Реферат: Making Utilities For MSDOS Essay Research Paper

Having seen that DOS external commands are not a good example for a program that

wants to be compatible with all DOS versions, I turned to Microsoft Network. One

of its utilities, REDIR, is very similar to SHARE in its operation. Like SHARE,

it accesses the DOS kernel variables by absolute offsets. I thought that unlike

SHARE, REDIR is not tied to a specific DOS version. Unfortunatelly, I wasn’t

able to disassemble it, because as a high school student, I don’t have a copy of

Microsoft Network. However, Geoff Chappell says that it has separate versions

for different versions of DOS, just like SHARE. Therefore, I turned to another

utility again.

My next stop was MSCDEX, the utility for accessing the High Sierra and ISO-9660

file systems used by CD-ROMs. Unlike SHARE and REDIR, MSCDEX is not tied to one

specific DOS version. I’m using MSCDEX version 2.21 with MS-DOS version 5.00,

but the same version of MSCDEX can be used with PC-DOS version 3.30. However, it

accesses the DOS kernel variables by absolute offsets in DOSGROUP, just like

SHARE and REDIR. Of course, my question was “How does it do that in a version-

independent manner?” When I disassembled it, I saw that it takes the flag at

offset 4 in DOSGROUP and uses it to determine the absolute offsets of all the

variables it needs. If this flag equals 0, MSCDEX assumes that all offsets it’s

interested in are the same as in DOS versions 3.10-3.30. If this flag equals 1,

MSCDEX assumes that all offsets it’s interested in are the same as in DOS

versions 4.00-5.00. For all other values of this flag MSCDEX refuses to load.

Sharp-eyed readers might notice that this check already makes MSCDEX potentially

incompatible with future DOS versions. The comments in the source code for MS-

DOS version 3.30 (DOS\MULT.INC file) refer to MSCDEX, therefore, it had existed

at the time of MS-DOS version 3.30. It is very doubtful that anyone, including

the author of MSCDEX, could know what offsets would the kernel variables in DOS

version 4.00 have at that time. If this is true, an MSCDEX version that predates

MS-DOS version 4.00 won’t run under DOS versions 4.00 and later.

MSCDEX uses the flag at offset 4 in DOSGROUP to determine not only the absolute