Status Bit Definitions, Status Information Definition

Instructions for the 8080 require from one to five machine cycles for complete execution. The 8080 sends out 8 bit of status informatton on the data bus at the beginning of each machine cycle (during SYNC time). The following table defines the status information.

	STATUS INFORMATION DEFINITION
Symbols	Data Bus
Symbols	Bit	Definition
INTA*	00	Acknowledge signal for INTERRUPT re-
		quest. Signal should be used to gate a re-
		start instruction onto the data bus when
		OBIN is active.
	0,	Indicates that the operation in the current
		machine cycle will be a WR ITE memory
		or OUTPUT function (WO = O).Otherwise,
		a REAO memory or INPUT operation will
	°2	be executed.
STACK		Indicates that the address bus holds the
		pushdown stack address from the Stack
	°3	Pointer.
HLTA	°3	Acknowledge signal for HALT instruction.
HLTA	°4	Acknowledge signal for HALT instruction.
OUT		Indicates that the address bus contains the
		address of an output device and the data
		bus will contain the output data when
M,	°5	WR is active.
M,		Provides a signal to indicate that the CPU
		is in the fetch cycle for the first byte of
	°6	an instruction.
INP*		Indicates that the address bus contains the
		address of an input device and the input
		data shou Id be placed on the data bus
		when OB IN is active.
MEMR*	0 7	Designates that the data bus will be used
		for memory read data.

*These three status bits can be used to control the flow of data onto the 8080 data bus.

STATUS WORD CHART

8080 STATUS LATCH

o10

2 7

		0 3		3
8080		0	4	4
8080		0 5		5
		0 5		5
		~6		6
			7
	SYNC -19
	OBIN ~
01	02					STATUS
						STATUS
22		15				LATCH
22		15			--2. 0 1
					--2. 0 1			DO	~ INTA
					5				~ W6
					~				~ STACK
					9				~ STACK
					9				~ HlTA
					16	8212			.lL. OUT
					18				.lL. OUT
					18				17	M1
				--	20				JL INP
CLOCK GEN.				--	22				JL INP
CLOCK GEN.				(olTTl)	r;;				~ MEMR
				~			I	I
						OS2 MO		OS,
& DRIVER						CLR
						13	12	Y1

DBIN

SYNC

OATA 1------'' --- t -- t --

STATUS ......-.----+001'----1

TYPE OF MACHINE CYCLE

DO INTA

02STACK

03HLTA

04OUT

06 INP

07MEMR

Table 2-1. 8080 Status Bit Definitions

2-6

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.

Intel 8080 manual Status Bit Definitions, Status Information Definition, Status Word Chart

Models: 8080

STATUS INFORMATION DEFINITION

INTA*

OUT

INP*

MEMR*

STATUS WORD CHART

JL INP

Table 2-1. 8080 Status Bit Definitions

8080 specifications