MAMEWorld Forums - News - Re: Going old-school

> The only low level programming I'm familiar with is x86. I never bothered learning
> Z80. If that were the case I could had started with Z80 even if it isn't the first.
> While a 4004 won't look like 8086, a 4 bit processor will be enough to know the
> roots.

The 4004 is actually a bit of an anomaly. The design is heavily optimised to minimise pin count, and it's designed to scale from a very small 64 nybbles of RAM up to 1,024 nybbles of RAM with no additional hardware besides the 4001 RAM chips themselves. I/O scales with RAM and ROM as the RAM chip provide outputs and the ROM chips provide inputs and/or outputs. It's clearly designed for low-cost applications where minimising chip count, keeping pin count down and simplifying board layout are important.

It achieves this by distributing CPU functionality across the chips. All the chips in the system work in lockstep in the 8-phase instruction cycle, and RAM and ROM chips need to decode I/O instructions. The CPU doesn't really even address RAM and I/O as such. There's an instruction that places the contents of a register on the bus during certain phases of the instruction cycle. Peripheral chips need to detect this and latch the value as an address. Then when they detect a RAM or I/O instruction, they need to either read a value off or place a value on the bus during the appropriate cycle. Hence it isn't really a single-chip microprocessor.

The CPUs that came before the it have far more in common with modern Intel microprocessors than the 4004 itself. Even the 8008, which is still heavily optimised for low pin count, is a far more conventional architecture and could be described as a single-chip microprocessor.

The 4004 is interesting in and of itself, due to the rather unique constraints that it was designed for. But if you're looking to find the roots of the x86 family tree, you won't find them there.