Added GDT setup code.

kernel/arch/i386/gdt.nim

@@ -0,0 +1,60 @@
+type
+  GDTEntry{.packed.} = object
+    limit_low: uint16
+    base_low: uint16
+    base_middle: uint8
+    access: uint8
+    granularity: uint8
+    base_high: uint8
+  GDTPointer{.packed.} = ptr object
+    limit: uint16 # Size of GDT
+    base: uint32 # GDT memory location
+  GDTEntries = ptr array[0..2, GDTEntry]
+
+var
+  gdt: GDTEntries
+  gp: GDTPointer
+
+let gpAddr = cast[uint32](addr(gp))
+
+{.push stackTrace:off.}
+proc gdtFlush(){.asmNoStackFrame, inline.} =
+  asm """
+    lgdt [`gpAddr`]
+    mov %ax, 0x10
+    mov %ds, %ax
+    mov %es, %ax
+    mov %fs, %ax
+    mov %gs, %ax
+    mov %ss, %ax
+  """
+{.pop.}
+
+proc gdtSetGate(num: int, base: uint16, limit: uint32, access: uint8, granularity: uint8) =
+  # Setup the descriptor base address
+  gdt[num].base_low = (base and 0xFFFF)
+  gdt[num].base_middle = (base shr 16) and 0xFF
+  gdt[num].base_high = (base shr 24) and 0xFF
+
+  # Setup the descriptor limits
+  gdt[num].limit_low = (limit and 0xFFFF)
+  gdt[num].granularity = ((limit shr 16) and 0x0F)
+
+  # Finally, set up the granularity and access flags
+  gdt[num].granularity = gdt[num].granularity or (granularity and 0xF0)
+  gdt[num].access = access
+
+proc gdtInstall*() =
+    gp.limit = uint16(sizeof(GDTEntry) * 3) - 1;
+    gp.base = cast[uint32](addr(gdt));
+
+    # NULL descriptor
+    gdtSetGate(0, 0, 0, 0, 0);
+
+    # The second entry is our Code Segment
+    gdtSetGate(1, 0'u16, 0xFFFFFFFF'u32, 0x9A'u8, 0xCF'u8);
+
+    # The third entry is our Data Segment
+    gdtSetGate(2, 0'u16, 0xFFFFFFFF'u32, 0x92'u8, 0xCF'u8);
+
+    gdtFlush();

kernel/kernel.nim

@@ -1,6 +1,7 @@
-import tty
+import tty, gdt
 
 proc kernelMain() {.exportc: "kernel_main"}=
+  gdt.gdtInstall()
   terminalInitialize()
   terminalWrite("Hello World!\L")
   terminalWrite("MAX_INT:")