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Corrects Electron Mode 3 timing.

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Thomas Harte 2017-11-23 16:06:05 -05:00
parent 1d821ad459
commit c7dd6247f0
1 changed files with 23 additions and 4 deletions
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27
Machines/Electron/Video.cpp
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@ -376,18 +376,37 @@ VideoOutput::Interrupt VideoOutput::get_next_interrupt() {
unsigned int VideoOutput::get_cycles_until_next_ram_availability(int from_time) {
unsigned int result = 0;
int position = output_position_ + from_time;
int position = (output_position_ + from_time) % cycles_per_frame;
// Apply the standard cost of aligning to the available 1Mhz of RAM bandwidth.
result += 1 + (position&1);
// In Modes 0–3 there is also a complete block on any access while pixels are being fetched.
if(screen_mode_ < 4) {
const int current_column = graphics_column(position + (position&1));
int current_line = graphics_line(position);
if(current_column < 80 && current_line < 256) {
// Mode 3 is a further special case: in 'every ten line block', the final two aren't painted,
// so the CPU is allowed access. But the offset of the ten-line blocks depends on when the
// user switched into Mode 3, so that needs to be calculated relative to current output.
if(screen_mode_ == 3) {
// Get the line the display was on.
int output_position_line = graphics_line(output_position_);
int implied_row = current_character_row_ + (current_line - output_position_line) % 10;
if(implied_row < 8)
result += static_cast<unsigned int>(80 - current_column);
int implied_row;
if(current_line >= output_position_line) {
// Get the number of lines since then if still in the same frame.
int lines_since_output_position = current_line - output_position_line;
// Therefore get the character row at the proposed time, modulo 10.
implied_row = (current_character_row_ + lines_since_output_position) % 10;
} else {
// If the frame has rolled over, the implied row is just related to the current line.
implied_row = current_line % 10;
}
// Mode 3 ends after 250 lines, not the usual 256.
if(implied_row < 8 && current_line < 250) result += static_cast<unsigned int>(80 - current_column);
}
else result += static_cast<unsigned int>(80 - current_column);
}