bradgrantham-apple2e/notes.vblank

per-scanline modes
    Actual Apple II TEXT/GR and HGR video address patterns are in Understanding the Apple II
        Not for DHGR or 80-col but they're probably just same address twice alternating banks
    tracking clock and reporting video memory contents
        clock
            one tick of ~1MHz clock maps to one tick of address counter
                so is 7 HGR bits or one GR/TEXT horizontal cell
                "most 6502 cycles are 978 nS long [14 14mhz clocks], but every 65th cycle is 1117 nS long [16 14mhz clocks]."
            source clock is 14.31818MHz
                unsigned long would overflow after 20 minutes
                    Could modulo byteclock by 14318180 at iterate() and not overflow
                    as long as it doesn't take longer than 20 minutes between calls to APPLE2Einterface::iterate than 20 minutes...)
                Could have clk be in 14MHz ticks.
                    unsigned long long will last 40853 years (could be slow on embedded systems)
                    Keep track of phase at clk delta time, and add 14 usually but 16 every 65th
                    Then wall clock is just always ticks / 14318180.0
                    phase_hpe initially 0
                    add_cpu_cycles(elapsed_cpu) {
                        clock_cpu += elapsed_cpu
                        clock_14mhz = elapsed_cpu * 14 + (elapsed_cpu + phase_hpe) / 65 * 16
                        phase_hpe = (phase_hpe + elapsed_cpu) % 65
                    }
            clock_14mhz is 14mhz ticks
            clock_cpu is CPU clocks, averaging 1.023MHz
            phase_hpe is clock_cpu within horizontal line
            Current CPU clk (in 1MHz CPU cycles) is used for
                which audio sample (at libAO sample rate) in the audio waveform we should fill
                    OK - should work with higher fixed rate
                mark after how many cycles the open apple key should be raised
                    OK - should work with higher fixed rate
                mark paddle timer
                    OK - should work with higher fixed rate
                incrementing CPU instruction cycles
                    OK - call add_cpu_cycles()
        address generator
            every line is 65 bytes, there are 262 lines, so there are 17030 bytes per field
            honor softswitches
            convert clock_cpu to address
            blanking and offscreen addressing during HIRES MIXED
                160 through 191 and 224 through 261
                TEXT addressing at 160, HPE' + 1 through at 192, HPE'
                argh
        read empty memory address on bus no longer fails but instead
            generate address, get byte from RAM and return it
    recording softswitch mode changes
        store vector of switch or mode change and byteclock
        instead of setting before APPLE2Einterface::iterate, pass into iterate
    honoring softswitch change in APPLE2Einterface
        initialize scanline mode array to text
        receive vector of tuple of byteclock and modes
        fill scanline mode array from passed in vector
        draw framebuffer by scanline