Programming Forums > Application Development > Assembly

conbrio · Jul 5th, 2005, 12:42 PM

This question doesn't have anything directly to do with assembly code but is "essential," I would think, to assembly (and is from what I've seen a topic in many assembly tutorials).

Anyway, my question is relating to the area of the descriptor table entry called the "limit." This limit is supposed to hold a 20-bit number describing the size of the segment, in bytes (or 4K if granularity bit is enabled). Now, nearly everywhere I've read says that in protected mode segments can be up to 4GB. Yet 2^20=1,048,576B and hence 1MB segments at most seem possible. I don't understand how this can be. I think the discrepancy is obvious.

The information on the 'net regarding memory logic layout & low level CPU stuff is pretty scant. It's either usually outdated and maybe only covers real mode and old processors or really condensed. If anyone has any good links, pass them along

Help is appreciated. I feel like I ask too many questions here.

DaWei · Jul 5th, 2005, 2:35 PM

What you're talking about is platform and tool dependent. From what does your question arise, exactly?

conbrio · Jul 5th, 2005, 6:38 PM

Hmm, platform and tool dependent? Not sure what you mean by tool, but I was of the thought that OSes only managed what were already fixed CPU-memory relationships (ie, the segmented real and protected modes and flat memory model). The question arises mainly out of curiosity, but I suppose it'd help whenever I desire to start learning assembly lang. too.

DaWei · Jul 5th, 2005, 7:28 PM

I presume, possibly erroneously, that you're referring to descriptor tables for some memory management approaches. Not every system indulges in memory management or segmented addressing schemes, or even "real" vs. "protected" modes. It's very implementation dependent. Despite the huge number of desktop machines out there running a few "common" operating systems, there are even more non-desktop microprocessor "computers" or systems. If you want to learn assembly language, pick a processor -- they differ, often radically. When you have a processor in hand (or mind), then familiarize yourself with what IT does and how it does it. A great deal of system software is an adjunct constrained by the capabilities of the underlying machine.

conbrio · Jul 5th, 2005, 7:55 PM

You're correct. I was talking specifically about PC memory management/CPUs. I thought "PC" was assumed by default on the forum, but apparently not, which is probably a good thing.

DaWei · Jul 5th, 2005, 8:42 PM

A very large amount of commercial programming is not in "PCs". Neither does PC mean "IBM PC compatible". The world of the microprocessor is actually quite diverse and probably more widespread than frequently thought. I don't have the usual picture of the market, I suppose; in 35 years of microprocessor programming interspersed throughout other engineering duties, "desktop" programs are but a tiny fraction of the whole.

omega_red · Jul 6th, 2005, 3:41 AM

Quote:

Originally Posted by conbrio

Anyway, my question is relating to the area of the descriptor table entry called the "limit." This limit is supposed to hold a 20-bit number describing the size of the segment, in bytes (or 4K if granularity bit is enabled). Now, nearly everywhere I've read says that in protected mode segments can be up to 4GB. Yet 2^20=1,048,576B and hence 1MB segments at most seem possible. I don't understand how this can be. I think the discrepancy is obvious.

From Intel's manual (vol. 3):

Quote:

Segment limit field
Specifies the size of the segment. The processor puts together the two segment limit fields to form a 20-bit value. The processor interprets the segment limit in one of two ways, depending on the setting of the G (granularity) flag:
• If the granularity flag is clear, the segment size can range from 1 byte to 1
MByte, in byte increments.
• If the granularity flag is set, the segment size can range from 4 KBytes to
4 GBytes, in 4-KByte increments.

And that's it

Either you have byte-resolution and 1MB max size, or 4kB (page) resolution and 4GB size (this is used by most OSes).

Jul 5th, 2005, 12:42 PM	#1
conbrio Newbie Join Date: Apr 2005 Posts: 10 Rep Power: 0	Descriptor tables This question doesn't have anything directly to do with assembly code but is "essential," I would think, to assembly (and is from what I've seen a topic in many assembly tutorials). Anyway, my question is relating to the area of the descriptor table entry called the "limit." This limit is supposed to hold a 20-bit number describing the size of the segment, in bytes (or 4K if granularity bit is enabled). Now, nearly everywhere I've read says that in protected mode segments can be up to 4GB. Yet 2^20=1,048,576B and hence 1MB segments at most seem possible. I don't understand how this can be. I think the discrepancy is obvious. The information on the 'net regarding memory logic layout & low level CPU stuff is pretty scant. It's either usually outdated and maybe only covers real mode and old processors or really condensed. If anyone has any good links, pass them along Help is appreciated. I feel like I ask too many questions here.

Jul 5th, 2005, 2:35 PM	#2
DaWei Resident Grouch Join Date: Jun 2005 Posts: 6,453 Rep Power: 10	What you're talking about is platform and tool dependent. From what does your question arise, exactly? __________________ Abstraction doesn't make it impossible to write bad code; it makes it possible to write superior code. Contributor's Corner: Grumpy on C++ Exceptions DaWei on Pointers

Jul 5th, 2005, 7:28 PM	#4
DaWei Resident Grouch Join Date: Jun 2005 Posts: 6,453 Rep Power: 10	I presume, possibly erroneously, that you're referring to descriptor tables for some memory management approaches. Not every system indulges in memory management or segmented addressing schemes, or even "real" vs. "protected" modes. It's very implementation dependent. Despite the huge number of desktop machines out there running a few "common" operating systems, there are even more non-desktop microprocessor "computers" or systems. If you want to learn assembly language, pick a processor -- they differ, often radically. When you have a processor in hand (or mind), then familiarize yourself with what IT does and how it does it. A great deal of system software is an adjunct constrained by the capabilities of the underlying machine. __________________ Abstraction doesn't make it impossible to write bad code; it makes it possible to write superior code. Contributor's Corner: Grumpy on C++ Exceptions DaWei on Pointers

Jul 5th, 2005, 8:42 PM	#6
DaWei Resident Grouch Join Date: Jun 2005 Posts: 6,453 Rep Power: 10	A very large amount of commercial programming is not in "PCs". Neither does PC mean "IBM PC compatible". The world of the microprocessor is actually quite diverse and probably more widespread than frequently thought. I don't have the usual picture of the market, I suppose; in 35 years of microprocessor programming interspersed throughout other engineering duties, "desktop" programs are but a tiny fraction of the whole. __________________ Abstraction doesn't make it impossible to write bad code; it makes it possible to write superior code. Contributor's Corner: Grumpy on C++ Exceptions DaWei on Pointers

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Jul 5th, 2005, 6:38 PM	#3
conbrio Newbie Join Date: Apr 2005 Posts: 10 Rep Power: 0	Hmm, platform and tool dependent? Not sure what you mean by tool, but I was of the thought that OSes only managed what were already fixed CPU-memory relationships (ie, the segmented real and protected modes and flat memory model). The question arises mainly out of curiosity, but I suppose it'd help whenever I desire to start learning assembly lang. too.

Jul 5th, 2005, 7:55 PM	#5
conbrio Newbie Join Date: Apr 2005 Posts: 10 Rep Power: 0	You're correct. I was talking specifically about PC memory management/CPUs. I thought "PC" was assumed by default on the forum, but apparently not, which is probably a good thing.

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Programming Forums > Application Development > Assembly

Descriptor tables