6510 Copy data

// Simply copy routine by Scan
//
// Copies every 16th byte from $1000-$2000 to $2000-
// Uses the undocumented opcodes AXS and ISC
// Reference: No More Secrets http://csdb.dk/release/?id=161035
// Copy routine to zero page for less cycle consumption
.const ENDPAGE = $20

BasicUpstart2(main)

main:   ldy #$00
        ldx #$00
.label xhi= *+2
repeat: lda $1000,x
.label yhi= *+2
        sta $2000,y
        txa
        axs #$f0        // X= A&X-#imm, adds #$10, carry is always clear before operation
        bcc !+          // carry set if overflow
        lda #ENDPAGE
        sec
        isc xhi
        beq done
!:      iny
        bne repeat
        inc yhi
        bcc repeat
done:   rts

.pc = $1000
.fill $1000,i