separate notes on VBLANK and mode switch

notes.vblank

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+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
+