The events that take place during the T3 state are determined by the kind of machine cycle in progress. In a FETCH machine cycle, the processor interprets the data on its data bus as an instruction. During a MEMORY READ or a STACK READ, data on this bus is interpreted as a data word. The processor outputs data on this bus during a MEMORY WRITE machine cycle. During I/O operations, the processor may either transmit or receive data, de- pending on whether an OUTPUT or an INPUT operation is involved.

Figure 2-6 illustrates the timing that is characteristic of a data input operation. As shown, the low-to-high transi- tion of <P2 during T2 clears status information from the pro- cessor's data lines, preparing these lines for the receipt of incoming data. The data presented to the processor must have stabilized prior to both the "¢1-data set-up" interval (tDS1), that precedes the falling edge of the ¢1 pulse defin- ing state T3, and the "¢2-data set-up" interval (tDS2), that precedes the rising edge of <P2 in state T 3. This same

data must remain stable during the "data hold" interval (tDH) that occurs following the rising edge of the <P2 pulse. Data placed on these Iines by memory or by other external devices will be sampled during T3.

During the input of data to the processor, the 8080 generates a DBIN signal which should be used externally to enable the transfer. Machine cycles in which DB IN is avail- able include: FETCH, MEMORY READ, STACK READ, and INTERRUPT. DBIN is initiated by the rising edge of ¢2 during state T2 and terminated by the corresponding edge of <P2 during T3. Any TW phases intervening between T2 and T3 will therefore extend DBIN by one or more clock periods.

Figure 2-7 shows the timing of a machine cycle in wh ich the processor outputs data. Output data may be des- tined either for memory or for peripherals. The rising edge of <P2 within state T2 clears status information from the CPU's data lines, and loads in the data which is to be output to external devices. This substitution takes place within the

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NOTE; ® Refer to Status Word Chart on Page 2-6.

Figure 2-5. Basic 8080 Instruction Cycle

2-8

Page 22
Image 22
Intel 8080 manual Rr

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.