Instruction type ADD R in 8085 Microprocessor


In 8085 Instruction set, ADD R is a mnemonic that stands for “Add contents of R to Accumulator”. As addition is a binary operation, so it requires two operands to be operated on. So input operands will reside on Accumulator and R registers and after addition the result will be stored back on to Accumulator. In this case, “R” stands for any of the following registers or memory location M pointed by HL pair.

R = A, B, C, D, E, H, L, or M

It is 1-Byte instruction so occupies only 1-Byte in memory. As R can have any of the eight values, there are eight opcodes for this type of instruction.

Mnemonics, Operand Opcode(in HEX) Bytes
ADD A 87 1
ADD B 80 1
ADD C 81 1
ADD D 82 1
ADD E 83 1
ADD H 84 1
ADD L 85 1
ADD M 86 1

As example of this category, let us consider the instruction ADD E. It is a 1-Byte instruction. Let us consider that Accumulator is having initial content 02H and E register is having initial content 03H. so after summation 05H will remain on the Accumulator updating its previous content. Here is the tracing table for us to have better understanding.

Before After
(E) 03H 03H
(A) 02H 05H

Address Hex Codes Mnemonic Comment
2004 83 ADD E Accumulator = Accumulator + Register E

Here is the timing diagram of the instruction ADD E execution.


Summary − So this instruction ADD E requires 1-Byte, 1 Machine Cycles (Opcode Fetch) and 4 T-States for execution as shown in the timing diagram.

Published on 30-Nov-2018 12:34:29