At 10:58 pm +0100 27/8/99, Thomas Harte wrote: > First : are there any speed differences between the graphics modes? I
Mode 1 is a special slow case, but modes 2 to 4 should all have the same effect on memory accesses. >understand that Mode 1 is slowed down deliberately to loosely approximate a ZX >Spectrum, but are there any other differences in other graphics modes? How >many t-states do I get per scanline, and are there still 312 of them? There are 312 scanlines of 384 tstates each. (See my answer to your third question). > Second : does altering the palette have any effect in Modes 1 & 2? Yes, exactly as you would expect from having used Mode 4. > Does Mode >3 simply only look at the first 4 palette entries, or is it more complex than >that? Well, yes it is more complicated but the chances are you'll never really need to worry about that.... There's a fairly useless feature involving bits 5 and 6 of the HMPR - by setting these bits, the pixels in mode 3 will be displayed using palette entries 4 to 7, 8 to 11, or 12 to 15 instead of 0 to 3. It is also worth noting that screen bits %10 are displayed as palette entry %01, and %01 is displayed as %10. Mode 3 has always been a bit quirky! > Third : is whichever page of memory which doesn't contain any of >the screen >still contended? No, all RAM pages are affected in the exactly same way, regardless of which page is displayed at the paricular time. > Am I right in thinking that the only effect of contention is >to frequently round t-states up to the nearest 4? Well, no... it also halves the speed of memory accesses during the time that the screen is displayed. In effect nearly all instructions take twice as long to execute during that time, so it is easiest to keep the number of t-states per instruction constant, and consider lines of 256 "usable" t-states. But a few instructions which don't need to access memory will appear to "gain" 4 tstates over normal execution timings. Andrew -- -- Andrew Collier ([EMAIL PROTECTED]) -- My other -- http://mnemotech.ucam.org -- .sig is a -- Part 3 Materials Science, Cambridge -- PDF file --
