On Tue, 14 Aug 2007 09:35:43 +0200, Palenicek Jan <[EMAIL PROTECTED]> wrote:
> Thanks Marcel! > >> I'm no hardware guy, but I'm pretty sure the ULA locks ALL internal >> memory during screen refresh, i.e. depending on model (EU/US) 50 or 60 >> times per second. The CPU cannot access anything at all during these >> times. > > OK, I will measure read/write time in the vram anyway. ULA should > somehow share bus with CPU, it should not lock memory the whole TV frame > refresh. The actual sharing is quite complex and effectively slows down the CPU access buy some 40% or so, unless there is shadowing involved, like with the GC and SGC. The ULA actually requests the bus only during the active display portion of the whole screen raster generated. The total raster generated is 640x312 of which 512x256 is used, so the ULA requests the bus about 66% of each frame (one frame takes 20ms). During this time the ULA reads screen RAm bytes in bursts of 4 bytes each, to it's internal buffer. It uses page mode DRAM access to do so, otherwise it would take approximately twice the time. Depending on the synch between the CU bus cycles and the ULA timing, at best the CPU can get 50% of available memory bandwidth, and usually less. When all is said and done, you come out with about 50-60% bandwidth long term. It shoul dbe mnoted that this is true for the whole 128k of internal RAM, even though the screen memory cannod reside in the second 64k - the ULA logic must have been a really tight squeeze because it would have been very easy to add a screen 2 and 3... >> Some memory expansions like the Trump Card are not as limited, you >> do not get any delays accessing the extended RAM there. Most RAM expansions (the notable exception being some schemes which used internal piggy-back RAM expansion also driven by the ULA) are considerably faster. Expanderam/TrumpCard are not as fast as they could be but a great deal faster than the ULA addressed RAM. SuperQboard was known as the fastest, running at the full speed of the CPU. Best, N _______________________________________________ QL-Users Mailing List http://www.q-v-d.demon.co.uk/smsqe.htm
