This chapter will illustrate, in detail, how to interface the 8080 CPU with Memory and I/O. It will also show the benefits and tradeoffs encountered when using a variety of system architectures to achieve higher throughput, de- creased component count or minimization of memory size.

8080 Microcomputer system design lends itself to a simple, modular approach. Such an approach will yield the designer a reliable, high performance system that contains a minimum component count and is easy to manufacture and maintain.

The overall system can be thought of as a simple block diagram. The three (3) blocks in the diagram repre- sent the functions common to any computer system.

CPU Module* Contains the Central Processing Unit, system timing and interface circuitry to Memory and I/O devices.

Memory Contains Read Only Memory (ROM) and Read/Write Memory (RAM) for program and data storage.

I/OCon"tains circuitry that allows the computer system to communicate with devices or structures existing outside of the CPU or Memory array.

for example: Keyboards, Floppy Disks, Paper Tape, etc.

There are three busses that interconnect these blocks:

Data Bust A bi-directional path on which data can flow between the CPU and Memory or I/O.

Address Bus A uni-directional group of lines that identify a particular Memory location or I/O device.

*"Module" refers to a functional block, it does not ref- erence a printed circuit board manufactured by INTE L.

t"Bus" refers to a set of signals grouped together because of the similarity of their functions.

Control Bus A uni-directional set of signals that indicate the type of activity in current process.

Type of activities: 1. Memory Read 2. Memory Write 3. I/O Read

4. I/O Write

5. Interrupt Acknowledge

CPU

MODULE

Figure 3-1. Typical Computer System Block Diagram

Basic System Operation

1.The CPU Module issues an activity command on the Control Bus.

2.The CPU Module issues a binary code on the Address Bus to identify which particular Memory location or I/O device will be involved in the current process activity.

3.The CPU Module receives or transmits data with the selected Memory location or I/O device.

4.The CPU Module returns to CD and issues the next activity command.

It is easy to see at this point that the CPU module is the central element in any computer system.

3-1

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Intel 8080 manual Basic System Operation, Typical Computer System Block Diagram
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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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