do using polling

actually this does not work, because polling requires:
        in
        andi
        brne
which takes 4 clock cycles, that, on average, means a 2 clock offset,
while interrupts occur during a rjmp, which takes 2 clocks, which means,
on average, only 1 clock cycle of offset

I am not interested in higher or lower overhead: I am just interested in
minimizing its variance.
Being late of a few clock cycles, is not a problem: it just means that
the picture will be slightly shifted toward the right border; but,
starting the picture 1 or 2 clock cycles before (on one line), and 1 or
2 clock cycles after, on another line, means that it just wiggles sligthly
left and right

main.S

@@ -8,24 +8,8 @@
 main:
     ldi r16, 0x30       ; port B, pin 4 and 5 as output
     sts DDRB, r16
-
-    ; set interrupt vectors at address 0x0, not bootloader
-    ; timing is important, see atmel datasheet
-    ldi r16, (1 << IVCE)
-    ldi r17, 0
-    out IO(MCUCR), r16
-    out IO(MCUCR), r17
-
-    ldi r16, 0xa        ; external interrupt 0 and 1, falling edge
-    sts EICRA, r16
-
-    ldi r16, 0x3        ; external interrupt 0 and 1, mask enable
-    sts EIMSK, r16
-
-    ldi r16, 0x02       ; don't connect output pins to timer, CTC[1:0] mode
-    sts TCCR0A, r16
-    ldi r16, 0x01       ; CTC[2] mode, no prescaler
-    sts TCCR0B, r16
+    cbi IO(DDRD), 2     ; port D, pin 2 and 3 as input
+    cbi IO(DDRD), 3
 
     ; init variables
     ldi r16, 0
@@ -35,85 +19,68 @@ main:
     ldi r16, 1
     sts line, r16
 
+    cli                 ; global interrupt disable
+
     call setup_c
 
-    sei                 ; global interrupt enable
-
-1:
-    call loop_c
-    rjmp 1b
-
-.global int_horizontal_sync
-int_horizontal_sync:    ; +3
-    push r31            ; +5
-    in r31, IO(SREG)    ; +6, status register
-    push r31            ; +8
-    push r30            ; +10
-
+wait_next_line:
+    in r16, IO(PIND)
+    andi r16, 0x08
+    brne wait_next_line
+/*
     ; if (line >= VERTICAL_OFFSET), then enter
-    lds r30, line       ; +12
-    lds r31, line + 1   ; +14
-    adiw z, 1           ; +16
-    sts line, r30       ; +18
-    sts line + 1, r31   ; +20
-    cpi r31, 0          ; +21
-    brne enter                      ; +22, +23
-    cpi r30, VERTICAL_OFFSET        ; +23
-    brlo int_horizontal_sync_end    ; +24
+    lds r30, line               ; +2, 2
+    lds r31, line + 1           ; +2, 4
+    adiw z, 1                   ; +2, 6
+    sts line, r30               ; +2, 8
+    sts line + 1, r31           ; +2, 10
+    cpi r31, 0                  ; +1, 11
+    brne enter                  ; +2, 13
+    cpi r30, VERTICAL_OFFSET    ; TODO timing bad at 256 lines
+    brsh enter
+    jmp horizontal_line_end
+*/
 
 enter:
-    ; here, +23 or +24 cycles have passed since horizontal sync
-    ; so, there are still ~168 cycles before first useful data
-    ldi r31, 0
-    sts TCNT0, r31
-    ldi r31, HORIZONTAL_OFFSET  ; set counter TOP
-    sts OCR0A, r31
+    ; here, +13 cycles have passed since horizontal sync
+    ; so, there are still (192 - 13) = 179 cycles before first useful data
+    ldi r31, 64         ; approx
+1:
+    dec r31             ; 1
+    brne 1b             ; 2
 
-    ldi r31, 0x7        ; clear any pending interrupt
-    sts TIFR0, r31
-
-    lds r31, TIMSK0
-    ori r31, 0x02       ; mask enable interrupt timer A
-    sts TIMSK0, r31
-
-int_horizontal_sync_end:
-    pop r30
-    pop r31
-    out IO(SREG), r31
-    pop r31
-    reti
-
-.global int_timer_0
-int_timer_0:
     ; here we are at the beginning of the visible line
-    push r31
-    in r31, IO(SREG)
-    push r31
-    push r30
-    push r29
+    ldi r31, 16
+1:
+    dec r31
+    brne 1b
 
-    ; turn off interrupt
-    lds r31, TIMSK0
-    andi r31, 0xfd      ; mask disable interrupt timer A
-    sts TIMSK0, r31
+    sbi IO(PORTB), 5
 
-    ; draw things
-    ldi r30, lo8(line_buffer)
-    ldi r31, hi8(line_buffer)
-
-.rept LINE_BUFFER_SIZE
-    ld r29, z+
-    out IO(PORTB), r29
+    sbi IO(PORTB), 4
     nop
-.endr
+    nop
+    nop
+    nop
+    cbi IO(PORTB), 4
 
-    pop r29
-    pop r30
-    pop r31
-    out IO(SREG), r31
-    pop r31
-    reti
+#    ; draw things
+#    ldi r30, lo8(line_buffer)
+#    ldi r31, hi8(line_buffer)
 
+#.rept LINE_BUFFER_SIZE
+#    ld r29, z+
+#    out IO(PORTB), r29
+#    nop
+#.endr
+
+    cbi IO(PORTB), 5
+
+horizontal_line_end:
+    jmp wait_next_line
+
+
+/*
 .global int_vertical_sync
 int_vertical_sync:
     push r31
@@ -139,4 +106,4 @@ int_vertical_sync_end:
     pop r31
     reti
 
-
+*/

main.c

@@ -7,23 +7,13 @@ volatile uint16_t frame;
 volatile uint16_t line;
 volatile char line_buffer[LINE_BUFFER_SIZE];
 
-ISR(INT0_vect, ISR_NAKED) {
-    asm("jmp int_vertical_sync");
-}
-ISR(INT1_vect, ISR_NAKED) {
-    asm("jmp int_horizontal_sync");
-}
-ISR(TIMER0_COMPA_vect, ISR_NAKED) {
-    asm("jmp int_timer_0");
+void setup_c() {
+    for (int i = 0; i < LINE_BUFFER_SIZE; ++i) {
+        line_buffer[i] = (i > 50 && i < 100) ? '1' : 'A';
+    }
 }
 
-void setup_c() {
-    /*
-    for (int i = 0; i < LINE_BUFFER_SIZE; ++i) {
-        line_buffer[i] = (i % 2) ? 0x0 : 0xff;
-    }
-    */
-}
+/*
 
 void loop_c() {
     for (;;) {
@@ -53,3 +43,4 @@ void loop_c() {
         }
     }
 }
+*/