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GDT, segmentation and long mode:
in real mode (16 bit), the address is generated with SegBaseReg:Offset
PhyAddr = (SegBaseReg << 4) + Offset SegBase:Offset could be CS:IP for
instructions or DS for data etc. DS:SI for data.
in protected/long mode, the segment base registers are not directly used
to generate address. Instead they index into GDT. The pointed GDT entry
describes the logical "Segment", including the "base" and "limit". PhyAddr
= GDT[SegBaseReg].base + Offset
Segmentation is obsolete and we will assume a "flat memory model", as most
compilers do. That is, all the segments cover the full address space. The
"Base" part of the segmentation is ignored and offset represents the full
logical address. Nevertheless we can't really disable segmentation, so
some we still have to manually set up some segmentations in the GDT.
GDT Entry (Segment Descriptor) Format
-------------------------------------
word 0 limit [0:15]
word 1 base [0:15]
word 2 base [16:23]
AccessByte [0:7]
word 3 limit [16:19]
flags [0:3]
base [24:31]
-------------------------------------
limit: 20-bit value. unit either 1 byte or 4K page
base : 32-bit value.
to use a flat memory model (as we do here), set limit to 0xFFFF, base to
0, and use 4k granularity in the access byte.
AccessByte:
0 - A Accessed bit
1 - RW Readable bit for code seg, or Writable for data seg.
Write access is never allowed for code. Read access is always
allowed for data
2 - DC For data, Direction bit, 1 for grow down
For code, Conforming bit
3 - E Executable
4 - S Descriptor type. 0 for system segment and 1 for code/data segment
[6:5] DPL Descriptor Privilege Level. ring 0~3
7 - P Present Bit. Must be 1 for valie entries
flags:
0 Res 0
1 - L Long-mode code, 1 for 64-bit code. When L set, DB MBZ
2 - DB Size Flag for protected mode seg. 0 for 16 bit and 1 for 32 bit
3 - G Granularity of Limit. 0 for 1 byte and 1 for 4KiB
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