8085 Microprocessor Basics, 8085 Microprocessor tutorial

Although nowadays we use microcontrollers such as 8051 and 8052 which are much faster & better than 8085 microprocessor, we must still learn the basics of 8085 so as to get an idea of microprocessor's instruction and programming basics.

The 8085 microprocessor family can address 64K bytes of memory which is used for both code space and data space.

Memory is accessed by 20 pins. 8 address high pins and 8 pins are used both for the 8 address low signals and for the 8 data signals.4 pins are used for control purpose.

8085 requires only a +5V power supply. This microprocessor has seven 8-bit registers named A, B, C, D, E, H, and L, where A is the 8-bit accumulator and the other six can be used as independent byte-registers or as three 16-bit register pairs, BC, DE, and HL.
Accumulator is an 8-bit register that is part of the arithmetic/logic unit (ALU). This register is used to store 8-bit data and to perform arithmetic and logical operations. The result is stored in the accumulator. The accumulator is also called register A.

Stack pointer is a 16 bit register.

This register is always incremented/decremented by 2.

Program counter is a 16-bit register.
Instruction set of Intel 8085 microprocessor:

Data moving instructions. Arithmetic - add, subtract, increment and decrement.
Logic - AND, OR, XOR and rotate. Control transfer - conditional, unconditional, call subroutine, return from subroutine and restarts. Input/Output instructions. Other - setting/clearing flag bits, enabling/disabling interrupts, stack operations, etc.

READY (pin 35) allows memory or I/O access to slow down the data transfer.

HOLD (pin 39) allows peripheral hardware to take over the memory bus.

The 8085 Microprocessor has 5 interrupts.

INTR is maskable 8080A compatible interrupt. .

RST5.5 is a maskable interrupt.

RST6.5 is a maskable interrupt.

RST7.5 is a maskable interrupt.

Trap is a non-maskable interrupt.

All maskable interrupts can be enabled or disabled using EI and DI instructions. RST 5.5, RST6.5 and RST7.5 interrupts can be enabled or disabled individually using SIM instruction.

Addressing modes in 8085:

Register Addressing mode:

- tells about the data in a register or in a register pair.

Register indirect Addressing mode

- instruction specifies register pair containing address, where the data is located.

Direct Addressing mode. & Immediate Addressing mode

- 8 or 16-bit data.

The 8085 microprocessor has 5 flags as stated below:

Zero flag: The zero flag is set, when the ALU operation results a zero .

Carry flag: If an arithmetic operations results in a carry, this flag is set.

Parity flag: This flag is set, when an arithmetic or logical operation results in a data, which has even number of 1s. If otherwise, it is reset.

Sign flag: After performing arithmetic or logic operations, if D7 bit of the accumulator is 1, it indicates a negative number and this flag is set. If otherwise, it is reset.

Auxiliary Carry flag: used for BCD Operations, If D3 bit produces a carry then the AC bit set as1, otherwise the bit is 0.

Carry Flag: when a carry is generated by digit D7, then the carry flag set as 1, otherwise the bit will be 0.

8085 Microprocessor Program to Multiply two 8 bit no.s

LDA 2200H
            MOV E, A

            MVI D, 00

            LDA 2201H

            MOV C, A

            LXI H, 0000H

BACK:            DAD D

            DCR C

            JNZ BACK

            SHLD 2300H

            HLT

8085 Microprocessor Tutorial

8085 Pin Diagram

Addressing modes in 8085:

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