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Support half-clocked mode.

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This commit is contained in:
Thomas Harte 2023-12-06 09:59:14 -05:00
parent 561e50a24d
commit f455f07ba2
1 changed files with 25 additions and 18 deletions
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43
Machines/PCCompatible/CGA.hpp
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@ -32,8 +32,10 @@ class CGA {
// CGA is clocked at the real oscillator rate of 14 times that.
// But there's also an internal divide by 8 to align to the fetch clock.
full_clock_ += 7 * cycles.as<int>();
crtc_.run_for(Cycles(full_clock_ / 4));
full_clock_ %= 4;
const int modulo = 4 * outputter_.clock_divider;
crtc_.run_for(Cycles(full_clock_ / modulo));
full_clock_ %= modulo;
}
template <int address>
@ -95,9 +97,11 @@ class CGA {
control_ = control;
if(control & 0x2) {
mode_ = (control & 0x10) ? Mode::Pixels640 : Mode::Pixels320;
mode_ = Mode::Pixels;
clock_divider = 1 + !(control & 0x10);
} else {
mode_ = (control & 0x01) ? Mode::Text80 : Mode::Text40;
mode_ = Mode::Text;
clock_divider = 1 + !(control & 0x01);
}
}
@ -112,21 +116,19 @@ class CGA {
((state.display_enable && control_&0x08) ? OutputState::Pixels : OutputState::Border);
// Upon either a state change or just having accumulated too much local time...
if(new_state != output_state || count > 882) {
if(new_state != output_state || pixels_divider != clock_divider || count > 912) {
// (1) flush preexisting state.
if(count) {
switch(output_state) {
case OutputState::Sync: crt.output_sync(count); break;
case OutputState::Border: crt.output_blank(count); break;
case OutputState::Pixels:
crt.output_data(count);
pixels = pixel_pointer = nullptr;
break;
case OutputState::Sync: crt.output_sync(count * clock_divider); break;
case OutputState::Border: crt.output_blank(count * clock_divider); break;
case OutputState::Pixels: flush_pixels(); break;
}
}
// (2) adopt new state.
output_state = new_state;
pixels_divider = clock_divider;
count = 0;
}
@ -137,7 +139,7 @@ class CGA {
// Flush any period where pixels weren't recorded due to back pressure.
if(pixels && count) {
crt.output_blank(count);
crt.output_blank(count * clock_divider);
count = 0;
}
}
@ -192,14 +194,17 @@ class CGA {
// Output pixel row prematurely if storage is exhausted.
if(output_state == OutputState::Pixels && pixel_pointer == pixels + DefaultAllocationSize) {
crt.output_data(count);
flush_pixels();
count = 0;
pixels = pixel_pointer = nullptr;
}
}
void perform_bus_cycle_phase2(const Motorola::CRTC::BusState &) {}
void flush_pixels() {
crt.output_data(count * pixels_divider, size_t(count));
pixels = pixel_pointer = nullptr;
}
Outputs::CRT::CRT crt;
enum class OutputState {
@ -209,7 +214,8 @@ class CGA {
uint8_t *pixels = nullptr;
uint8_t *pixel_pointer = nullptr;
static constexpr size_t DefaultAllocationSize = 720;
int pixels_divider = 1;
static constexpr size_t DefaultAllocationSize = 320;
const uint8_t *ram = nullptr;
std::vector<uint8_t> font;
@ -217,8 +223,9 @@ class CGA {
uint8_t control_ = 0;
enum class Mode {
Pixels320, Pixels640, Text80, Text40,
} mode_ = Mode::Text80;
Pixels, Text,
} mode_ = Mode::Text;
int clock_divider = 1;
} outputter_;
Motorola::CRTC::CRTC6845<CRTCOutputter, Motorola::CRTC::CursorType::MDA> crtc_;