Vertical Timings

When the vertical position register VPOS is set to zero by the CPU, it actually is storing a compare value of 128, since the MSB of VPOS is inverted. This actually corresponds to raster count 256, since the vertical event counter is counting half-lines. When the vertical event counter matches the VPOS register, the vertical synccounter is reset to zero. Multiply the desired line for each event by 2 and subtract the nominal VPOS value of 256 to get the desired decode. If the result is negative add the modulo of the vertical event counter, which is 525 for NTSC and 625 for PAL. The "line" in these tables refer to raster lines, where line 50 is the first displayed line in a 25 row display.

NTSC

    Event         line  v count - vpos  decode
    ------        ----  ------- ------  ------
    VSYNC START   11    22      -234    291
    VSYNC STOP    14    28      -228    297
    VEQU START    8     16      -240    285
    VEQU STOP     17    34      -222    303
    VBLANK START  8     16      -240    285
    VBLANK STOP   28    56      -200    325
    EARLY START   64    128     -128    397
    EARLY STOP    11    22      -234    291
    LATE START    11    23      -233    292
    LATE STOP     3     6       -250    275

PAL -- timings begin 25 lines before NTSC because of 50 extra lines

    Event         line  v count - vpos  decode
    -----         ----  ------- ------  ------
    VSYNC START   -14  -29      -285    340
    VSYNC STOP    -11  -24      -280    345
    VEQU START    -17  -34      -290    335 *equ/sync is 15 half-lines
    VEQU STOP     -9   -19      -275    350 *for pal
    VBLANK START  -17  -34      -290    335
    VBLANK STOP   3    6        -250    375
    EARLY START   39   78       -176    447
    EARLY STOP    -14  -29      -285    340
    LATE START    -14  -28      -284    341
    LATE STOP     -22  -44      -300    325

Divide ratios (including external sync values)

    
    Counter         Normal Early    Late    Gross
    -------         ------ ----     ----    -----
    NTSC vertical   525     524     526     540
    PAL vertical    625     624     626     640
    NTSC horiz      910     908     912
    PAL horiz       908     906     910
    horiz counter   520     519     521