The PAGE operator is PAGE.
PAGE Byte: PAGE(<operand>)
This operator returns the page byte of the address of a memory location.
Assume data1 is a word located at address $28050 in the memory.
LDA #PAGE(data1)
This instruction will load the immediate value of the page byte of the address of data1 ( $2).
LDA PAGE(data1)
This instruction will load the direct value at memory location of the page byte of the address of data1 (i.e., the value in memory location $2).