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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.
The TR register is a 16-bit register which holds a segment selector for the TSS. It may be loaded through the LTR instruction. LTR is a privileged instruction and acts in a manner similar to other segment register loads.
The lowest two bits (bit 1 and bit 0) of the selector are combined to define the privilege of the request, where the values of 0 and 3 represent the highest and the lowest privilege, respectively. This means that the byte offset of descriptors in the descriptor table is the same as the 16-bit selector, provided the lower three bits are zeroed.
Otherwise it's the 16-bit max 0x0000ffff. Essentially the same meaning as "D". L=Long If set, this is a 64-bit segment (and D must be zero), and code in this segment uses the 64-bit instruction encoding. "L" cannot be set at the same time as "D" aka "B". (Bit 21 in the image) AVL=Available For software use, not used by hardware (Bit 20 in the ...
Should 16-bit code need to run in a 32-bit environment while sharing memory (this happens e.g. when running OS/2 1.x programs on OS/2 2.0 and later), the LDT must be written in such a way that every flat (paged) address has also a selector in the LDT (typically this results in the LDT being filled with 64 KiB entries).
x86 assembly language is the name for the family of assembly languages which provide some level of backward compatibility with CPUs back to the Intel 8008 microprocessor, which was launched in April 1972. [1][2] It is used to produce object code for the x86 class of processors. Regarded as a programming language, assembly is machine-specific ...
Far pointer. In a segmented architecture computer, a far pointer is a pointer to memory in a specific context, [ 1] such as a segment selector making it possible to point to addresses outside of the default segment. Comparison and arithmetic on far pointers is problematic: there can be several different segment-offset address pairs pointing to ...
Will change OperandSize from 16-bit to 32-bit if CS.D=0, or from 32-bit to 16-bit if CS.D=1. 67h: AddressSize override. Will change AddressSize from 16-bit to 32-bit if CS.D=0, or from 32-bit to 16-bit if CS.D=1. The 80386 also introduced the two new segment registers FS and GS as well as the x86 control, debug and test registers.