6-5 Descriptor Segment Privilege attributes Privilege Level(DPL) 4 Segment S =1 Code or Data segment descriptor Descriptor(S) S =0 System Segment or Gate Descriptor 3 Executable(E) E = 0 Descriptor type is data segment E = 1 Descriptor type is code segment 2 Expansion ED =0 Expand up(data) segment, offsets must be ≤ Direction(ED) for limit. data ED =1 Expand down(stack) segment, offsets must be > limit. Confirming(C) Code segment may only be executed when CPL ≥DPL when E=1 for code and CPL remains unchanged. 1 Writeable (W) for W = 0 Data segment are read only data W = 1 Data segment may be written into. Readable (R) for R = 0 Code segment may not be read(execute only) code R = 0 Code segment may be read(execute/read) 0 Access Bit (A) A = 0 Segment has not been accessed A = 1 Segment has been accessed System Segment Descriptor Paging Paging is used for virtual memory multitasking operating system. Pages are fixed size portions of the module or program data The and only a few pages complete task need not be in physical memory at any time are required. Hence the remaining space can be allocated for other tasks and thus multitasking can be achieved Paging Organization Page Mechanism Intel386DX uses a two level table mechanism to convert linear address to physical address. There are 3 components in paging mechanism: Page Directory
6-5 Descriptor Privilege Level(DPL) Segment Privilege attributes 4 Segment Descriptor(S) S =1 Code or Data segment descriptor S =0 System Segment or Gate Descriptor 3 Executable(E) E = 0 Descriptor type is data segment E = 1 Descriptor type is code segment 2 Expansion Direction(ED) for data ED =0 Expand up(data) segment, offsets must be ≤ limit. ED =1 Expand down(stack) segment, offsets must be > limit. Confirming(C) when E=1 for code Code segment may only be executed when CPL ≥DPL and CPL remains unchanged. 1 Writeable (W) for data W = 0 Data segment are read only W = 1 Data segment may be written into. Readable (R) for code R = 0 Code segment may not be read(execute only) R = 0 Code segment may be read(execute/read) 0 Access Bit (A) A = 0 Segment has not been accessed A = 1 Segment has been accessed System Segment Descriptor Paging Paging is used for virtual memory multitasking operating system. Pages are fixed size portions of the program module or data The complete task need not be in physical memory at any time and only a few pages are required. Hence the remaining space can be allocated for other tasks and thus multitasking can be achieved Paging Organization Page Mechanism Intel386DX uses a two level table mechanism to convert linear address to physical address. There are 3 components in paging mechanism: Page Directory
80386-summary1.docx