Sascha,
If you speak of 51, here is a quotation:
"The instructions that read the latch rather than the pin are the ones that read a value , possibly change it, and then rewrite it to the latch. These are called "read-modify-write" instructions. The instructions listed below are read-modify-write instructions. When the destination operand is a port, or a port bit, these instructions read the latch rather than the pin:
ANL
ORL
XRL
JBC
CPL bit
INC
DEC
DJNZ
MOV bit,C
CLR bit
SETB "
Philips, book IC20, page 2-22.
Michael.