GetStringLength: ;$00 and $01 make up the pointer to the string's base address.
                 ;(Of course, any two consecutive zero-page memory locations can fulfill this role.)
LDY #0           ;Y is both the index into the string and the length counter.

loop_getStringLength:
LDA ($00),y
BEQ exit
INY
JMP loop_getStringLength

exit:
RTS               ;string length is now loaded into Y.