Status Bit Definitions | Intel MCS-80/85 guide

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

	STATUS INFORMATION DEFINITION
	Data Bus
Symbols	Bit	Definition
INTA*	DO	Acknowledge signal for INTE R R UPT re-
		quest. Signal should be used to gate a re-
		start instruction onto the data bus when
		DBIN is active.
	Dl	Indicates that the operation in the current
		machine cycle will be a WR ITE memory
		or OUTPUT function (WO = 0). Otherwise,
		a READ memory or INPUT operation will
		be executed.
STACK	D2	Indicates that the address bus holds the
		pushdown stack address from the Stack
		Pointer.
HLTA	D3	Acknowledge signal for HALT instruction.
OUT	D4	Indicates that the address bus contains the
		address of an output device and the data
		bus will contain the output data when
		WR is active.
M,	D5	Provides a signal to indicate that the CPU
		is in the fetch cycle for the first byte of
		an instruction.
INP*	De	Indicates that the address bus contains the
		address of an input device and the input
		data should be placed on the data bus
		when DBIN is active.
MEMR*	D7	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

/too 8080 STATUS LATCH

	D3	3
	D,
8080	D,	4
8080	D5	5
	D5	5
	~!
	~!	6

SYNC ~

,DBIN r22-

STATUS

LATCH

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SYNC

ST A TUS f ------ f . J' ----- 1

TYPE OF MACHINE CYCLE

INTA

STACK HLTA OUT M1 INP MEMR

DBIN


Do	INTA			000000010
	WO	1	1	0	1	0	1 0		1	1
	STACK	000			1	100			0	0	o
	HLTA	o	0	0	0	0	0	0	0	1
	OUT	o	0	000			0	1	0	0	o
		1	0	0	0	0	0	0	1	0

De	INP	o	0	000			1	000			o
	MEMR	110100001									o

Table 4-1. 8080 Status Bit Definitions

4-6

MCS-80/85 specifications

The Intel MCS-80/85 family, introduced in the late 1970s, is a seminal collection of microprocessors that played a pivotal role in the early days of computing. The MCS-80 series, initially targeting embedded systems and control applications, gained remarkable attention due to its innovative architecture and flexible programming capabilities.

The MCS-80 family is anchored by the 8080 microprocessor, which was one of the first fully integrated 8-bit microprocessors. Released in 1974, the 8080 operated at clock speeds ranging from 2 MHz to 3 MHz and featured a 16-bit address bus capable of addressing up to 64KB of memory. The processor’s instruction set included around 78 instructions, providing extensive capabilities for data manipulation, logic operations, and branching.

Complementing the 8080 was a suite of support chips, forming the MCS-80 platform. The most notable among them was the 8155, which integrated a static RAM, I/O ports, and a timer, tailored for ease of designing systems around the 8080. Other support chips included the 8085, which provided improvements with an integrated clock generator, making it compatible with more modern designs and applications.

The MCS-85 series, on the other hand, revolves around the 8085 microprocessor, which provided a more advanced architecture. The 8085 operated at clock speeds of up to 6 MHz and came with a 16-bit address bus, similar to its predecessor. However, it introduced more sophisticated features, including an enhanced instruction set and support for interrupt-driven programming. These enhancements made the 8085 especially appealing to developers working in real-time processing environments.

The MCS-80/85 family utilized NMOS technology, known for its lower power consumption and higher performance compared to previous technologies like TTL. The family’s architecture allowed for easy interfacing with a variety of peripherals, making it a favorite for educational institutions and hobbyists embarking on computer engineering projects.

With its robustness, versatility, and affordability, the Intel MCS-80/85 microprocessors laid the groundwork for many subsequent microcomputer systems and applications. The legacy of this powerful family continues to influence modern microprocessor design, emphasizing the importance of reliable architecture in a rapidly evolving technology landscape.

Intel MCS-80/85 manual Status Bit Definitions

Models: MCS-80/85

INTA*

OUT

INP*

MEMR*

~ g~; MO

Table 4-1. 8080 Status Bit Definitions

MCS-80/85 specifications