8085 Microprocessor

Intel 8085

• Intel 8085 Pin Configuration

The 8085 and Its Buses

• The 8085 is an 8-bit general purpose microprocessor that can address 64K Byte of memory.

• It has 40 pins and uses +5V for power. It can run at a maximum frequency of 3 MHz.

• The pins on the chip can be grouped into 6 groups:

• Address Bus.

• Data Bus.

• Control and Status Signals.

• Power supply and frequency.

• Externally Initiated Signals.

• Serial I/O ports.

The Address and Data Bus Systems• A8-A15(output): The address bus has 8 signal lines A8 – A15

which are unidirectional.These are address bit 8-bits used for the most significant bits of memory address.

• AD0-AD7(input/output): these are the address/data bus.i.e. they can be used for both address as well as 8-bit data. So, the bits AD0 – AD7 are bi-directional and serve as A0 – A7 and D0 –D7 at the same time.During the execution of the instruction, these lines carry the address bits during the early part, then during the late parts of the execution, they carry the 8 data bits.

The Control and Status Signals• There are 4 main control and status signals. These are:

• ALE(output): Address Latch Enable. This signal is a pulse that become 1 when the AD0 – AD7 lines have an address on them. It becomes 0 after that. This signal can be used to enable a latch to save the address bits from the AD lines.

• IO/M(output): This signal specifies whether the operation is a memory operation (IO/M=0) or an I/O operation (IO/M=1).

• S1 and S0(output) :These are the Status signals to specify the kind of operation being performed. Usually not used in small systems.

S1 S0 output

0 0 halt

0 1 write

1 0 read

1 1 fetch

RD: Read(output).When this signal low theselected memory or I/O devices is read.( Activelow)

WR: Write(output): When it goes low the dataon the data bus is written into the selectedmemory or I/O location

• READY (Input): It is used by the microprocessor to see whether a peripheral (I/O Device) is ready to transfer data or not. If the signal is high the peripheral is ready. If it is low the microprocessor waits till its goes high.

• HOLD(Input):This signal is used during DMA(Direct memory Access) Operation. This Signal indicates that another device is requesting the use of address and data bus.So this device gives hold signal to the microprocessor .

• HLDA(Output):It is signal for HOLD acknowledgement. It indicates that the HOLD request has been received.

• INTR(Input):It is an interrupt request signal. An interrupt is used by I/O devices to transfer data to microprocessor.

• INTA(Output): It is an interrupt acknowledgement sent by the microprocessor after INTR is received.

• RST 5.5,6.5,7.5 and TRAP(Inputs).These are Interrupts.

• RESET IN(Output): It resets the program counter to zero.

• RESET OUT(Output): It indicates that CPU is being reset.

• X1,X2(Input):These are terminals to an external crystal oscillator to produce clock signal for the operation of microprocessor

• CLK(Output):It is a clock output, which can beused for other digital ICs.(Integrated Circuits)

• SID(Input): It is a data line for serialtransmission of data to microprocessor.

• SOD(Output): It is a data line for serialtransmission of data From themicroprocessor.

• Vcc+5V Supply(d.c.)

• Vss Ground

General Purpose Registers

• 8085 consists of 6 special types of registers called General Purpose Registers. These General Purpose Registers are used to hold data like any other registers .

• The GPR in 8085 processors are B,C,D,E,H,L.Each registers can hold 8-bit data. They can work in pair such as B-C,D-E,H-L to store 16-bit data.

Temporary Register

This register acts as a temporary memory during the arithmetic and logical operations. Unlike other registers, this temporary registers can only be accessed by the microprocessor and it is completely inaccessible to programmers. Temporary register is an 8-bit register.

PSW

• The combination of these 8-bits is called Program Status Word(PSW).PSW and the accumulator as a 16-bit unit for the stack operation.

Architecture of 8085

8085

ALU

The ALU performs the arithmetic and logicaloperations. The operations performed by ALUof 8085 are addition, subtraction, increment,decrement, logical AND, OR, EXCLUSIVE -OR,compare, complement and left / right shift .

Accumulator and temporary register • The accumulator and temporary register are

used to hold the data during an arithmetic /logical operation. After an operation theresult is stored in the accumulator and theflags are set or reset according to the result ofthe operation.

TIMING & CONTROL UNIT

The timing and control unit synchronizes all the

microprocessor operations with the clock and

generates the control signals necessary for

communication between the microprocessor

and peripherals.

INSTRUCTION REGISTER & DECODER• When an instruction is fetched from memory it

is placed in instruction register. Then it is decoded and encoded into various machine cycles.

STACK POINTER (SP)• It is also a 16-bit register which is used as a memory

pointer.

• It maintains the address of the last byte that isentered into stack.

PROGRAM COUNTER (PC)

• The program counter (PC) keeps track ofprogram execution.

• It is a special purpose register.

• A program counter stores the address of thenext instruction to be executed.

FLAG REGISTER

There are five flags in 8085, which are sign flag

(8), zero flag (Z), auxiliary carry flag (AC), parity

flag (P) and carry flag (CY). The bit positions

reserved for these flags in the flag register are

shown in figure below.

The Flags register• There is also a flag register whose bits are affected by

the arithmetic & logic operations.

• S-sign flag

• The sign flag is set if bit D7 of theaccumulator is set after an arithmetic or logicoperation.

• Z-zero flag

• Set if the result of the ALU operation is 0.Otherwise is reset. This flag is affected byoperations on the accumulator as well as otherregisters. (DCR B).

• AC-Auxiliary Carry

• This flag is set when a carry is generated frombit D3 and passed to D4 . This flag is usedonly internally for BCD operations.

• P-Parity flag

• After an ALU operation, if the result has an even #of 1s, the p-flag is set. Otherwise it is cleared. So,the flag can be used to indicate even parity.

• CY-carry flag

• This flag is set when a carry is generated from bitD7 after an unsigned operation.

• OV-Overflow flag

• This flag is set when an overflow occurs after asigned operation.

Unspecified

7 6 5 4 3 2 1 0SF ZF X AF X PF X CF

General Purpose Registers

• 8085 consists of 6 special types of registers calledGeneral Purpose Registers. These General PurposeRegisters are used to hold data like any otherregisters .

• The GPR in 8085 processors are B,C,D,E,H,L.Eachregisters can hold 8-bit data.

• They can work in pair such as B-C,D-E,H-L to store16-bit data.

Temporary Register

This register acts as a temporary memory during

the arithmetic and logical operations.

Unlike other registers, this temporary registers

can only be accessed by the microprocessor and

it is completely inaccessible to programmers.

Temporary register is an 8-bit register.

PSW

• The combination of these 8-bits is calledProgram Status Word(PSW).PSW and theaccumulator as a 16-bit unit for the stackoperation.