Реферат: Making Utilities For MSDOS Essay Research Paper

intentionally in order to get an unfair advantage over its competitors. My

reasoning for this was that Microsoft’s own utilities have never stopped working

with a new DOS version.

To find the magic of “correct” use of DOS internals, I started disassembling

Microsoft’s utilities. First I looked at three DOS external commands, SHARE,

JOIN, and SUBST. All three programs check for exact DOS version number match.

This means that they can work only with one specific version of MS-DOS. This

makes sense, given that these utilities are bundled with MS-DOS and can be

considered to be parts of MS-DOS. One of the utilities, SHARE, unlike other DOS

external commands, accesses the DOS kernel variables by absolute offsets in

DOSGROUP, the DOS kernel data segment, in addition to getting pointers to

certain DOS internal structures and accessing their fields. SHARE not only

checks the MS-DOS version number, but also checks the flag at offset 4 in

DOSGROUP. In DOS Internals, Geoff Chappell says that this flag indicates the

format (or style) of DOSGROUP layout (501). If you look at the MS-DOS source

code (I’ll explain how to do it in a few paragraphs), you’ll see that programs

like SHARE access the kernel variables in the following way:

The kernel modules defining these variables in DOSGROUP are linked in with

SHARE’s own modules. Since the assembler always works the same way, the DOS

kernel variables get the same offsets in the SHARE’s copy of DOSGROUP as in the

DOS kernel’s copy. When SHARE needs to access a DOS kernel variable, it loads

the real DOSGROUP segment into a segment register, tells the assembler that the

segment register points to SHARE’s own copy of DOSGROUP, and accesses the

variable through that segment register. Although the segment register points to

one copy of DOSGROUP and assembler thinks that it points to another one,

everything works correctly because they have the same format. The reader can

drawn the following conclusion from this aside: MS-DOS designers have made the

MS-DOS internal structures accessible to other programs only for DOS’own use

(since linking DOS modules in with a program is acceptable only for the parts of