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Merge pull request #229 from TomHarte/Skew

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Adds an initial implementation of display skew to the 6845
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Thomas Harte 2017-08-29 22:32:26 -04:00 committed by GitHub
commit c88d627b4e
1 changed files with 16 additions and 2 deletions
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18
Components/6845/CRTC6845.hpp
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@ -45,6 +45,7 @@ class BusHandler {
enum Personality {
HD6845S, // Type 0 in CPC parlance. Zero-width HSYNC available, no status, programmable VSYNC length.
// Considered exactly identical to the UM6845, so this enum covers both.
UM6845R, // Type 1 in CPC parlance. Status register, fixed-length VSYNC.
MC6845, // Type 2. No status register, fixed-length VSYNC, no zero-length HSYNC.
AMS40226 // Type 3. Status is get register, fixed-length VSYNC, no zero-length HSYNC.
@ -86,6 +87,15 @@ template <class T> class CRTC6845 {
0xff, 0x1f, 0x7f, 0x1f, 0x3f, 0xff, 0x3f, 0xff
};
// Per CPC documentation, skew doesn't work on a "type 1 or 2", i.e. an MC6845 or a UM6845R.
if(selected_register_ == 8 && personality_ != UM6845R && personality_ != MC6845) {
switch((value >> 4)&3) {
default: display_skew_mask_ = 1; break;
case 1: display_skew_mask_ = 2; break;
case 2: display_skew_mask_ = 4; break;
}
}
if(selected_register_ < 16) {
registers_[selected_register_] = value & masks[selected_register_];
}
@ -156,12 +166,13 @@ template <class T> class CRTC6845 {
private:
inline void perform_bus_cycle_phase1() {
bus_state_.display_enable = character_is_visible_ && line_is_visible_;
// Skew theory of operation: keep a history of the last three states, and apply whichever is selected.
character_is_visible_shifter_ = (character_is_visible_shifter_ << 1) | static_cast<int>(character_is_visible_);
bus_state_.display_enable = (character_is_visible_shifter_ & display_skew_mask_) && line_is_visible_;
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_);
}
@ -253,6 +264,9 @@ template <class T> class CRTC6845 {
uint16_t line_address_;
uint16_t end_of_line_address_;
uint8_t status_;
int display_skew_mask_ = 1;
int character_is_visible_shifter_ = 0;
};
}