C64 FLD Test

/* ------------------------------------------------------------------------------------------------
Empty project
------------------------------------------------------------------------------------------------ */

// here you put consts, vars etc.
NumFLDLines:    .byte 0


BasicUpstart2(start)

* = $3400 "Main Code"

start:

main:
    sei
loop1:
    bit $d011               // Wait for new frame
    bpl loop1
loop4:
    bit $d011
    bmi loop4

    lda #$1b                // Set y-scroll to normal position (because we do FLD later on..)
    sta $d011

    jsr CalcNumLines        // Call sinus substitute routine

    lda #$40                // Wait for position where we want FLD to start
wait_fld_position:
    cmp $d012
    bne wait_fld_position

    ldx NumFLDLines
    beq loop1               // Skip if we want 0 lines FLD
loop2:
    lda $d012               // Wait for beginning of next line
loop3:
    cmp $d012
    beq loop3

    clc                     // Do one line of FLD
    lda $d011
    adc #1
    and #7
    ora #$18
    sta $d011

    dex                     // Decrease counter
    bne loop2               // Branch if counter not 0

    jmp loop1               // Next frame







CalcNumLines:
    lda #0
    bpl *+4
    eor #$ff
    lsr
    lsr
    lsr
    sta NumFLDLines
    inc CalcNumLines+1
    rts