[Freedos-devel] re: HIMEM64.EXE

[Eric A. <eric@CoLi.Uni-SB.DE>]

Hi, A20 problems? I may be repeating myself, but I recommend that
XMS memory managers do a TEST whether the A20 switching has worked
every time and do some extra WAITING if not. Some of our memory
managers just have a delay loop, and as AT style switching is slow,
you may have to configure a longer delay. Bad idea, most users will
not think that far. I think that would be worth "wasting" 20 or more
bytes of resident driver size for the sake of secure operation.

(Most suggestions for XMS drivers first have to pass the evil "but
the driver has to be VERY small in memory" barrier... My suggestion
in return: Use JMP SHORT to the exit-from-call code of one-out-of-3
functions and save the space that would be needed for the exit-from-
call code in two-out-of-3 functions that way... I think FDXMS already
does use this trick, but HIMEM does not...)

As far as I know, AT style switching will always work after testing
it once and PS2 style switching will always work after testing it
once, so you only have to decide on switching style (PS2 preferred)
at driver load time. Later only timing should be a problem. DURING
the initial test you (of course) must not wait infinitely for the
switch to work because that may never happen (e.g. PS2 style never
works on very old computers...). Hm. Already wrote too much. You get
the idea anyway :-).


PS: Did you know that FreeDOS is a VIRUS? Well, Trend ChipAway Virus
Guard thinks so. It checksums the boot sector before booting and compares
it to a list of known boot sectors, it seems! Older BIOS virus protection
systems just block the BIOS call "write to sector 0/0/1", so THOSE will
not complain if you wait with enabling them until AFTER SYSing your disk.
In both protection variants, the alarm will be a popup text window with
some "Virus!? Proceed? Y/N" style user interface. EXTRA experience from
that: The KERNEL does not clear the extra lines in video RAM if you select
an higher video mode in config sys! At least the extra lines (e.g. what
the virus guard has backed up before poping up) will scroll into the visible
area later that way (not really scroll, but "jump", when the visible area
suddenly grows by the mode change).

Eric.