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A Brief History of Early Microprocessors

The first microprocessor was developed by what was then a small company called Intel (short for Integrated Electronics) in the early 1970s.  The client, a Japanese company called Busicon, declined to buy the chipset and Intel, faced with a development cost and no customer, decided to market the chipset as a "general purpose" microprocessing system for use in applications where digital logic chips would have been used.  The chipset was a success and within a short while Intel developed a general purpose 4 bit microprocessor called the 4004.

In April 1972 Intel and TI co-produced an 8 bit CPU with a 14-bit data bus that could address 16KB of memory. It was originally known as the 1201, and the chip was commissioned by Computer Terminal Corporation (CTC) to implement an instruction set designed for their Datapoint 2200 programmable terminal, but CDC dropped the project and allowed Intel the IP on the 1201, and Intel released it as the 8008.  This was quickly followed by the Intel 8080.  Both the 8008 and the 8080 operated from +5V, -5V and +12V supplies (using NMOS technology).

At about the same time Motorola released its first microprocessor, the 6800, which was also an 8 bit processor with about the same processing power as that of the intel 8080.

The architectures used in the Intel 8080 and the Motorola 6800 were very different.

The Intel 8080 used a register based architecture with registers AX, BX, CX, DX, and HL, all 16 bit but capable of being used as 8 bit register pairs so that the AX register could be used as two separate registers AH and AL.  AH was really just the higher byte of register AX; and AL the lower byte.  In this way, the AX, BX, CX, DX, and HL registers could be used as AH, AL, BH, BL, CH, CL, DH, DL, H, and L 8 bit registers.

Another thing about the 8080 was its separate I/O map.  This meant that to perform byte-wide input/output to hardware, special instructions were used: IN to input from byte-wide input ports, OUT to output to byte-wide output ports.  Access to memory involved access to a different memory map using typically the MOV instruction.

The Motorola 6800 microprocessor used what is called "Memory Mapped I/O" which means that both memory and byte-wide input/output share the same memory map.

The register set was much smaller, consisting of two 8 bit accumulators (A and B) and a 16 bit index register called X.  These registers could however support a range of addressing modes which, in effect, made up for the fewer registers (and also made for simpler programming).  Both microprocessors had other registers but we will ignore these since we have not yet formally looked at the microprocessor in detail.

To input data from memory or from I/O requires the use of the LDAA instruction, to write data to memory or I/O requires the use of the STAA instruction.  Access to the X register was via its own set of instructions, ie. LDX and STX.

Intel and Motorola have maintained the fundamental differences in architecture during the development of later microprocessors.

In the case of the Intel range of microprocessors, the 8080 evolved into the 8085 (also 8 bit like the 8080 but the first Intel 5V only CPU - hence the '5' in its name), in 1977 and was binary compatible with the 8080 but much more integrated. It included the 8224 clock gen and 8228 system controller, and introduced the multiplexed address data bus (with the ALE signal). This then evolved into the third generation 16 bit 8086 microprocessor which, in its 8088 pseudo 16 bit form, was used in the first IBM PCs.  The 8088 was an 8086 but with only an 8 bit data bus.  This made it easier to interface to the common 8 bit peripheral devices available at the time.  In time this was followed by the 80186, the 80286, the 800386 ( a 32 bit processor), and 80486, leading to the Pentium range of microprocessors (64 bit processors) available today.  The 80x86 and Pentium processors have all been designed for use in personal computer type applications and have large memory maps.

The Motorola range of microprocessors followed a similar path with the 6800 replaced by the 6809 (8 bit), then the 68000 (16 bit), the 68010, 68020, and 68030 used in many workstations and of course the Apple MAC range of personal computers.

In due course the Intel 8080 core processor was used for a range of microcontrollers (8048 and 8051 to name but two).  The 8051 microcontroller survives today but is now manufactured by Philips.  Intel have gone on to develop a range of other microcontrollers that are more complex than the basic original devices. More details at (products/embedded/microcontrollers).

Motorola followed in a similar vein with a range of microcontrollers based on the 6800 (6805, 6808, 6811 which survive to this day).

So, many of today's popular microcontrollers are based around two core architectures - that of the 8080 and that of the 6800 microprocessors.

Other manufacturers such as Rockwell produced microprocessors based on the 6800 architecture (6502), whilst Zilog developed the Z80 (based on the 8080 architecture).

The development of more recent microprocessor architectures such as the Harvard architecture and the use of Reduced Instruction Set Computers (RISC) have led to the development of microcontrollers such as the Microchip PIC.

[link to the 'History of the PIC development']

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Updated 24.05.07 ML

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