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The memory model concept derives from the setup of the segment registers. For example, in the tiny model CS=DS=SS, that is the program's code, data, and stack are all contained within a single 64 KB segment. In the small memory model DS=SS, so both data and stack reside in the same segment; CS points to a different code segment of up to 64 KB.
The x86-64 architecture does not support segmentation in "long mode" (64-bit mode). [18] Four of the segment registers: CS, SS, DS, and ES are forced to 0, and the limit to 2 64. The segment registers FS and GS can still have a nonzero base address. This allows operating systems to use these segments for special purposes such as thread-local ...
if X = 0 then C is the direction bit: if C = 0 then the segment grows up; if C = 1 then the segment grows down. X is the Executable bit: [38] 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 ...
x86-64 is a 64-bit extension of x86 that almost entirely removes segmentation in favor of the flat memory model used by almost all operating systems for the 386 or newer processors. In long mode, all segment offsets are ignored, except for the FS and GS segments.
The x86 and x86-64 segment descriptor has the following form: [3] Where the fields stand for: Base Address Starting memory address of the segment. Its length is 32 bits and it is created from the lower part bits 16 to 31, and the upper part bits 0 to 7, followed by bits 24 to 31.
The alternate wording b-bit aligned designates a b/8 byte aligned address (ex. 64-bit aligned is 8 bytes aligned). A memory access is said to be aligned when the data being accessed is n bytes long and the datum address is n-byte aligned. When a memory access is not aligned, it is said to be misaligned. Note that by definition byte memory ...
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 ...
The five-volume set of the x86-64 Architecture Programmer's Manual, as published and distributed by AMD in 2002. x86-64 (also known as x64, x86_64, AMD64, and Intel 64) [note 1] is a 64-bit version of the x86 instruction set, first announced in 1999. It introduced two new modes of operation, 64-bit mode and compatibility mode, along with a new ...