unreal gdt now has 2 selectors 1 code, 1 data

ss, and e/f/g segments are loaded to the same flat 4gb segment
moved gdt notes to documentation

docs/gdt.txt

@@ -0,0 +1,61 @@
+GDT documentation below:
+
+   Pr: Present bit. This must be 1 for all valid selectors.
+
+   Privl: Privilege, 2 bits. Contains the ring level,
+          0 = highest (kernel), 3 = lowest (user applications).
+
+   S: Descriptor type. This bit should be set for code or data segments
+      and should be cleared for system segments (eg. a Task State Segment)
+
+   Ex: Executable bit. If 1 code in this segment can be executed
+       ie. a code selector. If 0 it is a data selector.
+
+   DC: Direction bit/Conforming bit.
+       Direction bit for data selectors: Tells the direction.
+       0 the segment grows up. 1 the segment grows down, ie. the offset has to be greater than the limit.
+
+       Conforming bit for code selectors:
+           If 1 code in this segment can be executed from an equal or lower privilege level.
+              For example, code in ring 3 can far-jump to conforming code in a ring 2 segment.
+              The privl-bits represent the highest privilege level that is allowed to execute the segment.
+                  For example, code in ring 0 cannot far-jump to a conforming code segment with privl==0x2
+                  while code in ring 2 and 3 can. Note that the privilege level remains the same
+                  ie. a far-jump form ring 3 to a privl==2-segment remains in ring 3 after the jump.
+
+           If 0 code in this segment can only be executed from the ring set in privl.
+
+   RW: Readable bit/Writable bit.
+       Readable bit for code selectors: Whether read access for this segment is allowed. Write access is never allowed for code segments.
+       Writable bit for data selectors: Whether write access for this segment is allowed. Read access is always allowed for data segments.
+
+   Ac: Accessed bit. Just set to 0. The CPU sets this to 1 when the segment is accessed.
+
+   Gr: Granularity bit. If 0 the limit is in 1 B blocks (byte granularity), if 1 the limit is in 4 KiB blocks (page granularity).
+
+   Sz: Size bit. If 0 the selector defines 16 bit protected mode. If 1 it defines 32 bit protected mode.
+      You can have both 16 bit and 32 bit selectors at once.
+
+   AvL: Availible to software bit, the CPU does not use this field and software can read/write to it
+
+   D/B bit: The default operand-size bit is found in code-segment and data-segment descriptors but not in system-segment descriptors. Setting
+             this bit to 1 indicates a 32-bit default operand size, and clearing it indicates a 16-bit default size.
+
+   E bit: Expand down bit: Setting this bit to 1 identifies the data segment as expand-down.
+          In expand-down segments, the segment limit defines the lower segment boundary while the base is the upper boundary
+
+ A GDT entry is 8 bytes and is constructed as follows:
+ First DWORD
+   0-15	Limit 0:15	First 16 bits in the segment limiter
+   16-31	Base 0:15	First 16 bits in the base address
+ 
+ 2nd DWORD
+ 
+   0:7	    Base 16:23	Bits 16-23 in the base address
+   8:12	S/Type	    Segment type and attributes, S = bit 12, Type = 8:11, Type is either [1, DC, RW, Ac] <code> or [0, E, RW, Ac] <data>
+   13:14	Privl	    0 = Highest privilege (OS), 3 = Lowest privilege (User applications)
+   15	    Pr	        Set to 1 if segment is present
+   16:19	Limit 16:19	Bits 16-19 in the segment limiter
+   20:22	Attributes	Different attributes, depending on the segment type
+   23	    Gr	        Used together with the limiter, to determine the size of the segment
+   24:31	Base 24:31	The last 24-31 bits in the base address