Member Avatar for uu666

hello there!
lately i started learning assembly from a book that first gets you through many things about processors, memory, logical&physical addresses and many others... now, what i dont understand is:
any element from a segment is available through an logical address that has two elements: a selector and an offset. Selector is used as an index to select the descriptor from descriptors table. Where is located (if i can say thus) that logical address and the descriptors table (somehow in physical memory?) Thanks.

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Member Avatar for mostafadotnet

Hi uu666,
I'm not sure but as far as I know you can access physical memory when you're in real mode which there is nothing more than exact offset:segment or the reverse.But if you want to address more than the 1 MB limit of memory in 8bit real mode,you should define a descriptors table which addresses the memory logically in 32bit protected mode.

:D I don't know more.
Excuse me if these are incorrect.

Good luck.

Member Avatar for Ryu

Selector is used as an index to select the descriptor from descriptors table. Where is located (if i can say thus) that logical address and the descriptors table (somehow in physical memory?) Thanks.

I'm not 100% sure what your trying to ask, but I'm guessing its on the lines of "Where is the descriptor table located in linear memory?" Well yes it can reside in physical memory and generally is, but the location depends on what linear base address GDTR or LDTR or IDTR registers are set to, thus its possible to even set a base to a ROM address in memory. Basically you program these registers to tell where the descriptor table begins and end in linear address. Selectors can be thought of an offset to the descriptor table but not exactly with a few bits set on the side for flags such as TI and current privilege level RPL or CPL fields.

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