
THE 80286 AND 80386 PROCESSORS 523
15.9 | Calculating the Size of the Logical Address Space
In a segmented model of memory, the address space as viewed by an applications program
(called the logical address space) is a much larger space of up to 2
46
(64 terabytes). How is this
fi gure obtained? Let us make a calculation.
A selector has 13 bits as index for selecting one of the descriptors from a descriptor table.
us, it can access up to 2
13
descriptors with just one descriptor table. Since two descrip-
tor tables (the GDT and one LDT) are active at a time, 2 × 2
13
segments can be accessed.
Each descriptor corresponds to a segment with a maximum ...