blitscreen.asm

.const zptmp0 = $60
.const zptmp1 = $62
.const zptmp2 = $64
.const zptmp3 = $68

.const zp_dstptr = zptmp0
.const zp_srcptr = zptmp1
.const zp_nlines = zptmp2

.macro mov16imm(res, v) {
    lda #<v
    sta res+0
    lda #>v
    sta res+1
}

.macro add16_imm8(res, lo) {
    clc
    lda res
    adc #lo
    sta res+0
    lda res+1
    adc #0
    sta res+1
}


.macro copyscreen(buf0, buf1, starty, numlines) {
    lda dbl_buf
    and #1
    bne screen_0_to_1
    mov16imm(zp_srcptr, buf0 + starty*40+1)
    mov16imm(zp_dstptr, buf1 + starty*40)
    jmp start
screen_0_to_1:
    mov16imm(zp_srcptr, buf1 + starty*40+1)
    mov16imm(zp_dstptr, buf0 + starty*40)
start:
    lda #numlines
    sta zp_nlines
    jsr blit_screen
}

blit_screen:
    ldx #0
yloop:
    ldy #0
xloop:
    lda (zp_srcptr),y
    sta (zp_dstptr),y
    iny
    lda (zp_srcptr),y
    sta (zp_dstptr),y
    iny
    lda (zp_srcptr),y
    sta (zp_dstptr),y
    iny
    lda (zp_srcptr),y
    sta (zp_dstptr),y
    iny
    cpy #36
    bne xloop

    lda (zp_srcptr),y
    sta (zp_dstptr),y
    iny
    lda (zp_srcptr),y
    sta (zp_dstptr),y
    iny
    lda (zp_srcptr),y
    sta (zp_dstptr),y
    iny

    add16_imm8(zp_dstptr, 40)
    add16_imm8(zp_srcptr, 40)
    inx
    cpx zp_nlines
    bne yloop
    rts