[Gibbon1]

You are are correct, direct memory access on those machines were really cheap compared with today and even earlier. Modern day memory bus speeds haven't kept of with processor speeds (this is what sank RISC machines). Old core memory is slow. Drum memory is hella slow.

The other thing is there is a trade off between number of registers and instruction size. With 8 bit machines you see that for instance where only certain addressing modes can be used with certain registers. You don't have enough instruction space to encode for every addressing mode for all the registers you have.