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Explain the purpose here.

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This commit is contained in:
Thomas Harte 2023-01-01 21:20:30 -05:00
parent 5729ece7bb
commit da61909ec5
1 changed files with 28 additions and 8 deletions
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36
Components/9918/Implementation/ClockConverter.hpp
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@ -14,10 +14,30 @@
namespace TI {
namespace TMS {
/*!
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.
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.
*/
template <Personality personality> class ClockConverter {
public:
/*!
Converts a number of **half-cycles** to an internal number
Converts a number of extenral **half-cycles** to an internal number
of **cycles**.
*/
int to_internal(int source) {
@ -28,8 +48,9 @@ template <Personality personality> class ClockConverter {
}
/*!
Provides the number of external cycles that will need to pass in order to advance
_at least_ @c internal_cycles into the future.
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.
*/
HalfCycles half_cycles_before_internal_cycles(int internal_cycles) const {
return HalfCycles(
@ -37,9 +58,9 @@ template <Personality personality> class ClockConverter {
);
}
/*
/*!
Converts a position in internal cycles to its corresponding position
on the access-window clock.
on the memory-access clock.
*/
static constexpr int to_access_clock(int source) {
return source >> 1;
@ -52,9 +73,8 @@ template <Personality personality> class ClockConverter {
constexpr static int AccessWindowCyclesPerLine = 171;
private:
// This implementation of this chip officially accepts a 3.58Mhz clock, but runs
// internally at 5.37Mhz. The following two help to maintain a lossless conversion
// from the one to the other.
// Holds current residue in conversion from the external to
// internal clock.
int cycles_error_ = 0;
};