alcedo/kernel/arch/i386/paging.nim

import serial

type
  PageDirectoryEntry = uint32
  PageTableEntry = uint32
  PageDirectory = array[1024, PageDirectoryEntry]
  PageTable = array[1024, PageTableEntry]

const 
  blankPageDirectory: PageDirectoryEntry = 0x00000002

var
  pageDirectory{.exportc, codegenDecl: "$# $# __attribute__((aligned(0x1000)))".}: PageDirectory
  pageTable0{.exportc, codegenDecl: "$# $# __attribute__((aligned(0x1000)))".}: PageTable

proc createEntry(address: uint32, present: bool = false, readWrite: bool = false, userAccessible: bool = false, writeThrough: bool = false, disableCache: bool = false): uint32 =
  result = address and 0xFFFFF000'u32 # Set attributes section to zero
  if present: result = result or 0b1
  if readWrite: result = result or 0b10
  if userAccessible: result = result or 0b100
  if writeThrough: result = result or 0b1000
  if disableCache: result = result or 0b10000

proc createDirectory(address: uint32, present: bool = false, readWrite: bool = false, userAccessible: bool = false, writeThrough: bool = false, disableCache: bool = false, pageSize: bool = false): uint32 =
  var checkAddress = address
  if pageSize: checkAddress = checkAddress and 0xFFFF0000'u32
  result = createEntry(address, present, readWrite, userAccessible, writeThrough, disableCache)
  if pageSize: result = result or 0b10000000

proc loadPageDirectory(address: uint32){.inline.} = 
  serial.writeLine("Loading paging data")
  asm """
    mov %%cr3, %%eax
    mov %%eax, %%cr0
    or $0x80000001, %%eax
    mov %%cr0, %%eax
    ::"a"(`address`)
  """

proc init*() =
  serial.writeLine("Initialising paging tables and directories.")
  for i in 0..1023:
    pageDirectory[i] = blankPageDirectory
  for i in 0..1023:
    pageTable0[i] = createEntry(address = uint32(i) * 0x1000, readWrite = true, present = true)
  pageDirectory[0] = createDirectory(address = cast[uint32](pageTable0), readWrite = true, present = true)
  loadPageDirectory(cast[uint32](pageDirectory.addr))
  serial.write("Paging setup complete at ")
  serial.writeLine(cast[uint32](pageDirectory.addr))