To support these bit-wise operations the data memory of the 8051 (20H-2FH) is both bit and byte-addressable as we saw in the post on general memory structure of 8051. Moreover, a number ofspecial function registers in the RAM memory space are bit addressable which makes the life of the programmer so much easier.

The CY flag of the PSW register can be considered ground zero for all the bit-wise operations accounting for its extensive use during boolean operations in 8051.

Now that we have seen the rest of the instruction groups in this8051 course, we will specifically single out and enlist all of them that classify underboolean/bit-wise operations and are capable of working on single bits of data.

Let’s take a more in-depth look into each of these bit-wise operations.

Clear operation

The clear operation is used to clear the data bit at a particular address or in the carry flag.

An explanation for the size of an instruction

Any instruction in the 8051 microcontroller consists of two parts; an opcode and operand. As the 8051 has an 8-bit architecture each opcode is 8 bit in size (1 byte) but the size of instructions increases due to the size of the operands. In some cases, operands take no space whereas in some they can take up to 2 bytes of space (See the post onaddressing modes in 8051 for more details on the cases where instruction size increases due to operands.)

Let us look at an example to understand the above-mentioned concept.

The instruction CLR C is one byte in size as the opcode itself is enough to describe the instruction. But in the case of CLR 02H, the microcontroller needs two bytes to store the instruction as the opcode takes 1 byte and the address takes another.

In some cases this size increases to 3 bytes, this happens in the case of jump instructions where a relative address is also given to the microcontroller.

Examples

CLR C; Clears the data in the CY flag of the PSW registerCLR P1.7; Clears the bit at port 1.7

Complement instruction

Examples

CPL C; Complements the value stored in the carry flagCPL TF1; Complements the TF1 flag inTCON register

Set operation

SETB C; Sets the value in the carry flag to 1SETB TR1; Sets the value of TR1 flag in TCON register to 1(used to start the timer)

AND Operation

Examples

ANL C, 09HANL C, /05H

OR Operation

Examples

ORL C, 09HORL C, /07H

MOV Operation

Examples

MOV C, 09HORL 06H, C

Jump Operations

Any computing device performs operations in a sequential manner but in some cases, operations need to be performed in the form of a branch. There are a number of jump instructions which are used to perform this kind of branching, let us have a look at them.

Example

        MOV  TMOD , #10H ;The value 10H is 00010000B and selects mode 1 of timer 1        MOV R3 , #200H   ;used as a counter to create a delay of 14 secondsAGAIN : MOVTL1 , #08H    ;sets the value of 08H in the lower bit of timer 1 and the again keyword helps to create a loop.        MOV TH1 , #01H   ;sets the value of 01H in the lower bit of timer 1        SETB TR1         ;turns on the timerBACK:   JNB   TF1, BACK  ;checks the TF1 flag in TCON register continuously to know if overflow has occurred or not        CLR   TR1        ;clears TR1 flag to stop the timer        CLR   TF1        ;clears overflow bit as the timer is resetDJNZ    R3,  AGAIN       ;keeps looping the statements after again keyword till the value in R3 reduces to 0. DJNZ command decreases the value in r3 and checks if it zero

About the author

Nischay Khanna

An Engineering Graduate with a specialization in Embedded systems, who loves to dabble with IoT automation cause switching lights on with switches is to mainstream. A bibliophile who loves to travel and experience new cultures.