Here is a question re DOS/Windoze 3.1/Win 95/protected mode.



 What are the holes (if any) in PC Protected mode?



 I want to be able to run a piece of code under Windoze, etc, and have it

 achieve R/W access to _all_ of the PC's memory space, in flat, 32 bit mode.

 Ideally this should be done without relying on any DOS, BIOS, DPMI or

 Windows function calls or interrupts. The aim is for the code to be totally

 self-contained, and also to be as little contaminated with PC hardware

 specific 'features' like segmented memory pointers as possible.

 It should have sole control and knowledge of any GDT table entries and

 anything else it requires to achieve this. It should not depend on any

 particular compiler library functions, but should explicitly code all

 operations.



 At the moment when I write C code that needs to deal with memory, I use

 unsigned longs for all memory pointers, and just convert to far pointers

 when needed. I _never_ do address arithm on pointers, so I can ignore

 most of the stupidity of segmented addresses. For my purposes speed is

 not of great significance, but minimizing dependence on the PC so-called

 'architecture' is.

 I know a bit about the 80x86 addressing modes, enough to know how much

 work will be involved if I start to muck around with GDTs and privilege

 levels. Since my only interest in all this stuff is to effectively bypass

 it, I don't want to go to that trouble unless I have some confidence that

 what I want is achievable. This is a part time project, and I have very

 little time available for it, hence my apparent laziness in not just

 plowing into 'Undocumented DOS/PC/Windows' and spending weeks finding out

 for myself. I'm hoping that some kind soul who has already dealt with this

 problem can show me a quick way to do it.



 I think that what I need is a way for a piece of (my) code, running at

 privilege level 3, to be able to cause some of itself (or something it

 created) to run at privilege level 0 - system level.

 Of couse, if the protection scheme was perfect, this would be impossible.

 I'm hoping though, that the severe kludginess of the PC/DOS/Windows system

 has left at least one hole somewhere, and that I won't have to waste

 much time learning a vast amount of PC sludge to do it.

 

 I also loathe programming in 80x86 assembler, and go to great lengths to

 do even low level PC programming in C, rather than assembler. Actually,

 this turns out quite well once you decide to not use the native pointer

 types, and have a few basic functions to deal with all the ridiculous

 DOS memory structures. So hopefully whatever is needed to slip one past

 the memory protection scheme can be achieved in C too.



 What I need this for, generally, is an investigation of hardware

 independent, minimalist, interpreted operating systems.

 More as a learning experience for me than in the hope of producing

 anything of great significance.



 Any help would be greatly appreciated.

 

 Guy Dunphy





NB See article '32 bit tricks for embedded controllers' in DEC 95 issue of

Circuit Cellar Ink.  Way to get 32 bit addresses in DOS!

It's almost a reply to my question.