How compatible were "MS-DOS Compatible" machines?

[Tony Duell]

> 
> Fred Cisin wrote:
> > DOS calls will work for ANYTHING running MS-DOS, including grossly
> > incompatible machines, such as Victor 9000 (Sirius)
> 
> Since one of my requirements is manipulating the PC speaker to sound an 
> arbitrary tone of arbitrary duration -- something that is not provided 
> by the BIOS or MS-DOS -- should I worry about "grossly incompatible" 
> machines?  For example, does the Victor 9000 even *have* a speaker (or 
> 8253 timer?)?

The Victor 9000 (Sirius over here) does have a speaker. But it's not 
driven by an 8253 (or 8354), in fact there is no such chip in the machine.

It's driven from a PCM codec chip. The bitstream for that comes from a 
6852 (synchronous serial chip), the clock comes from one of the 
counter/timers (line PB7???) of the 'User' VIA. Yes, the input side is 
connected up as well, there's a pin header on the mainboard that can be 
used to feed audio into the codec, and the bitstream from that goes to 
the 6852.

Please don't ask me how to program that to make beeps ;-)

> That was also an issue with the AT&T PC 6300/Olivetti M24 if you were 
> using a monochrome monitor.  It was monochrome CGA, but the monitor 
> would do things like _underline_ the text based on the attribute.

That, IIRC, is what a real _MDA_ card does.

-tony