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Clarify and simplify half_cycles_before_internal_cycles.

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Thomas Harte 2023-01-09 22:55:46 -05:00
parent fd14829992
commit f1f16d1f9a
1 changed files with 18 additions and 34 deletions
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52
Components/9918/Implementation/ClockConverter.hpp
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@ -96,31 +96,9 @@ template <Personality personality> struct StandardTiming {
template <Personality personality> struct Timing: public StandardTiming<personality> {};
/*!
This implementation of the TMS, etc mediates between three clocks:
1) the external clock, which is whatever the rest of the system(s)
it plugs into run at;
2) the internal clock, which is used to time and place syncs, borders,
pixel regions, etc; and
3) a memory acccess clock, which correlates to the number of windows
available for memory accesses.
E.g. for both a regular TMS9918 and the Sega Master System, the external
clock is 3.58Mhz, the internal clock is 5.37Mhz and the memory access
clock is 2.69Mhz.
Or, put another way, for both a TMS9918 and Master System:
* 228 external cycles;
* is 342 internal cycles;
* which exactly covers 228 NTSC colour clocks; and
* contains 171 memory access windows.
Both the Yamaha extensions and the Mega Drive VDP are a bit smarter about
paged mode memory accesses, obviating any advantage to treating (3) as a
separate clock.
Provides a [potentially-]stateful conversion between the external and internal clocks.
Unlike the other clock conversions, this one may be non-integral, requiring that
an error term be tracked.
*/
template <Personality personality> class ClockConverter {
public:
@ -161,26 +139,32 @@ template <Personality personality> class ClockConverter {
}
/*!
Provides the number of complete external cycles that lie between now and
@c internal_cycles into the future. Any trailing fractional external cycle
is discarded.
Provides the number of external cycles that need to begin from now in order to
get at least @c internal_cycles into the future.
*/
HalfCycles half_cycles_before_internal_cycles(int internal_cycles) const {
// Logic here correlates with multipliers as per @c to_internal.
switch(personality) {
default:
return HalfCycles(
((internal_cycles << 2) + (2 - cycles_error_)) / 3
);
// Relative to the external clock multiplied by 3, it will definitely take this
// many cycles to complete a further (internal_cycles - 1) after the current one.
internal_cycles = (internal_cycles - 1) << 2;
// It will also be necessary to complete the current one.
internal_cycles += 4 - cycles_error_;
// Round up to get the first external cycle after
// the number of internal_cycles has elapsed.
return HalfCycles((internal_cycles + 2) / 3);
case Personality::V9938:
case Personality::V9958:
return HalfCycles((internal_cycles + 2) / 3);
case Personality::MDVDP:
return HalfCycles(
((internal_cycles << 1) + (1 - cycles_error_)) / 7
);
internal_cycles = (internal_cycles - 1) << 1;
internal_cycles += 2 - cycles_error_;
return HalfCycles((internal_cycles + 6) / 7);
}
}