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Update ClockConverter for potential alternative clocks.

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This commit is contained in:
Thomas Harte 2023-01-02 14:59:36 -05:00
parent dc3f8f5e42
commit 475440dc70
2 changed files with 83 additions and 30 deletions
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29
Components/9918/Implementation/9918.cpp
View File

@ -274,24 +274,21 @@ void TMS9918<personality>::run_for(const HalfCycles cycles) {
// ------------------------
// Perform memory accesses.
// ------------------------
#define fetch(function) \
if(final_window != this->clock_converter_.AccessWindowCyclesPerLine) { \
function<true>(first_window, final_window);\
} else {\
function<false>(first_window, final_window);\
#define fetch(function, converter) \
const int first_window = this->clock_converter_.converter(this->write_pointer_.column); \
const int final_window = this->clock_converter_.converter(end_column); \
if(first_window != final_window) break; \
if(final_window != TMSAccessWindowsPerLine) { \
function<true>(first_window, final_window); \
} else { \
function<false>(first_window, final_window); \
}
// Adjust column_ and end_column to the access-window clock before calling
// the mode-applicable fetch function.
const int first_window = this->clock_converter_.to_access_clock(this->write_pointer_.column);
const int final_window = this->clock_converter_.to_access_clock(end_column);
if(first_window != final_window) {
switch(line_buffer.line_mode) {
case LineMode::Text: fetch(this->template fetch_tms_text); break;
case LineMode::Character: fetch(this->template fetch_tms_character); break;
case LineMode::SMS: fetch(this->template fetch_sms); break;
case LineMode::Refresh: fetch(this->template fetch_tms_refresh); break;
}
switch(line_buffer.line_mode) {
case LineMode::Text: { fetch(this->template fetch_tms_text, to_tms_access_clock); } break;
case LineMode::Character: { fetch(this->template fetch_tms_character, to_tms_access_clock); } break;
case LineMode::SMS: { fetch(this->template fetch_sms, to_tms_access_clock); } break;
case LineMode::Refresh: { fetch(this->template fetch_tms_refresh, to_tms_access_clock); } break;
}
#undef fetch

84
Components/9918/Implementation/ClockConverter.hpp
View File

@ -14,6 +14,17 @@
namespace TI {
namespace TMS {
template <Personality personality> constexpr int cycles_per_line() {
switch(personality) {
default: return 342;
case Personality::V9938:
case Personality::V9958: return 1368;
case Personality::MDVDP: return 3420;
}
}
constexpr int TMSAccessWindowsPerLine = 171;
/*!
This implementation of the TMS, etc mediates between three clocks:
@ -47,12 +58,36 @@ template <Personality personality> class ClockConverter {
Given that another @c source external **half-cycles** has occurred,
indicates how many complete internal **cycles** have additionally elapsed
since the last call to @c to_internal.
E.g. for the TMS, @c source will count 456 ticks per line, and the internal clock
runs at 342 ticks per line, so the proper conversion is to multiply by 3/4.
*/
int to_internal(int source) {
// Default behaviour is top apply a multiplication by 3/4.
const int result = source * 3 + cycles_error_;
cycles_error_ = result & 3;
return result >> 2;
switch(personality) {
// Default behaviour is to apply a multiplication by 3/4;
// this is correct for the TMS and Sega VDPs other than the Mega Drive.
default: {
const int result = source * 3 + cycles_error_;
cycles_error_ = result & 3;
return result >> 2;
}
// The two Yamaha chips have an internal clock that is four times
// as fast as the TMS, therefore a stateless translation is possible.
case Personality::V9938:
case Personality::V9958:
return source * 3;
// The Mega Drive runs at 3420 master clocks per line, which is then
// divided by 4 or 5 depending on other state. That's 7 times the
// rate provided to the CPU; given that the input is in half-cycles
// the proper multiplier is therefore 3.5.
case Personality::MDVDP: {
const int result = source * 7 + cycles_error_;
cycles_error_ = result & 1;
return result >> 1;
}
}
}
/*!
@ -61,24 +96,45 @@ template <Personality personality> class ClockConverter {
is discarded.
*/
HalfCycles half_cycles_before_internal_cycles(int internal_cycles) const {
return HalfCycles(
((internal_cycles << 2) + (2 - cycles_error_)) / 3
);
// Logic here correlates with multipliers as per @c to_internal.
switch(personality) {
default:
return HalfCycles(
((internal_cycles << 2) + (2 - cycles_error_)) / 3
);
case Personality::V9938:
case Personality::V9958:
return HalfCycles(internal_cycles / 3);
case Personality::MDVDP:
return HalfCycles(
((internal_cycles << 1) + (1 - cycles_error_)) / 7
);
}
}
/*!
Converts a position in internal cycles to its corresponding position
on the memory-access clock.
on the TMS memory-access clock, i.e. scales down to 171 clocks
per line
*/
static constexpr int to_access_clock(int source) {
return source >> 1;
static constexpr int to_tms_access_clock(int source) {
switch(personality) {
default:
return source >> 1;
case Personality::V9938:
case Personality::V9958:
return source >> 3;
case Personality::MDVDP:
return source / 20;
}
}
/// The number of internal cycles in a single line.
constexpr static int CyclesPerLine = 342;
/// Indicates the number of access-window cycles in a single line.
constexpr static int AccessWindowCyclesPerLine = 171;
constexpr static int CyclesPerLine = cycles_per_line<personality>();
private:
// Holds current residue in conversion from the external to