An instruction is a binary pattern designed inside a microprocessor to perform a specific function. The entire group of instructions, called the instruction set, determines what functions the microprocessor can perform. These instructions can be classified into the following five functional categories: data transfer (copy) operations, arithmetic operations, logical operations, branching operations, and machine-control operations.

Data Transfer Croup

The data transfer instructions move data between registers or between memory and registers.

MOV Move

MVI Move Immediate

LDA Load Accumulator Directly from Memory STA Store Accumulator Directly in Memor

LHLD Load H & L Registers Directly from Memory SHLD Store H & L Registers Directly in Memory

An 'X' in the name of a data transfer instruction implies that it deals with a register pair (16-bits);

LXI Load Register Pair with Immediate data

LDAX Load Accumulator from Address in Register Pair STAX Store Accumulator in Address in Register Pair XCHG Exchange H & L with D & E

XTHL Exchange Top of Stack with H & L

Arithmetic Group

The arithmetic instructions add, subtract, increment, or decrement data in registers or memory.

ADD Add to Accumulator

ADI Add Immediate Data to Accumulator ADC Add to Accumulator Using Carry Flag

ACI Add Immediate data to Accumulator Using Carry SUB Subtract from Accumulator

SUI Subtract Immediate Data from Accumulator

SBB Subtract from Accumulator Using Borrow (Carry) Flag

SBI Subtract Immediate from Accumulator Using Borrow (Carry) Flag INR Increment Specified Byte by One

DCR Decrement Specified Byte by One INX Increment Register Pair by One DCX Decrement Register Pair by One

DAD Double Register Add; Add Content of Register Pair to H & L Register Pair

Logical Group

This group performs logical (Boolean) operations on data in registers and memory and on condition flags.

The logical AND, OR, and Exclusive OR instructions enable you to set specific bits in the accumulator ON or OFF.

ANA Logical AND with Accumulator

ANI Logical AND with Accumulator Using Immediate Data ORA Logical OR with Accumulator

OR Logical OR with Accumulator Using Immediate Data XRA Exclusive Logical OR with Accumulator

XRI Exclusive OR Using Immediate Data

The Compare instructions compare the content of an 8-bit value with the contents of the accumulator;

CMP Compare

CPI Compare Using Immediate Data

The rotate instructions shift the contents of the accumulator one bit position to the left or right:

RLC Rotate Accumulator Left RRC Rotate Accumulator Right RAL Rotate Left Through Carry RAR Rotate Right Through Carry

Complement and carry flag instructions:

CMA Complement Accumulator CMC Complement Carry Flag STC Set Carry Flag

Branch Group

The branching instructions alter normal sequential program flow, either unconditionally or conditionally. The unconditional branching instructions are as follows:

JMP Jump CALL Call RET Return

Conditional branching instructions examine the status of one of four condition flags to determine whether the specified branch is to be executed. The conditions that may be specified are as


NZ Not Zero (Z = 0) Z Zero (Z = 1)

NC No Carry (C = 0) C Carry (C = 1)

PO Parity Odd (P = 0) PE Parity Even (P = 1) P Plus (S = 0)

M Minus (S = 1)

Thus, the conditional branching instructions are specified as follows: Jumps Calls Returns

C CC RC (Carry)

INC CNC RNC (No Carry) JZ CZ RZ (Zero)

JNZ CNZ RNZ (Not Zero) JP CP RP (Plus)

JM CM RM (Minus)

JPE CPE RPE (Parity Even) JP0 CPO RPO (Parity Odd)

Two other instructions can affect a branch by replacing the contents or the program counter:

PCHL Move H & L to Program Counter RST Special Restart Instruction Used with Interrupts

POP Pop Two Bytes of Data off the Stack XTHL Exchange Top of Stack with H & L SPHL Move content of H & L to Stack Pointer I/0 instructions

IN Initiate Input Operation OUT Initiate Output Operation Machine Control instructions EI Enable Interrupt System

DI Disable Interrupt System HLT Halt

NOP No Operation