Solved What's written to disk during DOS boot?

March 20, 2015 at 00:55:27
Specs: Win98 DOS
In an embedded environment, I'd like DOS to run in a way that there is absolutely nothing written back to the disk it's running from. With setting the right flags in MSDOS.SYS etc., boot logging, disk scanning etc. can be disabled. So far, so good!
Unfortunately, I find that every single time I boot, the DOS image slightly changes. Specifically, on the entire disk, 2 bytes change!
My DOS is Win98 based and it only has the bare essentials to run with the sole purpose of running a script file when booting up.
Does anybody know why the DOS disk image changes during the boot? Is this some kind of boot counter or...? Can it be disabled?
If I use a hardware write-protected disk, there are no complaints, DOS runs OK and the disk image does not change. Unfortunately, I don't have the luxury of changing the disks to write protected, in my situation.
Any advice would be appreciated! Unfortunately, any search terms for this problem are very generic and I couldn't find anything and DOS boot descriptions do not go down to this level of detail.

message edited by xelA


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#1
March 20, 2015 at 01:21:13
To add a bit more context, the changing bytes are on the OS partition, not the bootblock. All files and directories have the read-only flag set.
Looking at the section with a hex editor (HxD), this is the context (in ASCII). It's a bit jibberishy but maybe the structure of it looks familiar to those in the know:
Case 1:
CONFIG SYS'.SÆtU9¢(..X`sFÊUk...åUTOEXECBAT .ÅïtU9ÄD..a\…Dÿ.V...
Case 2:
CONFIG SYS'.SÆtU9¡:..X`sFÊUk...åUTOEXECBAT .ÅïtU9ÄD..a\…Dÿ.V...
Notice the small difference not much long after the "CONFIG SYS" string.

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#2
March 20, 2015 at 01:53:41
✔ Best Answer
That's the Last Accessed Date. Check CONFIG.SYS for ACCDATE entries. This field doesn't exist in earlier versions of DOS.

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#3
March 20, 2015 at 05:10:19
Why do you use a hacked unofficial version of DOS?

Install instead FreeDOS, the open source free clone of MS DOS, that supports FAT32 partitions and is more suited to run on modern computers.


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#4
March 20, 2015 at 05:57:20
What are you talking about? MS-DOS version 7.1 was supplied with Windows 98. It's not hacked and is more "official" than FreeDOS.

(PS. And it supports FAT 32!)

message edited by ijack


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#5
March 20, 2015 at 06:45:30
DOS 7.1 was never released as a standalone system, just embedded as Windows 98 kernel; in that sense I talk about "hacked" and "unofficial".

The last version of MS DOS as self contained operating system is 6.22, released, as I am sure you perfectly know, in the spring of 1994. FreeDOS with its reach set of tools and drivers is an effective replacement for MS DOS family in full respect of license policies.


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#6
March 20, 2015 at 08:30:11
To call a Microsoft version of MS-DOS that was included as part of Windows 98 "unofficial" is just perverse. Just because it wasn't sold separately doesn't make it any less official and doesn't mean that it is in any sense "hacked". After all, FreeDOS was never sold separately. ;)

MS-DOS 7.1 is as "official" as Internet Explorer. That's more than can be said of FreeDOS, which could certainly accurately be described as a hacked, unofficial version.

The last version of DOS that was sold as a standalone product was not MS-DOS 6.22 (June 1994), but IBM's PC-DOS 7.0 (April 1995). Anyway, I don't think this off-topic digression is helping the OP with his question, which I answered in my first post.

message edited by ijack


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#7
March 22, 2015 at 16:22:02
All posts were very helpful - thanks IVO and ijack!
It's useful to know what was changing and I may also test FreeDOS as a possible alternative. I was using the Win98 version because... that has already been successfully used for the project and the whole thing is entangled with some secure booting issues.
The script I mentioned was for field upgrade, not run-time usage, so it is not critical what I use, as long as it does the deed.

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#8
March 23, 2015 at 00:21:46
I had a look at FreeDOS and I'm sorry but I have to side with ijack on this one. FreeDOS is the hack - be it a very clever one! ;-)
FreeDOS seems great if you want an all-singing-dancing DOS but if you just want the bare essentials, maybe not so much. If you are simply after the usual command.com and io.sys and a small number of config files with familiar options, you'll be in for a surprise. It's like you've asked for a slice of bread and you got a whole Sacher torte instead...

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#9
March 23, 2015 at 04:05:58
In the real world black is not black and white is not white, what rules is the scale of grays. So what I and ijack talk about DOS versions is much like your opinion about a movie: you may love or hate it. both opinions are true and false at the same time (Quantum Physics allows that!).

Many device producers, e.g. motherboards or hard disks, use a minimum FreeDOS system to package their hardware utilities to be executed standalone. So they respect Microsoft 's license... and to get a plain slice of bread just extract Kernel.sys and Command.com, any other module is unnecessary to boot FreeDOS.


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