28 Jan
2008
28 Jan
'08
2:19 p.m.
On Mon, 28 Jan 2008, Jim Leonard wrote:
I guess what I'm asking is something like,
"Is it worth attempting to
support 'MS-DOS only' machines made before Compaq, or were they just too
goofy and limited to bother with?" Maybe a follow-up question is, "How
many MS-DOS-only machines were made before 99.9% IBM PC compatibility
became the norm after Compaq's example?"
PS: I'm on a deadline for this project, which is why I care about
wasting time trying to support four different ways of doing the same
thing (like screen access, for example: 1. ansi.sys, 2. DOS calls, 3.
BIOS calls, 4. direct screen writes).
ansi.sys is NOT a good approach unless you control the boot, since a
significant percentage of users did NOT enable it in CONFIG.SYS
DOS calls will work for ANYTHING running MS-DOS, including grossly
incompatible machines, such as Victor 9000 (Sirius)
BIOS calls will work for anything claiming PC compatability (and some
others), but can not be counted on for machines that don't claim to be
compatible.
Some minor glitches may still show up. For example, the Corona and the PC
CGA had different ideas what to do about the intensity attribute bit for
reverse video.
Direct screen writes will only work for machines claiming PC compatability
("99 44/100% copatible")
But sometimes you can work around it. For example, Toshiba 300 will work
with DOS and BIOS calls, but their "CGA" video memory was at B000h (the
normal location for MDA). But THAT was the only change needed to get
PC-WRITE to run on them.
Even "100% compatible" machines had a few discrepancies. For example, it
wasn't until about 1984 that IBM documented that they used the second to
last byte of memory for a code identifying whether a machine was PC/XT v
PC/AT. So, even Compaq failed that.
Any code that presupposes the presence of the BASIC ROMs will fail on
EVERYTHING except IBM.
In approximately January 1982? PC-WORLD had a comparison of "compatibles".
As expected, the version of XenoCopy that was hard coded to only work on
the IBM PC (as part of publisher's attempt to peddle it to IBM) would only
work on the IBM PC, and they dubbed it "the acid test". (I then used that
as the subtitle for XenoPhobe, which was a crude compatability tester that
is not still around)
--
Grumpy Ol' Fred cisin at xenosoft.com