is this calculation correct? \ VOGONS

is this calculation correct?

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First post, by noshutdown

Posted on 2013-11-03, 13:57

noshutdown Offline

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an isa bus usually has 16MB/s bandwidth
a picture of 320*200*8bit resolution takes ~64kb in size, while 640*480*8bit and 640*480*16bit takes 300kb and 600kb respectively.
so in theory, an isa video card without 2d acceleration can play 320*200*8bit video at 250fps max, and 640*480*8bit or 16bit at 50 and 25fps.
well i know that the bandwidth of video ram also affects performance, but at least its faster than the isa bus, so it only comes in play when 2d acceleration is supported.

Reply 1 of 1, by idspispopd

Posted on 2013-11-03, 21:33

idspispopd Offline

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IIRC the theoretical maximum is only half of that, 8MB/s at 8MHz.
ISA has no burst mode like PCI which would enable a word transfer every cycle.
You are right about video ram bandwidth being greater than ISA bandwidth, but part of the VRAM bandwidth is needed for screen refresh, and a lot of chipsets are not fast enough to saturate the ISA bus.

Additionally the CPU needs time to calculate the displayed frames. It can't spend 100% of the time for transferring data to the video card.

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