19-11-2012, 01:48 PM
Intel Microprocessors: 8008 to 8086
[attachment=40149]
The Prophecy
Intel introduced the microprocessor in November 1971 with the advertisement, "Announcing a New Era in Integrated Electronics." The fulfillment of this prophecy has already occurred with the delivery of the 8008 in 1972, the 8080 in 1974, the 8085 in 1976, and the 8086 in 1978. During this time, throughput has improved 100-fold, the price of a CPU chip has declined from $300 to $3, and microcomputers have revolutionized design concepts in countless applications. They are now entering our homes and cars.
Each successive product implementation depended on semiconductor process innovation, improved architecture, better circuit design, and more sophisticated software, yet upward compatibility not envisioned by the first designers was maintained. This paper provides an insight into the evolutionary process that transformed the 8008 into the 8086, and gives descriptions of the various processors, with emphasis on the 8086.
8008 Objectives and Constraints
Late in 1969 Intel Corporation was contracted by Computer Terminal Corporation (today called Datapoint) to do a pushdown stack chip for a processor to be used in a CRT terminal. Datapoint had intended to build a bit-serial processor in TTL logic using shift-register memory. Intel counterproposed to implement the entire processor on one chip, which was to become the 8008. This processor, along with the 4004, was to be fabricated using the then-current memory fabrication technology, p-MOS. Due to the long lead time required by Intel, Computer Terminal proceeded to market the serial processor and thus compatibility constraints were imposed on the 8008.
Most of the instruction-set and register organization was specified by Computer Terminal. Intel modified the instruction set so the processor would fit on one chip and added instructions to make it more general-purpose. For although Intel was developing the 8008 for one particular customer, it wanted to have the option of selling it to others. Intel was using only 16- and 18-pin packages in those days, and rather than require a new package for what was believed to be a low-volume chip, they chose to use 18 pins for the 8008.
8008 Instruction-Set Processor
The 8008 processor architecture is quite simple compared to modern-day microprocessors. The data-handling facilities provide for byte data only. The memory space is limited to 16K bytes, and the stack is on the chip and limited to a depth of 8. The instruction set is small but symmetrical, with only a few operand-addressing modes available. An interrupt mechanism is provided, but there is no way to disable interrupts.
Memory and I/O Structure
The 8008 addressable memory space consists of 16K bytes. That seemed like a lot back in 1970, when memories were expensive and LSI devices were slow. It was inconceivable in those days that anybody would want to put more than 16K of this precious resource on anything as slow as a microprocessor.
The memory size limitation was imposed by the lack of available pins. Addresses are sent out in two consecutive clock cycles over an 8-bit address bus. Two control signals, which would have been on dedicated pins if these had been available, are sent out together with every address, thereby limiting addresses to 14 bits.
The 8008 provides eight 8-bit input ports and twenty-four 8-bit output ports. Each of these ports is directly addressable by the instruction set. It was felt that output ports were more important than input ports because input ports can always be multiplexed by external hardware under control of additional output ports.
One of the interesting things about that era was that, for the first time, the users were given access to the memory bus and could define their own memory structure; they were not confined to what the vendors offered, as they had been in the minicomputer era. As an example, the user had the option of putting I/O ports inside the memory address space instead of in a separate I/O space.
[attachment=40149]
The Prophecy
Intel introduced the microprocessor in November 1971 with the advertisement, "Announcing a New Era in Integrated Electronics." The fulfillment of this prophecy has already occurred with the delivery of the 8008 in 1972, the 8080 in 1974, the 8085 in 1976, and the 8086 in 1978. During this time, throughput has improved 100-fold, the price of a CPU chip has declined from $300 to $3, and microcomputers have revolutionized design concepts in countless applications. They are now entering our homes and cars.
Each successive product implementation depended on semiconductor process innovation, improved architecture, better circuit design, and more sophisticated software, yet upward compatibility not envisioned by the first designers was maintained. This paper provides an insight into the evolutionary process that transformed the 8008 into the 8086, and gives descriptions of the various processors, with emphasis on the 8086.
8008 Objectives and Constraints
Late in 1969 Intel Corporation was contracted by Computer Terminal Corporation (today called Datapoint) to do a pushdown stack chip for a processor to be used in a CRT terminal. Datapoint had intended to build a bit-serial processor in TTL logic using shift-register memory. Intel counterproposed to implement the entire processor on one chip, which was to become the 8008. This processor, along with the 4004, was to be fabricated using the then-current memory fabrication technology, p-MOS. Due to the long lead time required by Intel, Computer Terminal proceeded to market the serial processor and thus compatibility constraints were imposed on the 8008.
Most of the instruction-set and register organization was specified by Computer Terminal. Intel modified the instruction set so the processor would fit on one chip and added instructions to make it more general-purpose. For although Intel was developing the 8008 for one particular customer, it wanted to have the option of selling it to others. Intel was using only 16- and 18-pin packages in those days, and rather than require a new package for what was believed to be a low-volume chip, they chose to use 18 pins for the 8008.
8008 Instruction-Set Processor
The 8008 processor architecture is quite simple compared to modern-day microprocessors. The data-handling facilities provide for byte data only. The memory space is limited to 16K bytes, and the stack is on the chip and limited to a depth of 8. The instruction set is small but symmetrical, with only a few operand-addressing modes available. An interrupt mechanism is provided, but there is no way to disable interrupts.
Memory and I/O Structure
The 8008 addressable memory space consists of 16K bytes. That seemed like a lot back in 1970, when memories were expensive and LSI devices were slow. It was inconceivable in those days that anybody would want to put more than 16K of this precious resource on anything as slow as a microprocessor.
The memory size limitation was imposed by the lack of available pins. Addresses are sent out in two consecutive clock cycles over an 8-bit address bus. Two control signals, which would have been on dedicated pins if these had been available, are sent out together with every address, thereby limiting addresses to 14 bits.
The 8008 provides eight 8-bit input ports and twenty-four 8-bit output ports. Each of these ports is directly addressable by the instruction set. It was felt that output ports were more important than input ports because input ports can always be multiplexed by external hardware under control of additional output ports.
One of the interesting things about that era was that, for the first time, the users were given access to the memory bus and could define their own memory structure; they were not confined to what the vendors offered, as they had been in the minicomputer era. As an example, the user had the option of putting I/O ports inside the memory address space instead of in a separate I/O space.