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Diffstat (limited to 'docs')
| -rw-r--r-- | docs/x86_gdt.txt | 54 |
1 files changed, 54 insertions, 0 deletions
diff --git a/docs/x86_gdt.txt b/docs/x86_gdt.txt new file mode 100644 index 0000000..556881d --- /dev/null +++ b/docs/x86_gdt.txt @@ -0,0 +1,54 @@ + +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 PhyAddr += GDT[SegBaseReg].base + Offset + +Segmentation is obsolete and we will assume a "flat memory model", as most +compilers do.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 |