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@mos_8502 that'd do it.

another option would be to use the 3.3V rail off the ATX supply (assuming it's rated for 8A or so) and stabilise it by sticking a 100Ω 2W resistor over the 5V rail and a 330Ω 2W resistor over the 12V rail as a method of applying a minimum rail load. wastes a little power but keeps the design simpler and the BOM cheaper.

but if you do this, make sure the ATX voltage tolerance spec is ok for the types of parts you'd expect to see here (e.g. the Z80 chips)