Dixon,
Because JZ and JNZ check the contents of the accu, not some flag. As soon as the accu contains 0, JNZ will *not* branch, because the accu contains 0.
Most other processors contain a zero flag, but not the 8052.
While designing code for the 8052, keep good track of what every instruction does with the flags in PSW.
Another good example of awkward flag behaviour is that INC and DEC do not influence carry. There are more situations. For reference, use the PDF at http://www.semiconductors.philips.com/acrobat/v...UIDE_1.pdf