Attiny85 code on Attiny84
There are slight differences between registers, register names and registers bit names:


WDTCR is named WDTCSR
PCIE doesn't exist in GIMSK as there are two IO ports, so there are two pin change channels: PCIE0 and PCIE1
and PCMSK is similar to PCIE. There'll be two of them too.


Basically you have to look into both datasheets and compare what you are using and check if it's correct.