Search results
Results from the WOW.Com Content Network
The x86 and x86-64 segment descriptor has the following form: [3] ... If clear, this is a 16-bit code segment; if set, this is a 32-bit segment. B = Big: If set, the ...
Otherwise, address translation continues. The processor then takes the 32-bit or 16-bit offset and compares it against the segment limit specified in the segment descriptor. If it is larger, a GP fault is generated. Otherwise, the processor adds the 24-bit segment base, specified in descriptor, to the offset, creating a linear physical address.
The GDT is a table of 8-byte entries. Each entry may refer to a segment descriptor, Task State Segment (TSS), Local Descriptor Table (LDT), or call gate. Call gates were designed for transferring control between x86 privilege levels, although this mechanism is not used on most modern operating systems. There is also a Local Descriptor Table (LDT).
The default OperandSize and AddressSize to use for each instruction is given by the D bit of the segment descriptor of the current code segment - D=0 makes both 16-bit, D=1 makes both 32-bit. Additionally, they can be overridden on a per-instruction basis with two new instruction prefixes that were introduced in the 80386:
Four registers are used to refer to four segments on the 16-bit x86 segmented memory architecture. DS (data segment), CS (code segment), SS (stack segment), and ES (extra segment). Another 16-bit register can act as an offset into a given segment, and so a logical address on this platform is written segment:offset, typically in hexadecimal ...
if X = 1 then the segment is a code segment; if X = 0 then the segment is a data segment. S is the Segment type bit, which should generally be cleared for system segments; [38] DPL is the Descriptor Privilege Level; P is the Present bit; D is the Default operand size; G is the Granularity bit; Bit 52 of the 80386 descriptor is not used by the ...
Memory segmentation is an operating system memory management technique of dividing a computer 's primary memory into segments or sections. In a computer system using segmentation, a reference to a memory location includes a value that identifies a segment and an offset (memory location) within that segment. Segments or sections are also used in ...
More recent x86 chips provide a per-page no-execute bit in the PAE mode. The W^X, Exec Shield, and PaX mechanisms described above emulate per-page non-execute support on machines x86 processors lacking the NX bit by setting the length of the code segment, with a performance loss and a reduction in the available address space.