vertical line, scrolling from left to right

sufficiently handles the back/front porch without disrupting the sync
signal
todo improvement: compute a sensible amount of cycles to waste during
the porch

const.h

@@ -1,4 +1,4 @@
 #pragma once
 
 #define VERTICAL_OFFSET 30
-
+#define HORIZONTAL_OFFSET_CYCLE 50

main.S

@@ -58,30 +58,43 @@ int_horizontal_sync:
     in r31, IO(SREG)    ; status register
     push r31
 
-#if 0
     lds r31, vertical_offset
     cpi r31, 0
-    breq enter1
+    breq enter
     dec r31
     sts vertical_offset, r31
     jmp int_horizontal_sync_end
 
-enter1:
+enter:
     lds r31, line
     cpi r31, 0
     breq int_horizontal_sync_end
     inc r31
     sts line, r31
 
+    ldi r31, HORIZONTAL_OFFSET_CYCLE    ; skip back porch
+1:
+    dec r31
+    brne 1b
+
+    ; do things
     lds r31, frame
-    lsr r31
-    lsr r31
+    cpi r31, 0
     breq 2f
 1:
     dec r31
     brne 1b
-#endif
+2:
 
+    sbi IO(PORTB), 4
+    nop
+    nop
+    nop
+    nop
+    nop
+    cbi IO(PORTB), 4
+
+#if 0
     ldi r31, 140
 1:
     dec r31
@@ -99,6 +112,7 @@ enter1:
     nop
     nop
     cbi IO(PORTB), 4
+#endif
 
 int_horizontal_sync_end:
     pop r31