unsigned char new_data; // new port bits
unsigned char chg_mask; // 1's for bits to change
...
...
// in C
P1 = (P1 & ~chg_mask) | (new_data & chg_mask);

; in assembler
MOV A, chg_mask
CPL A
ANL A, P1
MOV B, A
MOV A, chg_mask
ANL A, new_data
ORL A, B
MOV P1, A


MICHAEL KARAS