Started paging code.

kernel/arch/i386/paging.nim

@@ -0,0 +1,39 @@
+import serial
+
+type
+  PageDirectoryEntry = uint32
+  PageTableEntry = uint32
+  PageDirectory = array[1024, PageDirectoryEntry]
+  PageTable = array[1024, PageTableEntry]
+
+const 
+  blankPageDirectory: PageDirectoryEntry = 0x00000002
+
+var
+  pageDirectory: PageDirectory
+  pageTable: 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 loadPageDirectory(address: uint32){.asmNoStackFrame.} = 
+  asm """
+    mov `address`, %cr3
+    mov %cr0, %eax
+    or $0x80000001, %eax
+    mov %eax, %cr0
+  """
+
+proc init*() =
+  serial.writeLine("Initialising paging tables and directories.")
+  for i in 0..1023:
+    pageDirectory[i] = blankPageDirectory
+  for i in 0..1023:
+    pageTable[i] = createEntry(address = uint32(i) * 0x1000, readWrite = true, present = true)
+  pageDirectory[0] = createEntry(address = cast[uint32](pageTable), readWrite = true, present = true)
+  loadPageDirectory(cast[uint32](pageDirectory.addr))