Instruction Categories; Data Transfer Instructions; Arithmetic Instructions

Instruction Categories

8085 instructions are classified into five major groups:

Category	Affects flags?	Purpose
Data Transfer	No	Copy data without modifying source
Arithmetic	Yes	Add, subtract, increment, decrement
Logical	Yes	AND OR, XOR, compare, rotate
Branch	No	Change program sequence (conditional/unconditional)
Machine Control	No	Halt, interrupt enable/disable, NOP

Data Transfer Instructions

Data transfer instructions copy data — they do NOT modify the source and do NOT affect flags.

Mnemonic	Format	Description	Example
MVI	MVI Rd, data	Move immediate 8-bit to register	MVI A, 50H
MOV	MOV Rd, Rs	Copy register to register	MOV B, A
MOV	MOV Rd, M / MOV M, Rs	Via memory pointer HL	MOV A, M
LXI	LXI Rp, 16-bit	Load immediate to register pair	LXI H, 2050H
LDA	LDA addr	Load accumulator direct	LDA 2005H
STA	STA addr	Store accumulator direct	STA 3000H
LDAX	LDAX B/D	Load A indirect via BC/DE	LDAX B
STAX	STAX B/D	Store A indirect via BC/DE	STAX D
LHLD	LHLD addr	Load HL pair direct from memory	LHLD 2050H
SHLD	SHLD addr	Store HL pair direct to memory	SHLD 2050H
XCHG	XCHG	Exchange DE and HL contents	XCHG
IN	IN port	Read from I/O port to A	IN 10H
OUT	OUT port	Write A to I/O port	OUT 05H

Arithmetic Instructions

Mnemonic	Description	Example
ADD	Add register/M to A	ADD B
ADI	Add immediate to A	ADI 50H
ADC	Add with carry	ADC C
SUB	Subtract register/M from A	SUB B
SUI	Subtract immediate from A	SUI 34H
SBB	Subtract with borrow	SBB D
INR	Increment register/M by 1	INR C
DCR	Decrement register/M by 1	DCR C
INX	Increment register pair by 1	INX H
DCX	Decrement register pair by 1	DCX B
DAD	Add register pair to HL	DAD D

Note: INR and DCR do NOT affect the carry flag — an important exam point.

Data Transfer Details

Data-transfer instructions copy data without conceptually changing the source. Register transfers use MOV. Immediate instructions such as MVI place a constant in a register or memory. LXI loads a 16-bit constant into a register pair. Direct instructions include LDA and STA. LDAX and STAX use the address in BC or DE. LHLD and SHLD transfer two consecutive memory bytes with the low byte at the lower address.

Most data-transfer instructions do not change flags. This distinction matters because a later conditional branch may still test flags produced by an earlier arithmetic or logical instruction.

Arithmetic Details

ADD and ADC add a register or memory value to the accumulator, with ADC including carry. ADI and ACI use immediate data. Subtraction instructions set carry to represent borrow according to 8085 convention. INR and DCR change an 8-bit operand without changing carry. INX and DCX change a register pair and do not affect flags.

DAD adds a register pair to HL and affects carry. DAA adjusts the accumulator after adding packed BCD digits by applying corrections to the lower and upper nibbles according to auxiliary carry, carry and digit values. Signed overflow is not represented by a dedicated 8085 flag and must be inferred when signed arithmetic is required.