.granularity_4k = granularity_4k
        self.bits32 = True
        self.bits64 = False
        self.present = True
        self.accessed = False

    def encode(self):
        access = (
            self.present         << 7 |
            self.privilege_level << 5 |
            (not self.system)    << 4 |
            self._type()
        )

        flags = (
            self.granularity_4k << 3 |
            self.bits32         << 2 |
            self.bits64         << 1
        )

        data = ", ".join(
            gdt_word if isinstance(gdt_word, str) else "0x{:04x}".format(gdt_word)
            for gdt_word in (
                self.limit if isinstance(self.limit, str) else self.limit & 0xFFFF,
                self.base if isinstance(self.base, str) else self.base & 0xFFFF,
                access << 8 | (0 if isinstance(self.base, str) else self.base >> 16 & 0xFF),
                (0 if isinstance(self.base, str) else self.base >> 24 & 0xFF) << 8 | flags << 4 | (0 if isinstance(self.limit, str) else self.limit >> 16 & 0xF),
            )
        )

        return "{name}_gdte: dw {data}".format(name=self.name, data=data)


class GdtCodeEntry(GdtEntry):
    def __init__(self, name, base, limit, *, conforming, readable, **kwargs):
        super().__init__(name, base, limit, system=False, granularity_4k=True, **kwargs)
        self.__conforming = conforming
        self.__readable = readable

    def _type(self):
        return (
            True              << 3 |
            self.__conforming << 2 |
            self.__readable   << 1 |
            self.accessed     << 0
        )


class GdtDataEntry(GdtEntry):
    def __init__(self, name, base, limit, *, writable, **kwargs):
        super().__init__(name, base, limit, system=False, granularity_4k=True, **kwargs)
        self.__grows_down = False
        self.__writable = writable

    def _type(self):
        return (
            False             << 3 |
            self.__grows_down << 2 |
            self.__writable   << 1 |
            self.accessed     << 0
        )

class GdtSystemEntry(GdtEntry):
    def __init__(self, name, base, limit):
        super().__init__(name, base, limit, system=True, privilege_level=0, granularity_4k=False)

    def _type(self):
        return 0x9


TEMPLATE = """\
; Generated by {generator}, DO NOT EDIT

gdt:
    null_gdte: dw 0x0000, 0x0000, 0x0000, 0x0000
    {entries}

gdtr:
    gdt_size: dw gdtr - gdt
    gdt_base: dd gdt
"""

def generate(path):
    encoded_entries = [entry.encode() for entry in [
        GdtCodeEntry("kernel_code", 0, 0xFFFFF, readable=True, conforming=False, privilege_level=0),
        GdtDataEntry("kernel_data", 0, 0xFFFFF, writable=True, privilege_level=0),
        GdtCodeEntry("user_code",   0, 0xFFFFF, readable=True, conforming=False, privilege_level=3),
        GdtDataEntry("user_data",   0, 0xFFFFF, writable=True, privilege_level=3),

        GdtSystemEntry("tss", "tss", "tss_end - tss"),
    ]]

    data = TEMPLATE.format(
        generator=os.path.basename(__file__),
        entries="\n    ".join(encoded_entries),
    )

    with open(path, "w") as file:
        file.write(data)

if __name__ == "__main__":
    generate(os.path.join(os.path.dirname(__file__), "gdt.asm"))