On Sat, 5 Aug 2000, Gary Gagnon wrote:
> Hello Joseph
> On 05-Aug-00, you wrote:
>
> > Having been able to buy some more memory recently for my A2000
> > (bringing it to 8 MB RAM), i reconfigured my workbench in 16 colors. I
> > discovered that this made my connection to my ISP fail. (As i was told
> > starting up MiamiInit as a test to identify the origin of my problems)
> >
> > Miami complains about my foremost screen only. I often use Dopus on an
> > other screen in 4 colors.
I presume you mean MiamiInit when you say "complains". It sounds like
MiamiInit implements a "rule of thumb" to warn you, since this is such a
common source of trouble.
Only the frontmost screen matters, since that what determines the display
mode.
> > Is the idea valid at all: will i experience communications problems
> > when i return to my 16 colors WB, after having started miami on its 4
> > colors screen. I don't understand really why Miami has this problem at
> > all, since i can browse on an other screen in 16 colors easily.
>
> Without a video card, the more colors you use, the less time the CPU
> will have available to handle the serial device.
>
> Direct Memory Access (DMA) is used by both the video and the serial
> but the video has priority so the serial suffers and characters get
> dropped. The more colors used, the more DMA required by the video.
Sort of. The serial port doesn't use DMA (if it did there would be no
problem), but the CPU has to access it over the same bus as the display
DMA. On OCS or ECS machines, 4 colors has very little effect on
performance, 8 colors shows noticeable slowdown in access to CHIP RAM but
still doesn't significantly impact serial performance (assuming you're not
*running* in CHIP RAM), but 16 colors uses 100% of the bandwidth during
the active display time, forcing all CPU accesses to CHIP RAM and custom
chips (including the serial port) to squeeze into the retrace intervals.
Even an 060 machine typically can't run the serial port reliably at higher
than 38.4K with a 16-color OCS/ECS (hires) screen. With AGA you don't get
into bad trouble until 256 colors.
> As it is a builtin hardware shortcoming in all Amigas, the only way
> to get fast serial to work with 8+ color screens is to use either
> an add-on serial card or to use a video display card such as a CGX
> or Picasso. The add-on graphics cards solve the problem by not
> using DMA thus allowing the serial device better Bus access. The
True, but even if they did use DMA, it wouldn't be *custom chip* DMA, and
it would still be OK.
> add-on serial cards solve the problem by not using DMA and also
> having a multi-character buffer (the built-in serial hardware can
> only buffer ONE character).
The add-on cards aren't accessed over the custom chip bus, which helps,
but the system interrupt control registers, which need to be accessed as
part of the interrupt service, still are. Thus the primary improvement is
the added buffering.
> In theory, if the 4-color DOpus screen is the only active screen,
> you should not lose serial data but as soon as a 16-color (or part
> of) gets displayed, serial data will probably get lost.
Yes, even a small portion of a 16-color screen is enough to cause trouble
at high baud rates.
Fred Wright
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