On Fri, Aug 27, 1999 at 06:29:08PM +0100, Thomas Harte wrote: > First : are there any speed differences between the graphics modes? I > understand that Mode 1 is slowed down deliberately to loosely approximate a ZX > Spectrum, but are there any other differences in other graphics modes?
Not really. Apparently the difference between mode 1 and any other mode is that the memory is contended as if all 312 lines were screen lines instead of there being 120 off-screen lines and 192 on-screen ones. > Third : is whichever page of memory which doesn't contain any of the > screen > still contended? Am I right in thinking that the only effect of contention is > to frequently round t-states up to the nearest 4? No. :-) All 32 RAM pages are contended whichever one the screen happens to be in. ROM and external RAM packs are not contended. If the screen is turned off then the CPU has to wait until the next multiple of 4 T-states for any memory access in an affected RAM page. This often has the effect of rounding the instruction timing up to the next multiple of 4. If the screen is on then for 256 out of the 384 pixels on a line for 192 of the 312 lines it works in multiples of 8 instead of 4. This doubles the time for memory-intensive operations; some instructions take less than double. "INC HL" is a rare example of an instruction that takes the same amount of time (8 cycles) whether or not the screen is active. imc
