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Revision as of 01:21, 13 January 2007 view source osdev>Jhawthorn No edit summary		Latest revision as of 23:35, 14 September 2018 view source osdev>Carver m use talk page
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Line 1: __NOTOC__ ==Descriptors==▼ {{Infobox Tutorial \| name=Babystep6 \| prev=[[Babystep5]] \| next=[[Babystep7]] }} === Entering protected mode === Entering [[Protected mode]] is actually done by switching a single bit in a special control register (cr0). (All the other stuff, like [[A20Line]], tasks, [[IDT]], call gates, etc. is additional stuff.) Line 5 ⟶ 13: However, before switching to pmode, you have to use the LGDT instruction to load another special register (gdtr) with the location of a table of data structures called descriptors that tell the process how to access memory. ▲==Descriptors== ~~We're arguing about whether GDT could be set up after switching to pmode in this thread~~ ~~--PypeClicker~~ Overview of bytes in the descriptor: Line 34 ⟶ 41: \|- \| <tt>6</tt> \| <tt>0x00</tt> = (bits) 0 - 0 - 0 - 0 - 0000 = G - D/B - 0R - AVL - Size \|- \| <tt>7</tt> Line 40 ⟶ 47: \|} ===Bits in Type (byte #5)=== ;"P"▼ ▲;"P" :Present (1 bit) = 1 means segment is in memory (accessing a non-present segment will raise an exception) ;"DPL" :Descriptor Privilege Level (2 bits) = 0 is most privileged and 3 is least. ;"S" :System (1 bit) = must be 0 in descriptors for Task State Segments (TSS), Interrupt Gate, Trap Gate, Task Gate, Call Gates. Otherwise, for code/data/stack segment descriptors, it will be 1. ;"Type" :Type (4 bits) = interpretation of these depends on whether S (above) is set or not. For S=0, the interpretation will be covered in specific instances of gates etc. Line 58 ⟶ 64: ;Type bit 2 :The next highest bit depends on the highest bit. If code segment, this next bit indicates whether the segment is 'Conforming' or not. This allows programs somewhere else that are LESS privileged to access this segment, then this segment conforms to the ~~priviledge~~privilege level of the calling ~~programm~~program. If it's a data segment, this bit specifies "Expand (up or down)" for when the segment is used as a stack. Expand-up (bit=0) is your normal stack behavior. [[Expand_Down\|Expand-down]] is used to prevent problems in stacks that are resized. ;Type bit 1 Line 66 ⟶ 72: :The lowest bit means that the segment has been accessed already (1) or not. ===Bits in ~~'flags'~~Flags (byte #6)=== ;G ~~"G" =~~ :Granularity (1 bit) = segment Size specified in bytes (0) or 4K pages (1) ;D/B ~~"D/B" =~~ :Default (code seg) / Big (data seg) = (1 bit) In a code segment (see "Type" above), this bits says default operand/address size is 32-bit (1) or 16-bit (0). For a data segment, it means stack pointer is 32-bit (1) or 16-bit (0). Also means something for expand-down stacks (see "Type" above), but we don't care. ;R ~~"0" =~~ :Reserved (1 bit) = belongs to the Intel of the future. ;AVL ~~"AVL" =~~ :Available (1 bit) = For your use. Go crazy. ;Size ~~"Size" =~~ :Top Nibble of Size (4 bits) = The size of the segment is 20-bits. This is the final four. Whether it means the highest possible segment size is 1 meg or 4 Gigs depends on Granularity above. ==See Also== ~~===Articles===~~ [[Global Descriptor Table]] [[Segmentation]] [[Category:Babystep]]