Yes, it is. Or better it is a semi cusom chip - like ASICs today. The Basic design is a standard compilation of TTL functions, only the last metal layer is custom. This was an early attempt by some chip manufacturers to cut design costs - only this last layer had to be designed for every customer. The technic was(is) called ULA Unassigned(?) Logic Array. And thats also the name used by ZX-fans for this Chip. The ULA replaces something like 15 or 20 standard TTL bugs from the ZX-80 design - the 81 is just a ZX-80 II :) Gruss H. P.S.: Don't assemble it ... I assembled 5 kits several years ago - and now I could cry for one :( -- Ich denke, also bin ich, also gut HRK