From dea9892a858618f7030ee8f43bb55017eab95076 Mon Sep 17 00:00:00 2001 From: Thomas Harte Date: Sun, 13 May 2018 18:53:32 -0400 Subject: [PATCH] Attempts to implement vapour lock bus behaviour. --- Machines/AppleII/AppleII.cpp | 17 +++++++++++++ Machines/AppleII/Video.hpp | 49 ++++++++++++++++++++++++++++++++++++ 2 files changed, 66 insertions(+) diff --git a/Machines/AppleII/AppleII.cpp b/Machines/AppleII/AppleII.cpp index cbc4a9100..7529c74f5 100644 --- a/Machines/AppleII/AppleII.cpp +++ b/Machines/AppleII/AppleII.cpp @@ -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)) { diff --git a/Machines/AppleII/Video.hpp b/Machines/AppleII/Video.hpp index 5e7ddba49..1e23d1f30 100644 --- a/Machines/AppleII/Video.hpp +++ b/Machines/AppleII/Video.hpp @@ -209,7 +209,56 @@ template 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(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((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(((video_page_+1) * 0x2000) + row_address + ((pixel_row&7) << 10)) : + static_cast(((video_page_+1) * 0x400) + row_address); + } + const int flash_length = 8406; BusHandler &bus_handler_; };