I read about setting the fuse bits of the at89c5131 in the bootloader-manual page 20. It says that all 4 fuse bits are set with one command. On page 27 are the corresponding api-calls. Now I am a little confused about the AND's and OR's in the rightmost collumn. I would like to implement functions for reading and writing these fusebits, but know about what can happen if I make a mistake... So I first want to ask you about some hints. Thanks in advance!