notes on IIc VBL

src/libsrc/apple/dgrlib.pla

@@ -494,7 +494,7 @@ asm _dgrFillTile
         RTS
 end
 //
-// Wait for VLB - Shouldn't work on //c, but seems to.
+// Wait for VLB - Only IIe/IIGS.
 //
 asm vlbWait#0
 -       LDA     $C019
@@ -503,6 +503,30 @@ asm vlbWait#0
         BPL     -
         RTS
 end
+// Wait for VBL on IIc is entirely different from others.
+// Setup, one-time *only* if polling will be used exclusively.
+// Otherwise requires restoring the mode on exit,
+// and setting up per-use instead.
+//   [php]
+//   sei         ; otherwise interrupt is still raised
+//   sta   $C07F ; enable access to VBL register
+//   sta   $C05B ; enable VBL polling
+//   sta   $C07E ; disable access to VBL register
+//
+// wait for VBL:
+// - bit   $C019
+//   bpl   -
+//   lda   $C070 ; $c019 bit 7 is sticky, reset it
+//
+// To allow other methods of VBL, restore access:
+// - bit   $C019
+//   bpl   -
+//   lda   $C070 ; $c019 bit 7 is sticky, reset it
+//   sta   $C07F ; enable access to VBL register
+//   sta   $C05A ; disable VBL polling
+//   sta   $C07E ; disable access to VBL register
+//   [plp]
+
 //
 // Clear the buffer
 //