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A segment value of 0Ch (12) would give a linear address at C0h (192) in the linear address space. The address offset can then be added to this number. 0Ch:0Fh (12:15) would be C0h+0Fh=CFh (192+15=207), CFh (207) being the linear address. Such address translations are carried out by the segmentation unit of the CPU.
However, on the 80386, with its paged memory management unit it is possible to protect individual memory pages against writing. [4] [5] Memory models are not limited to 16-bit programs. It is possible to use segmentation in 32-bit protected mode as well (resulting in 48-bit pointers) and there exist C language compilers which support that. [6]
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.
In these languages, including the line __DATA__ (Perl) or __END__ (Ruby, old Perl) marks the end of the code segment and the start of the data segment. Only the contents prior to this line are executed, and the contents of the source file after this line are available as a file object: PACKAGE::DATA in Perl (e.g., main::DATA ) and DATA in Ruby.
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 ...
These instructions are also available in 32-bit mode, in which they operate on 32-bit registers (eax, ebx, etc.) and values instead of their 16-bit (ax, bx, etc.) counterparts. The updated instruction set is grouped according to architecture ( i186 , i286 , i386 , i486 , i586 / i686 ) and is referred to as (32-bit) x86 and (64-bit) x86-64 (also ...
Virtual 8086 mode is not available in x86-64 long mode, although it is still present on x86-64 capable processors running in legacy mode. Intel VT-x brings back the ability to run virtual 8086 mode from x86-64 long mode, but it has to be done by transitioning the (physical) processor to VMX root mode and launching a logical (virtual) processor ...
The term "segment" comes from the memory segment, which is a historical approach to memory management that has been succeeded by paging.When a program is stored in an object file, the code segment is a part of this file; when the loader places a program into memory so that it may be executed, various memory regions are allocated (in particular, as pages), corresponding to both the segments in ...