74act2155 chips

Anyone know what a 74ACT2155 IC is? Memory of some sort, but is there anything special about them? I've managed to pull the NCR Tower into the middle of the garage and have pulled the covers off - I'll take the disks out later and see if they spin up first as the machine's somewhat useless if they don't! After that I'll reseat the boards, check the PSU etc. but I do remember the machine had developed some sort of fault when I parked it up years ago. ISTR it being *totally* dead, so there's likely a PSU problem. Anyway, there are quite a few of these 74ACT2155 chips on little modules on the system board; one of the modules is labelled as being cache so they're definitely memory of some flavour - I've just not come across these with a 74xxx part number before. NCR didn't half like their PAL chips. I counted over 40 on the system board alone. There's almost as many as there are typical logic ICs. cheers Jules