Intel ARCHITECTURE IA-32 User Manual Page 263

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Vol. 3A 6-21
TASK MANAGEMENT
Through segment descriptors in distinct LDTs that are mapped to common addresses
in linear address space — If this common area of the linear address space is mapped to
the same area of the physical address space for each task, these segment descriptors permit
the tasks to share segments. Such segment descriptors are commonly called aliases. This
method of sharing is even more selective than those listed above, because, other segment
descriptors in the LDTs may point to independent linear addresses which are not shared.
6.6 16-BIT TASK-STATE SEGMENT (TSS)
The 32-bit IA-32 processors also recognize a 16-bit TSS format like the one used in Intel 286
processors (see Figure 6-10). This format is supported for compatibility with software written
to run on earlier IA-32 processors.
The following information is important to know about the 16-bit TSS.
Do not use a 16-bit TSS to implement a virtual-8086 task.
The valid segment limit for a 16-bit TSS is 2CH.
The 16-bit TSS does not contain a field for the base address of the page directory, which is
loaded into control register CR3. A separate set of page tables for each task is not
supported for 16-bit tasks. If a 16-bit task is dispatched, the page-table structure for the
previous task is used.
The I/O base address is not included in the 16-bit TSS. None of the functions of the I/O
map are supported.
When task state is saved in a 16-bit TSS, the upper 16 bits of the EFLAGS register and the
EIP register are lost.
When the general-purpose registers are loaded or saved from a 16-bit TSS, the upper 16
bits of the registers are modified and not maintained.
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