Manfred,

1. If my program contains more than 1K program-code the program AEC_ISP says it is NOT possible – why?

Perhaps you have a limited AEC_ISP version. Try to split your big hex-file for several (at least for two) parts with size less than 1K and program the flash by all of this parts in the sequence.

2. I read, that it is possible to program single bytes in flash-memory– how can I do so
Try this way. Put in your program the following fragment for each single byte.

    org   0x1234    ;the address you needed
    db    0x56      ;the data you needed

Hope it helps.

Regards,
George