diff options
Diffstat (limited to 'cpu/access_control/gdt.txt')
| -rw-r--r-- | cpu/access_control/gdt.txt | 70 |
1 files changed, 70 insertions, 0 deletions
diff --git a/cpu/access_control/gdt.txt b/cpu/access_control/gdt.txt new file mode 100644 index 0000000..fcadb5d --- /dev/null +++ b/cpu/access_control/gdt.txt @@ -0,0 +1,70 @@ +Each entry is n*4 bits long, where n is the current mode in bits +TODO: Specifiy a limit for them + +It uses the following format: + (higher address) + +---------------------+-------------------------+ + | upper 4 bits: flags | lower 3/4: base address | + +---------------------+-------------------------+ + | upper 4 bits: type | lower 3/4: 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 1 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 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 0 0 0 0 0 0 0 0 + 0x900000 + +type: 0 0 0 1 + type 1: <normal> + +limit: 0 0 0 0 0 0 0 0 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 |