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51 Can a virus hide in a PC's Upper Memory or in High Memory Area?




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This article is from the Computer Viruses FAQ, by Nick FitzGerald n.fitzgerald@csc.canterbury.ac.nz with numerous contributions by others.

51 Can a virus hide in a PC's Upper Memory or in High Memory Area?

Yes, it is possible to construct a virus which will locate itself in
Upper Memory Blocks (UMBs--640K to 1024K) or in the High Memory Area
(HMA--1024K to 1088K). Some viruses (e.g. EDV) do hide in UMBs and at
least one, Goldbug, will use the HMA if it is available.

It might be thought that there is no point in scanning in these areas
for any viruses other than those that are specifically known to inhabit
them. However, there are cases when even ordinary viruses can be found
in Upper Memory. Suppose that a conventional memory-resident virus
infects a TSR program and this program is loaded high by the user (for
instance, from AUTOEXEC.BAT). Then the virus code will also reside in
Upper Memory. Therefore, an effective scanner must be able to scan this
part of memory for viruses too.

 

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