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|
Each entry is n*4 bits long, where n is the current mode in bits
TODO: Specify a limit for them
It uses the following format:
(higher address)
+---------------------+-------------------------+
| upper 4 bits: flags | lower 1/2: base address |
+---------------------+-------------------------+
| upper 4 bits: type | lower 1/2: limit |
+---------------------+-------------------------+
| permissions required to use |
+-----------------------------------------------+
| permissions not allowed to use |
+-----------------------------------------------+
(lower address)
Unspecified bits are ignored for the gdt's purposes
type:
0: null, inactive
1: <normal>
???
flags for <normal> type segments:
3: invert region access (specifies a region of memory not allowed for this entry)
2: readable
1: writable
0: executable
base address:
This number is added to the address for all pointers referencing this segment
limit:
This is the maximum value allowed for the address of the pointer referencing this segment
permissions required to use:
The active task must have all of the permissions set here to use this segment
permissions not allowed to use:
The active task must not have any of these permissions to use this segment
32-bit example:
31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16 15 14 13 12 11 10 09 08 07 06 05 04 03 02 01 00
+------------------------------------------------------------------------------------------------+
| 0 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 0 0 1 0 0 0 0 0 0 0 0 0 0 0 0 |
| 0 0 0 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 0 0 0 |
| 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 |
| 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 1 0 |
+------------------------------------------------------------------------------------------------+
flags: 0 1 0 0
not inverted
readable
not writable
not executable
base address: 1 0 0 1 0 0 0 0 0 0 0 0 0 0 0 0
0x9000
type: 0 0 0 1
type 1: <normal>
limit: 0 0 0 0 0 1 0 0 0 0 0 0 0 0 0 0
0x400
permissions required: 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
must have 31st permission to use this segment
permissions not allowed: 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 1 0
must not have 1st or 2nd permission to use this segment
Access MUST be checked prior to any action, speculative or otherwise
|
