On Sat, Apr 10, 1999 at 02:58:35PM +0100, Si Owen wrote: > LD r,r' says 8 t-states, but the docs I have say it's 7. JR c,e takes 8 > t-states if the condition is not met and 12 if it is, but the docs I have > say it's 7 and 13 respectively. etc.
> Could someone explain the 4 t-state rounding? (machine cycles?) I could > understand it being rounded up, but the conditional relative jump is > slightly shorter. I've always done video effects with official timings, and > manual tweaks if necessary! For the moment, assume that the screen is turned off or it happens to be displaying the border. At this time, all accesses to main RAM are restricted by the ASIC to occur only on every fourth cycle. (This does not apply to external RAM or to ROM). The usual effect of this is to round instructions up to the next multiple of 4, because each instruction starts with an opcode fetch and thus has to wait until the next "fourth" cycle. So, for example, if INC HL is executed then it takes 6 cycles of its own plus 2 cycles waiting for the next instruction fetch for a total of 8 cycles. Some instructions involve more than one memory access. In these cases the instruction timing depends on when exactly the Z80 requests the memory access. In the simplest cases of LD A,n, LD HL,nn and so on the accesses follow on from each other and so the time taken is just four cycles for each access. However, for PUSH HL I believe it goes something like this opcode is fetched 4 cycles SP register is decremented 1 cycle Z80 must wait for the next access 3 cycles H is stored at (SP) 3 cycles SP is decremented in parallel with the store so no cycles Z80 must wait for the next access 1 cycle L is stored at (SP) 3 cycles Z80 must wait for the next instruction fetch 1 cycle making 16 cycles in total. As for the JR instruction, you remember wrongly. Docs state that JR takes 12 cycles if the jump is taken, and that is still true. For DJNZ docs state that it takes 13 cycles if the jump is taken, and Sam rounds that up to 16. The Sam never takes less time than the docs say. Now, if the screen is turned on then it is slightly more complicated. The screen is displayed for 256 of every 384 cycles on each line, for 192 of the 312 lines (though this is different in mode 1). During that time memory access is restricted to every eighth cycle, so most instructions take twice as long. INC HL still only takes 8 cycles, though, since it doesn't require memory access once the instruction has been fetched. On the other hand, if a program is running in ROM then the Z80 doesn't have to wait for instruction fetches, so it runs faster. It does have to wait for each instruction which accesses the RAM, however. imc
