RE: Upgrade to XyWin or Not?

[Paul Thornett paul@xxxxxxxx]

> I'm coming down with a cold and so my brain may be firing on a mere three
> cylinders or so. But I thought RAM was more usually denominated in
> megabytes (or, of course, bytes, kilobytes, gigabytes, etc.) and that DOS
> was infamous for its limit of one measly megabyte of directly addressable
> RAM. A megabyte is 2^20 bytes; a byte is 2^3 bits--so isn't a megabyte
> 2^23 bits and DOS a "20-bit" (or "23-bit"?) OS?

Not really. The 1 meg limit comes originally from the limitations of the 8088
processor, which could address only 1024*1024 separate locations. Thus this
megabyte was divided into 16 64k segments (remember that a 16-bit system can
directly address only 64k bytes at a time). 6 of those segments were
pre-allocated to various system resources, with the user being granted use of
the remaining 10 (here's everyone's favourite 640k!). Later chips support
linear memory addressing, completely removing these artificial constraints.
But as Dos was designed for the original 8088 chip, its limitations largely
remain.


Paul Thornett