; main.S
;
; This file is part of OSDY VideoCharGen
; a do-it-yourself on-screen-display image generator
; for superimposition of analog PAL signals
;
#include <avr/io.h>
#include "macro.h"
#include "const.h"

.data
show_image:
.byte 0x00

button_next_image:
.byte 0x0

current_jump_table:
.word 0x0

.text

.global main_asm
main_asm:
    ; init variables
    ldi r16, 0
    sts show_image, r16

    ; r0 always holds 0
    clr r0

    ; global interrupt enable
    sei

    ; endless loop
1:
    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

    ; 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
    breq enter          ; +23
    jmp int_horizontal_sync_end

enter:
    ; here, +23 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

    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

    ; turn off interrupt
    lds r31, TIMSK0
    andi r31, 0xfd      ; mask disable interrupt timer A
    sts TIMSK0, r31

    lds r31, show_image
    cpi r31, 0x1
    brne jump_table_return_address

    lds zl, current_jump_table
    lds zh, current_jump_table + 1

;    ldi zl, pm_lo8(line_jump_table_0)
;    ldi zh, pm_hi8(line_jump_table_0)

    clr r0
    lds r29, line
    add zl, r29
    adc zh, r0
    add zl, r29
    adc zh, r0

    ijmp

.global jump_table_return_address
jump_table_return_address:
    pop r29
    pop r30
    pop r31
    out IO(SREG), r31
    pop r31
    reti

.global int_vertical_sync
int_vertical_sync:
    push r31
    in r31, IO(SREG)
    push zl
    push zh
    push yl
    push yh

    lds r31, frame + 1
    lds r30, frame
    adiw z, 1
    sts frame + 1, r31
    sts frame, r30

    ldi r30, 1
    ldi r31, 0
    sts line, r30
    sts line + 1, r31

    ; check button
    in r31, IO(PINB)
    andi r31, 0x08
    breq check_if_released
    ldi r31, 1
    sts button_next_image, r31
    jmp int_vertical_sync_end

check_if_released:
    lds r31, button_next_image
    cpi r31, 1
    brne int_vertical_sync_end

    ; here button is released
    ; read transition mode
    in r31, IO(PINB)
    andi r31, 0x04
    ; if transition mode == 1, then always show image
    ; Nice to have: maybe this can be written better.
    breq 1f
    ldi r31, 0
    sts show_image, r31

1:
    lds r31, show_image
    cpi r31, 1
    breq 1f

    ldi r31, 1
    sts show_image, r31
    rjmp 2f
1:
    ldi r31, 0
    sts show_image, r31
    rjmp end_button_release
2:
    ; show image

    ldi zl, pm_lo8(line_jump_table_0)
    ldi zh, pm_hi8(line_jump_table_0)

    clr yl
    lds yh, image
    inc yh
    sts image, yh

    add zh, yh
    add zh, yh

    sts current_jump_table, zl
    sts current_jump_table + 1, zh

    ldi r31, 0x01
    sts show_image, r31

end_button_release:
    clr yl
    sts button_next_image, yl

int_vertical_sync_end:
    pop yh
    pop yl
    pop zh
    pop zl
    out IO(SREG), r31
    pop r31
    reti