OSBYTE 161/162 notes

On entry:
A=161 or 162
X=logical non-volatile memory location or &FF to read highest location offset
Y=writing: data to write
Y=reading: ignored

On exit:
A=preserved
X=absolute location (eg entryX+14 for RTC RAM starting at &0E, entryX+0 for EEPROM starting at &00)
Y=writing: preserved
Y=reading: data read if valid location, preserved if invalid location

NB: intercept code needs to ensure CLV to allow calling from *FX.