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Breaks the 6845 bus cycle into a phase 1 and a phase 2 per the belief that sync line changes, which are observable, happen at the end of the first phase rather than at the beginning of the next. This may have interrupt timing effects, as machines often derive an interrupt from sync.

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This commit is contained in:
Thomas Harte 2017-08-26 13:56:23 -04:00
parent 6e99169348
commit 28550c0227
1 changed files with 24 additions and 6 deletions
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30
Components/6845/CRTC6845.hpp
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@ -28,7 +28,19 @@ struct BusState {
class BusHandler {
public:
void perform_bus_cycle(const BusState &) {}
/*!
Performs the first phase of a 6845 bus cycle; this is the phase in which it is intended that
systems using the 6845 respect the bus state and produce pixels, sync or whatever they require.
*/
void perform_bus_cycle_phase1(const BusState &) {}
/*!
Performs the second phase of a 6845 bus cycle. Some bus state — including sync — is updated
directly after phase 1 and hence is visible to an observer during phase 2. Handlers may therefore
implement @c perform_bus_cycle_phase2 to be notified of the availability of that state without
having to wait until the next cycle has begun.
*/
void perform_bus_cycle_phase2(const BusState &) {}
};
enum Personality {
@ -94,8 +106,8 @@ template <class T> class CRTC6845 {
end_of_line_address_ = bus_state_.refresh_address;
}
perform_bus_cycle();
bus_state_.refresh_address++;
perform_bus_cycle_phase1();
bus_state_.refresh_address = (bus_state_.refresh_address + 1) & 0x3fff;
// check for end-of-line
if(character_counter_ == registers_[0]) {
@ -106,6 +118,8 @@ template <class T> class CRTC6845 {
// increment counter
character_counter_++;
}
perform_bus_cycle_phase2();
}
}
@ -114,10 +128,14 @@ template <class T> class CRTC6845 {
}
private:
inline void perform_bus_cycle() {
inline void perform_bus_cycle_phase1() {
bus_state_.display_enable = character_is_visible_ && line_is_visible_;
bus_state_.refresh_address &= 0x3fff;
bus_handler_.perform_bus_cycle(bus_state_);
bus_handler_.perform_bus_cycle_phase1(bus_state_);
}
inline void perform_bus_cycle_phase2() {
bus_state_.display_enable = character_is_visible_ && line_is_visible_;
bus_handler_.perform_bus_cycle_phase2(bus_state_);
}
inline void do_end_of_line() {