Hi,

Most 80C51 will need to have the VDD in a valid opperating range and > 12 clk cycles for the reset to be valid.

Before that all SFRs will have undefined (random) values and not the Reset values.
Thefore port latches will have random values, and the ports will either be pulled down if a 0 is in the port latch or pulled up weakly if a 1 is in the port latch.

After reset is valid the port latches will have the reset values of FFh and the ports will be weakly pulled up to 1.

Regards,

Bauke