Re: 6551 driver questions

From: Andre Fachat (
Date: 2003-08-24 11:35:58

Hi Ullrich,

Assuming your knowledge I would say you write a driver for an existing 
deivce, rihgt? Otherwise I would like to be the badass and say, why not
use something decent instead of the 6551...... (like 16550)
At least I would for any new design.

IIRC I never used the ACIA with the NMI, only with the IRQ.
But even then I had- with my GeckOS - latency problems when
receiving bytes, i.e. due to the missing buffer the CPU was not
quick enough draining the received bytes, so they were lost.


On Sat, Aug 23, 2003 at 04:16:29PM +0200, Ullrich von Bassewitz wrote:
> I'm currently redesigning a 6551 driver and have some questions.
> Another question: I think it would be ok to accept a 19200 baud limit for the
> C64. Would it be possible to use the IRQ line instead of the NMI at this
> speed? 19200 baud means a receive interrupt is generated roughly every 500us.
> How long does the C64 ROM interrupt handler take to execute? Does the C64 ROM
> contain code blocks that run with interrupts disabled for more than 500
> cycles? Using the IRQ line would have the advantage of not interfering with
> the disk routines (doing disk I/O with NMIs in the background is impossible).
> Regards
>         Uz
> -- 
> Ullrich von Bassewitz                        
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