Must only be using 7400 and 7404s doing it the hard way. As far back as '68 ALU blocks were availble, sure they cost $4 each then but the chip savings was there. Likely you'll never build it. Moving up just one step integreation wise is the 7483 (it equivilent is PDP8 era) full adder. That alone will cut the ALU chip count. My fun was not replicating the archetecture in an acient way but using the most MSI TTL parts I could get to replicate it in a moden way. IE: can I build a TTL PDP-8 with 1kx4 MOS ram in minimum TTL count for everything else. One can simplify that to working replica rather than exact replica. The difference is 2 'ls273 for a 16 bit register or 8 LS74s. But it goes further with fewer sockets, pins and wires, power and debug time. That also relects reliability once working as used parts are going to be a bit shakey untill (re)infant mortaility is again worked out from removal stresses. That may be minor but with a 200-400 peice TTL system you would be surprized unpleasently with how bad bring up can be if there is any uncertanty in the parts used. When I did the 200 peice TTL system it was 1972 into 73 and we were using the then common silicone plastic TTL aka the gray plastic and those were not reliable. That system took over a month to debug between wiring errors, timing errors (races and spikes) and new but partially dead chips. and after it was working for about three months after it was cranky when hot till we weeded out a few more bad actors. One thing I'd suggest for those building something greater 50 TTL is lots of LEDs to indicate the state or status of a block of logic plus the ability to slow the clock if possible to near DC so you can watch it do stuff without a fast logic analyser. It's a great fault finding tool. Theses days free PC power supplies (usually free scrap AT form factor) and cheap brick form factor switchers make 20A at 5 volts a trivial deal. Allison