With the basics of boot sectors behind us, let’s explore a
sophisticated boot sector virus that will overcome the rather glaring
limitations of the KILROY virus. Specifically, let’s look at a virus
which will carefully hide itself on both floppy disks and hard disks,
and will infect new disks very efficiently, rather than just at boot
time.
Such a virus will require more than one sector of code, so
we will be faced with hiding multiple sectors on disk and loading
them at boot time. To do this in such a way that no other data on a
disk is destroyed, while keeping those sectors of virus code well
hidden, will require some little known tricks. Additionally, if the
virus is to infect other disks after boot-up, it must leave at least a
portion of itself memory-resident. The mechanism for making the
virus memory resident cannot take advantage of the DOS Keep
function (Function 31H) like typical TSR programs. The virus must
go resident before DOS is even loaded, and it must fool DOS so
DOS doesn’t just write over the virus code when it does get loaded.
sophisticated boot sector virus that will overcome the rather glaring
limitations of the KILROY virus. Specifically, let’s look at a virus
which will carefully hide itself on both floppy disks and hard disks,
and will infect new disks very efficiently, rather than just at boot
time.
Such a virus will require more than one sector of code, so
we will be faced with hiding multiple sectors on disk and loading
them at boot time. To do this in such a way that no other data on a
disk is destroyed, while keeping those sectors of virus code well
hidden, will require some little known tricks. Additionally, if the
virus is to infect other disks after boot-up, it must leave at least a
portion of itself memory-resident. The mechanism for making the
virus memory resident cannot take advantage of the DOS Keep
function (Function 31H) like typical TSR programs. The virus must
go resident before DOS is even loaded, and it must fool DOS so
DOS doesn’t just write over the virus code when it does get loaded.
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