Undocumented Instructions, Why?

If you haven't heard of undocumented instructions, here's a quick primer.

They are opcodes that used to be present in early microprocessors due to side effects of different wiring circuits in them. They were usually left undocumented or rather cleanly left out from the official documentation and hence the name. But such policies were not to deter the exquisite brains of the geeks and they found out ways to exploit them. These instructions were not guaranteed hundred percent success. Usually, they amounted to a system crash, but in some rare cases, they produced something of value.

Early game developers of systems like Amiga, Commodore 64 etc used such instructions to speed up certain time critical process and hence optimized the system.
Certain instructions

Such instructions usually provided the programmer shorter ways to attain results. These instructions were actually curses to the chip designers. With each revision, such instructions were bound to change and resulted in crashing a perfectly working program.

Designers later began to trap such instructions in design phase itself and as a result, the volume of such instructions have sunk to a minimum. Certain instructions like SALC (set the AL register depending on the Carry Flag, opcode D6), AAM immd8 (Divide AL by immd8, put quotient in AH, Remainder in AL, opcode : D4) from the x86 family were later documented.

Fuzzy Logic

Fuzzy logic is giving new dimensions to everything around us. Lets first focus on what fuzzy logic is. As humans, we think and operate. Our process of thinking can be said to be a form of fuzzy logic. I.e., all process and applications that are not linear and can’t be treated as non linear can be said to contain no clear cut logic. The logic involved is fuzzy in nature.

What does that have to do with designs? Well, the world and its applications are changing. Electronics have been applied to even very obscure applications. Certain applications lack a clear cut logic or mathematical model to be correctly implemented. Such applications which have been dubbed to be beyond the scope of normal electronics are now possible through application of fuzzy logic to solve the problems.

Put simply, fuzzy logic revolves around the basic principle that “X and Y equals Z”. How much X and how much Y is application specific and can be found out either through trial and error or through normal use of the application. It may seem like the concepts of self learning programs. IT may a bit, because in today’s world, its hard to distinguish one from another. Neural Programs and fuzzy logic have been tightly integrated today, so that it is impossible, virtually to separate them.

A number of factors or variables are considered in generating fuzzy logic. The idea behind a fuzzy engine is the definition of proper variables and the rules governing them which effect other process. One of the most important variable, for example may be an error function which calculates the difference between the actually obtained value and the required value. A rule is to be set so as to reduce error function within a specified percentage of actual value. The differential of error function can provide the rate at which the system responds, thereby providing a measure of the stability or responsively of the system.

What you just read was a primer on fuzzy logic. If you find the topic interesting, feel free to mail me or comment here on this blog to which I can promptly respond and of course you can go after the countless books that talk about fuzziness and fuzzy logic.

That brings an interesting question. Any ideas to generate a fuzzy cryptographic scheme?

Comment !!

Enabling Linux style file control in XP

No doubt many of you have dreamt of the user level file control present in Linux/POSIX type systems. Make no mistakes, XP has got the same feature too. But perhaps, its a little underpublished.

Here's how :

1. Open an explorer window.
2. Click on View
3. Click on Folder Options
4. Click on the view tab.
5. Scroll down below and uncheck "Use Simple File Sharing(Recommended)".

and voila, you got a per user controllable XP. To add permissions on whom to use and all, right click the concerned file and click on security.
Here, you can control permissions, set and deny access and permissions.

Nice, ain't it?

More info is available at Microsoft Page here

And finally, a note of warning. It is found that this feature is best available on a drive running NTFS.