On Fri, 10 Jan 2020 at 17:19, Adam Thornton <athornton@gmail.com> wrote:

> A) The original WD UART chip had very limited buffering.   The timing was
> such that as high rates it could not empty accept a second character
> without the first being overwritten.  This was a long-standing issue for
> many UARTs long in the 1990s.  The original chip NS built and IBM used on
> the PC (the NS8250) was notorious for the same problem.  By the time of
> Motorola's 6881, it had 8 characters of buffering IIRC.
>
> Great, now I'm having flashbacks to upgrading my 4-port serial card with
> 16450s and then 16550s in the early 90s.
>

Yep, same sorts of memories here.  I recall the 16450 upgrade being a big
deal for Internet connectivity in that a PC lacking the extra bytes of
buffering in the UART would find that the 80386 was having clock cycles
eaten nearly completely by PPP connections.
It was amazing to realize how a few bytes of memory lurking in a crucial
system interface could affect performance in such dramatic ways.
Tagged command queueing on SCSI controllers had a slightly less dramatic
effect on I/O performance, but again, a few hundred bytes of memory in the
right spot could nevertheless have dramatic effects.