SCHOTTKY BIPOLAR 8214

INTERRUPTS IN MICROCOMPUTER SYSTEMS

Microcomputer system design requires that I/O devices such as keyboards, displays, sensors and other components re- ceive servicing in an efficient method so that large amounts of the total systems tasks can be assumed by the micro- computer with little or no effect on throughput.

The most common method of servicing such devices is the Polled approach. This is where the processor must test each device in sequence and in effect "ask" each one if it needs servicing. It is easy to see that a large portion of the main program is looping through this continuence polling cycle and that such a method would have a serious, detrimental effect on system throughput thus limiting the tasks that could be assumed by the microcomputer and reducing the cost effectiven.ess of using such devices.

A more desireable method would be one that would allow the microprocessor to be executing its main program and only stop to service peripheral devices when it is told to do so by the device itself. In effect, the method would provide an external asynchronous input that would inform the processor that it should compiete whatever instruction that is currently being executed and fetch a new routine that will service the requesting device. Once this servicing is com- plete however the processor would resume exactly where it left off.

This method is called Interrupt. It is easy to see that system throughput would drastically increase, and thus more tasks could be assumed by the microcomputer to further enhance its cost effectiveness.

The Priority Interrupt Control Unit (PICU) functions as an overall manager in an Interrupt-Driven system environment. It accepts requests from the peripheral equipment, determ- ines which of the incoming requests is of the highest im- portance (priority), ascertains whether the incoming request has a higher priority value than the level currently being serviced and issues an Interrupt to the CPU based on this determination.

Each peripheral devi~e or structure usually has a special program or "routine" that is associated with its specific functional or operational requ irements; this is referred to as a "service routine". The PICU, after issuing an Interrupt to the CPU, must somehow input information into the CPU that can "point" the Program Counter to the service rou- tine associated with the requesting device. The PICU en- codes the requesting level into such information for use as a "vector" to the correct Interrupt Service Routine.

CPU·DRIVEN

MULTIPLEXOR

CPU

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RAM

ROM

Polled Method

CPU INT

RAM

ROM

Interrupt Method

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Intel 8080 manual Interrupts in Microcomputer Systems, Polled Method, Interrupt Method
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8080 specifications

The Intel 8085 and 8080 microprocessors were groundbreaking innovations in the world of computing, paving the way for future microprocessor development and personal computing.

The Intel 8080, introduced in 1974, was an 8-bit microprocessor that played a fundamental role in the early days of personal computing. With a 16-bit address bus, it had the capability to address 64 KB of memory. Running at clock speeds of 2 MHz, the 8080 was notable for its instruction set, which included 78 instructions and 246 opcodes. It supported a range of addressing modes including direct, indirect, and register addressing. The 8080 was compatible with a variety of peripherals and played a crucial role in the development of many early computers.

The microprocessor's architecture was based on a simple and efficient design, making it accessible for hobbyists and engineers alike. It included an 8-bit accumulator, which allowed for data manipulation and storage during processing. Additionally, the 8080 featured registers like the program counter and stack pointer, which facilitated program flow control and data management. Its ability to handle interrupts also made it suitable for multitasking applications.

The Intel 8085, introduced in 1976, was an enhancement of the 8080 microprocessor. It maintained a similar architecture but included several key improvements. Notably, the 8085 had a built-in clock oscillator, simplifying system design by eliminating the need for external clock circuitry. It also featured a 5-bit control signal for status line management, which allowed for more flexible interfacing with peripheral devices. The 8085 was capable of running at speeds of up to 3 MHz and had an extended instruction set with 74 instructions.

One of the standout features of the 8085 was its support for 5 extra instructions for stack manipulation and I/O operations, which optimized the programming process. Additionally, it supported serial communication, making it suitable for interfacing with external devices. Its 16-bit address bus retained the 64 KB memory addressing capability of its predecessor.

Both the 8080 and 8085 microprocessors laid the groundwork for more advanced microprocessors in the years that followed. They demonstrated the potential of integrated circuits in computing and influenced the design and architecture of subsequent Intel microprocessors. Their legacy endures in the way they revolutionized computing, making technology accessible to a broader audience, and their influence is still felt in the design and architecture of modern microprocessors today.
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