"Efficient", as measured by what? It's a couple thousand gates or so, which is enough to do an 8 bit microcontroller that doesn't have to worry about having a p

"Efficient", as measured by what? It's a couple thousand gates or so, which is enough to do an 8 bit microcontroller that doesn't have to worry about having a protected mode and such. Also, there are not many registers, so the latch count is pretty small. As measured by die-area per delivered functionality, it does pretty well, and if you compare 6502 code with 6800 or 1802 code, it is pretty dense for the functionality that you get out of it. That said, I always had my complaints about the 6502, mainly lack of 16 bit index registers that could reach the entire memory space. That forced a coding idiom that relied on indirect addressing through page 0. 6800 had a 16 bit index register. If you look at the instruction set, you can see that it is relatively horizontal and pretty straight forward to decode and implement with a simple sequencer. Of course, it was implemented in NMOS like most of the contemporary microcontrollers, so it was a power hog by today's standards, but the transistor count per gate was lower. The 1802 was CMOS, but it was aimed at applications that required space-grade parts, so it was low power and had transistors the size of a cow turd in order to be more resistant to alpha-particle hits. It was also slow. For comparison, at the time I was playing with 6502's as a hobby, I was at my day job a mainframe CPU logic designer working in 100K ECL -- I was working on a machine roughly equivalent to the Cray-1, and our gate count was roughly 250,000 gates. The logic design for a CPU the size of a 6502 is really not too bad -- sort of the scale of a largish homework assignment as a semester final -- constructing something like it in a gate level simulator might be an interesting exercise for the motivated.