The first single chip CPU, Intel 4004 was invented in 1971, this was called the calculator. Instructions were processed in four bits, but were 8 bits long.
Texas Instruments followed the Intel 4004 with 4-bit TMS 1000. This was the first microprocessor to include RAM, and also having space for ROM, allowing operation without many support chips.
The 8080 was the improved of April 1972. The 8080 had an eight bit data bus and a sixteen bit address. Inside it had seven 8 bit registers, a program counter, and 16 bit pointer to memory. It has several I/O ports allowing the stack to occupy from a separate memory bank. Intel updated with the 8085 in 1976.
The Z-80 was intended to be an improvement of the 8080 in 1976. It used 16 bit addressing and eight bit data, and could execute all of the op codes. The register set doubled, with multiple banks of data registers that could be switched between. The Z-80 was popular due to the memory interface. The CPU generated its own refresh signals, which meant lowering system cost.
The 6800 was introduced by Motorola in 1975. MOS Technologies designers introduced the 650x series which included the 6501 and the 6502. This is the first chip you could get for less. This was the CPU of choice for many home-based computers.
The 6809 can make a single 16 bit register from two eight bit accumulators. It features two index registers and two stack pointers, allowing advancing for addressing modes. The 6809 was made from Motorola’s 6800.