[TUHS] screen editors / machine load
Christopher Browne
cbbrowne at gmail.com
Sat Jan 11 10:30:30 AEST 2020
On Fri, 10 Jan 2020 at 17:19, Adam Thornton <athornton at 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.
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