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back porch skipping, wasting clock cycles, basically

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giomba 2021-06-28 21:54:25 +02:00
parent 1f3f3f36f2
commit 48da25958e
2 changed files with 18 additions and 15 deletions
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2
const.h
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@ -1,4 +1,4 @@
#pragma once #pragma once
#define VERTICAL_OFFSET 30 #define VERTICAL_OFFSET 30
#define HORIZONTAL_OFFSET_CYCLE 50 #define HORIZONTAL_OFFSET_CYCLE 56

31
main.S
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@ -36,24 +36,27 @@ main:
rjmp 1b rjmp 1b
.global int_horizontal_sync .global int_horizontal_sync
int_horizontal_sync: int_horizontal_sync: ; +3
push r31 push r31 ; +5
in r31, IO(SREG) ; status register in r31, IO(SREG) ; +6, status register
push r31 push r31 ; +8
push r30 push r30 ; +10
; if (line >= VERTICAL_OFFSET), then enter ; if (line >= VERTICAL_OFFSET), then enter
lds r30, line lds r30, line ; +12
lds r31, line + 1 lds r31, line + 1 ; +14
adiw z, 1 adiw z, 1 ; +16
sts line, r30 sts line, r30 ; +18
sts line + 1, r31 sts line + 1, r31 ; +20
cpi r31, 0 cpi r31, 0 ; +21
brne enter brne enter ; +22, +23
cpi r30, VERTICAL_OFFSET cpi r30, VERTICAL_OFFSET ; +23
brlo int_horizontal_sync_end brlo int_horizontal_sync_end ; +24
enter: enter:
; here, +23 or +24 cycles have passed since horizontal sync
; so, there are still ~168 cycles before first useful data
; or 56 3-cycle instructions (HORIZONTAL_OFFSET_CYCLE)
ldi r31, HORIZONTAL_OFFSET_CYCLE ; skip back porch ldi r31, HORIZONTAL_OFFSET_CYCLE ; skip back porch
1: 1:
dec r31 dec r31