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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 ...
In real mode or V86 mode, the size of a segment can range from 1 byte up to 65,536 bytes (using 16-bit offsets). The 16-bit segment selector in the segment register is interpreted as the most significant 16 bits of a linear 20-bit address, called a segment address, of which the remaining four least significant bits are all zeros.
In a system using segmentation, computer memory addresses consist of a segment id and an offset within the segment. [3] A hardware memory management unit (MMU) is responsible for translating the segment and offset into a physical address, and for performing checks to make sure the translation can be done and that the reference to that segment and offset is permitted.
Another example in the same computer family was the 16-bit protected mode of the 80286 processor, which, though supporting only 16 MB of physical memory, could access up to 1 GB of virtual memory, but the combination of 16-bit address and segment registers made accessing more than 64 KB in one data structure cumbersome.
In memory addressing for Intel x86 computer architectures, segment descriptors are a part of the segmentation unit, used for translating a logical address to a linear address. Segment descriptors describe the memory segment referred to in the logical address. [1] The segment descriptor (8 bytes long in 80286 and later) contains the following ...
In computer programming, thread-local storage (TLS) is a memory management method that uses static or global memory local to a thread. The concept allows storage of data that appears to be global in a system with separate threads. Many systems impose restrictions on the size of the thread-local memory block, in fact often rather tight limits.
For example, in an Intel 8086, as well as in later processors running 16-bit code, a far pointer has two parts: a 16-bit segment value, and a 16-bit offset value. A linear address is obtained by shifting the binary segment value four times to the left, and then adding the offset value. Hence the effective address is 21 bits [Note 1]. There can ...
Early x86 processors use the segmented memory model addresses based on a combination of two numbers: a memory segment, and an offset within that segment. Some segments are implicitly treated as code segments, dedicated for instructions, stack segments, or normal data segments.