Snasm: 65816

I've done most of the assembler stuff - not all, but most - to the point that I can now build simple programs. So I've built a simple 65816 program, and it appears to be falling over; and I've no idea why! These kinds of things suck coz I can't debug it at all - just try new things over and over until I get somewhere....

Heres the code....

        opt    prg
        opt    C64=START     ; Insert BASIC header, and jump to start of the code
        opt    A65816        ; Set 65816 assembler mode

 
        org $1000
START:
        longa off
        longi off
        sei
        clc            ; Set processor to 65816
        xce            ; Set 16bit processor mode 
 
        rep   #$30
        longi on
        longa on

        lda   #$0000
        sta   $20
 
Here:
        lda   $20
        ldx   #$1000
Here2:
        sta   $0400,x
        dex
        bne   Here2
 
        inc   $20
        jmp   Here   
 

If I dont put it into 16bit mode, it appears to work fine, but execute the rep #$30, and it dies.... I'm not sure why... If I didnt know better, Id say there was an interrupt going off - I wonder if the NMI's are still on....