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Attempts to implement vapour lock bus behaviour.

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This commit is contained in:
Thomas Harte 2018-05-13 18:53:32 -04:00
parent b9b6327707
commit dea9892a85
2 changed files with 66 additions and 0 deletions
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17
Machines/AppleII/AppleII.cpp
View File

@ -193,6 +193,23 @@ class ConcreteMachine:
if(isReadOperation(operation)) *value = block->read_pointer[address];
else if(block->write_pointer) block->write_pointer[address] = *value;
} else {
// Assume a vapour read unless it turns out otherwise; this is a little
// wasteful but works for now.
//
// Longer version: like many other machines, when the Apple II reads from
// an address at which no hardware loads the data bus, through a process of
// practical analogue effects it'll end up receiving whatever was last on
// the bus. Which will always be whatever the video circuit fetched because
// that fetches in between every instruction.
//
// So this code assumes that'll happen unless it later determines that it
// doesn't. The call into the video isn't free because it's a just-in-time
// actor, but this will actually be the result most of the time so it's not
// too terrible.
if(isReadOperation(operation) && address != 0xc000) {
*value = video_->get_last_read_value(cycles_since_video_update_);
}
switch(address) {
default:
if(isReadOperation(operation)) {

49
Machines/AppleII/Video.hpp
View File

@ -209,7 +209,56 @@ template <class BusHandler> class Video: public VideoBase {
}
}
/*!
Obtains the last value the video read prior to time now+offset.
*/
uint8_t get_last_read_value(Cycles offset) {
// Rules of generation:
// (1) a complete sixty-five-cycle scan line consists of sixty-five consecutive bytes of
// display buffer memory that starts twenty-five bytes prior to the actual data to be displayed.
// (2) During VBL the data acts just as if it were starting a whole new frame from the beginning, but
// it never finishes this pseudo-frame. After getting one third of the way through the frame (to
// scan line $3F), it suddenly repeats the previous six scan lines ($3A through $3F) before aborting
// to begin the next true frame.
//
// Source: Have an Apple Split by Bob Bishop; http://rich12345.tripod.com/aiivideo/softalk.html
// Determine column at offset.
int mapped_column = column_ + offset.as_int();
// Map that backwards from the internal pixels-at-start generation to pixels-at-end
// (so what was column 0 is now column 25).
mapped_column += 25;
// Apply carry into the row counter.
int mapped_row = row_ + (mapped_column / 65);
mapped_column %= 65;
mapped_row %= 262;
// Apple out-of-bounds row logic.
if(mapped_row >= 256) {
mapped_row = 0x3a + (mapped_row&255);
} else {
mapped_row %= 192;
}
// Calculate the address and return the value.
uint16_t read_address = static_cast<uint16_t>(get_row_address(mapped_row) + mapped_column - 25);
return bus_handler_.perform_read(read_address);
}
private:
uint16_t get_row_address(int row) {
const int character_row = row >> 3;
const int pixel_row = row & 7;
const uint16_t row_address = static_cast<uint16_t>((character_row >> 3) * 40 + ((character_row&7) << 7));
GraphicsMode pixel_mode = ((!mixed_mode_ || row < 160) && use_graphics_mode_) ? graphics_mode_ : GraphicsMode::Text;
return (pixel_mode == GraphicsMode::HighRes) ?
static_cast<uint16_t>(((video_page_+1) * 0x2000) + row_address + ((pixel_row&7) << 10)) :
static_cast<uint16_t>(((video_page_+1) * 0x400) + row_address);
}
const int flash_length = 8406;
BusHandler &bus_handler_;
};