bradgrantham-apple2e/notes.vblank
2018-08-02 16:25:25 -07:00

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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