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History of Microprocessors:
The Microprocessor was first developed in the early 1970's by a company called Intel (Integrated Electronics). Intel was a small company and its client refused to buy the product. So Intel decided to market its microprocessor as a general-purpose chipset, which was extensively used wherever digital logical chips were used. The success of this microprocessor triggered Intel to research more on microprocessors and very soon it developed a 4-bit microprocessor for general purpose. 4004 is much more advanced and it is designed and marketed as a "general purpose" chipset.
Later, Intel in association with TI produced an 8-bit central processing unit (CPU) with a 14-bit data memory bus, which can address up to 16KB of memory. The CTC (computer Terminal Corporation) ordered Intel and Texas Instruments to build them an 8-bit processor that can be used in terminals. Soon CTC gave up its idea. Even then Intel and TI kept working on the project and released a microprocessor 8008, which is able to address 16KB of memory and operation speed of 300 000 operations per second. Later an updated model of 8008, i.e. 8080 was very soon released which has operating range of +5V, -5V and +12V using NMOS technology.
Observing these developments in technology, Motorola released its first ever microprocessor, the 6800. Motorola was the only company to make other peripherals such as 6820 and 6850. 6800 is an 8-bit with almost the same specifications as of 8080. The architectures used in 8080 and 6800 differ a lot. The architecture used in Intel 8080 is a register based. The registers are from AX, BX, CX, DX, and HL. All are 16-bit but are capable of being used as an 8-bit register pairs. So the register AX can be used as two separate registers, i.e. AX can be used as AH and AL. Where AH is a byte higher than AX, where as AL is the previous lower byte. In the similar way each and every register BX, CX, DX, and HL can be used as BH, BL, CH, CL, DH, DL, H, and L.
Also Intel 8080 has separate I/O map. This is given separately to provide byte-wide input/output to hardware. Some special instructions are used to perform either input or output to the hardware. The instruction for accepting a byte-wide input from it's input port is IN. Similarly OUT is to output a byte-wide output to the output port. The MOV instruction is used to get access to memory from a different memory map.
Motorola 6800 used "Memory Mapped Input/Output". Memory mapped input/output is nothing but sharing of the same memory map by both memory and byte-wide input and output, which is different from Intel 8080. Also the register set consisted of two 8-bit accumulators, i.e. A and B, and an index register X. The index register X is of 16-bits and this makes the register set much smaller than Intel 8080. However small, these registers can support a range of addressing modes. These addressing modes are made up for few registers and simple programming.
The data input from the memory or from input source requires the use of LDAA. This is used to write data to the memory or input/output requires the use of STAA instruction. The access to register X is was through an own set of instructions, i.e. LDX and STX.
As before both the companies Intel and Motorola have maintained differences in their development of further microcontrollers.
The Intel upgraded it's 8080 to 8085, in 1977which is also 8-bit processor like 8080 and is first 5V CPU gave '5' in the place of '0'. Intel was very keen on upgrading microprocessors. Very soon 8086, 8088 and later 80186 were released. 800386 is a 32-bit processor and 80486 leading to the design of Pentium range of microprocessors which are 64-bit processors. 80x86 and Pentium range processors are specially designed for personal computer applications. They have large memory maps.
Motorola's microprocessors followed similar path by replacing 6800 with 6809, an 8-bit and then by 68000, a 16-bit. The microprocessors 68010, 68020, 68030 were used in many workstations and also in personal computers like Apple MAC.
The most recent microprocessors in the present market use Harvard architecture and use of Reduced Instruction Set Computers (RISC) have led to the development of microcontrollers such as Microchip PIC.