Yes, stealth viruses did report memory without their space, by monitoring the read, and returning the "clean" value instead. Boot sector viruses couldn't do an

Yes, stealth viruses did report memory without their space, by monitoring the read, and returning the "clean" value instead. Boot sector viruses couldn't do anything like that because they load and run before DOS enters the picture. So that was one of the things my boot sector looked for. File infectors had a number of tricks at their disposal, one of which you just mentioned. Other methods are still similar, just different means to that end.