Chapter: Microprocessor and Microcontroller

8051 Instructions

8051 has about 111 instructions. These can be grouped into the following categories 1. Arithmetic Instructions 2. Logical Instructions 3. Data Transfer instructions 4. Boolean Variable Instructions

8051 Instructions


8051 has about 111 instructions. These can be grouped into the following categories


1.     Arithmetic Instructions

2.     Logical Instructions

3.     Data Transfer instructions

4.     Boolean Variable Instructions


5.     Program Branching Instructions

The following nomenclatures for register, data, address and variables are used while write instructions.


A: Accumulator


B: "B" register


C: Carry bit


Rn: Register R0 - R7 of the currently selected register bank


Direct: 8-bit internal direct address for data. The data could be in lower 128bytes of RAM (00 - 7FH) or it could be in the special function register (80 - FFH).


@Ri: 8-bit external or internal RAM address available in register R0 or R1. This is used for indirect addressing mode.


#data8: Immediate 8-bit data available in the instruction.


#data16: Immediate 16-bit data available in the instruction.


Addr11: 11-bit destination address for short absolute jump. Used by instructions AJMP & ACALL. Jump range is 2 kbyte (one page).


Addr16: 16-bit destination address for long call or long jump.


Rel: 2's complement 8-bit offset (one - byte) used for short jump (SJMP) and all conditional jumps.


bit: Directly addressed bit in internal RAM or SFR


Arithmetic Instructions

Example programs


Character transmission using a time delay


A program shown below takes the character in 'A' register, transmits it, delays for transmission time, and returns to the calling program. Timer-1 is used to set the baud rate, which is 1200 baud in this program


The delay for one character transmission (in Mode 1 i.e.10 bits) is 10/2400 = 0.00833 seconds


Or, 8.33 milliseconds

Hence software delay of 10ms is used.

Timer-1 generates a baud rate close to 1200. Using a 12MHz crystal, the reload value is

This gives rise to an actual baud rate of 1202.

SMOD is programmed to be 0.


Assembly language Program is as follows


; Code to wait for the transmission to complete


The subroutine TRMITTIME generates a delay of about 10ms. With a clock of 12MHz, one instruction cycle time is

The loop "MILSEC" generates a delay of about 1 x 10-3 sec. This gets executed 10 times for a total delay of 10 x 10-3 sec or 10ms

Interrupt driven character transmission


In 8051, when a character is transmitted, SBUF register becomes empty and this generates a serial port interrupt (TI). TI and RI both point to the vector location 0023H in the program memory. An interrupt service routine can be written at 0023H to send the next character.


A program is written here to transmit a character say 'A' continuously based on interrupt. The microcontroller uses a clock of 12MHz with a baud rate of 1202. The program is executed following a hardware reset.


Assembly language program is as follows.

Interrupt driven data reception


When a character is received, if receive mode is enabled, RI flag is set. This leads to the interruption of the main program and the processor goes to the interrupt vector location, i.e.

0023H for serial port. The interrupt service routine at 0023H gets executed to read the character so that the next character can be received. The following program receives a character on interrupt basis and outputs the character to port-1, possibly for a display. The crystal frequency is12MHz and baud rate is set at 1202 baud.


Assembly language program is as follows





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