Re:Windows 3.x looks for 3 occurrences of “CON” 59 bytes apart
by userbinator on Saturday June 29, 2019 @09:29PM
(#20315845)
Is there maybe a copy of this text on a plain webpage anywhere that I can read instead?
Unfortunately not; this seems to be one of those obscure and unusual facts that may or may not have been well-known 26 years ago, and something I couldn't find a reference to anywhere else on the Internet (but that may be due to other things[1].) I've reproduced the interesting part here:
The size of an SFT entry is fixed for every DOS version up to 5.0, but Windows attempts to determine the SFT size by observation. It scans conventional memory seeking three evenly spaced occurrences of the string "CON" and takes the distance between them to be the size of one SFT entry. Since DOS itself opens three consecutive SFT entries for the CON device, this technique usually works. But if a utility program moves that portion of the SFT to upper memory (memory above 640K and below 1MB), Windows won't find it.
HIFILES.BAT overcomes this problem by placing a trio of ersatz SFT entries in conventional memory, where Windows can find them. All that's important is that the characters C, O, N, and space appear three times, starting 59 bytes apart.
Unfortunately not; this seems to be one of those obscure and unusual facts that may or may not have been well-known 26 years ago, and something I couldn't find a reference to anywhere else on the Internet (but that may be due to other things[1].) I've reproduced the interesting part here:
The size of an SFT entry is fixed for every DOS version up to 5.0, but Windows attempts to determine the SFT size by observation. It scans conventional memory seeking three evenly spaced occurrences of the string "CON" and takes the distance between them to be the size of one SFT entry. Since DOS itself opens three consecutive SFT entries for the CON device, this technique usually works. But if a utility program moves that portion of the SFT to upper memory (memory above 640K and below 1MB), Windows won't find it.
HIFILES.BAT overcomes this problem by placing a trio of ersatz SFT entries in conventional memory, where Windows can find them. All that's important is that the characters C, O, N, and space appear three times, starting 59 bytes apart.
Here are some images that may be clearer:
https://imgur.com/a/isxaP9h
(The article starts one page earlier, at "Simulating SFT Entries".)
[1] https://news.ycombinator.com/item?id=16153840