# Minimum number of transistors to build a CPU

I remember reading about this a while back. Someone decided to figure out what was the bare minimum number of transistors necessary to build a functioning CPU. IIRC, it was 87 or something similar but I’m having problems finding out where I read it.

One, would be my answer. You need to determine a state: so it’s either on or off; hot or cold, up or down, in or out. This is the primary function of the CPU. Now if you had to determine all of the above, plus take into consideration the day of the week, the time of day, the interest rates from the bank, the outside temperature, and the name of the user then more processing power would be required.

But really, a CPU is only a device that interrogates states, manipulates data, and responds according to pre-programmed instructions.

The 4004 circuit was built of 2,300 transistors, and was followed the next year by the first ever 8-bit microprocessor, the 3,300 transistor 8008
http://en.wikipedia.org/wiki/Intel_4004

Only if you disregard what a CPU is. I really hate it when someone tries to be clever by redefining a word in a way that makes no sense whatsoever. In the modern world, a CPU is a Turing-complete machine with:
[ul]
[li]Access to at least one bank of storage von Neumann machines have one address space, Harvard architectures have two[/li][li]At least one ALU Arithmetic and Logic Unit, the heart of the CPU that does the heavy lifting on the Turing-completeness front[/li][li]At least one register needed to form addresses into storage. Even the most primitive CPUs will have at least two: A program counter (PC) to address the current opcode and an accumulator to hold the result of the last computation. The 4004 had 16 registers (not counting the PC).[/li][li]A defined ISA (Instruction Set Architecture), which means the CPU can be controlled by software, as opposed to requiring physical modification to do a different job.[/li][/ul]Additionally, most CPUs made within the last 30 years are etched onto a single piece of silicon (called the die) alongside a substantial amount of on-die storage (either writable, in which case it’s called cache, or not, in which case it’s a program or other data in ROM for a system-on-a-chip).