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Boolean (bitwise) instructions in 8051 for bit manipulation

Most microcontrollers frequently deal with bits of data rather than bytes. There are many occasions where performing a particular operation in a bit of data in a memory location is more convenient than operating on the entire byte. It allows designers to be more resource optimum and reduce unnecessary overhead.

The MCS-51 was one of the first microcontrollers to have a dedicated boolean architecture which was responsible for making it exceptionally fast when it came to processing a single bit.

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 of special 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 this 8051 course, we will specifically single out and enlist all of them that classify under boolean/bit-wise operations and are capable of working on single bits of data.

[C]= Carry flag in PSW register;

Operation Mnemonics Description
Clear
CLR C [CY]<-0
CLR Address [Address]<-0
Complement
CPL C [CY]<-[CY]’
CPL Address [Address]<-[Address]’
Setting value
SETB C [CY]<-1
SETB Address [Address]<-1
AND
ANL C, Address [CY]<-[CY] AND [Address]
ANL C, /Address [CY]<-[CY] AND [Address]’
OR
ORL C, Address [CY]<-[CY] OR [Address]
ORL C, /Address [CY]<-[CY] OR [Address]’
Move
MOV C, Address [CY]<-[Address]
MOV Address,C [Address]<-[CY]
Jump
JC Address Jump to address if [C]=1
JNC Address Jump to address if [C]=0
JNB Address, Address Jump to destination address if source address =0
JB Address, Address Jump to destination address if source address =1
JBC Address, Address Jump to destination address if source address =1 and sets carry flag to 0

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.

Opcode
Operand
Description
Size
Execution Time
Flags affected
Carry Overflow Auxilary carry
CLR
C Clears the data stored in the CY flag of the accumulator 1 byte 12 clock cycles set to 0 Unaffected Unaffected
Address Clears the data bit stored at a particular address 2 bytes 12 clock cycles Unaffected Unaffected Unaffected

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 on addressing 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 register
CLR P1.7; Clears the bit at port 1.7

Complement instruction

Opcode
Operand
Description
Size
Execution Time
Flags affected
Carry Overflow Auxilary carry
CPL
C Complements the data stored in the CY flag of the accumulator 1 byte 12 clock cycles Affected Unaffected Unaffected
Address Complements the data bit stored at a particular address 2 bytes 12 clock cycles Unaffected Unaffected Unaffected

Examples

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

Set operation

Opcode
Operand
Description
Size
Execution Time
Flags affected
Carry Overflow Auxilary carry
SETB
C Sets the data stored in the CY flag of the PSW register to 1 1 byte 12 clock cycles 1 Unaffected Unaffected
Address Sets the data bit stored at a particular address to 1 2 bytes 12 clock cycles Unaffected Unaffected Unaffected
SETB C; Sets the value in the carry flag to 1
SETB TR1; Sets the value of TR1 flag in TCON register to 1(used to start the timer)

AND Operation

Opcode
Operand
Description
Size
Execution Time
Flags affected
Carry Overflow Auxilary carry
ANL
C, Address Performs AND operation between the data stored in the carry flag and the given address 2 byte 24 clock cycles Affected Unaffected Unaffected
C, /Address Performs AND operation between the data stored in the carry flag and the complement of the data at any given address 2 bytes 24 clock cycles Unaffected Unaffected Unaffected

Examples

ANL C, 09H
ANL C, /05H

OR Operation

Opcode
Operand
Description
Size
Execution Time
Flags affected
Carry Overflow Auxilary carry
ORL
C, Address Performs OR operation between the data stored in the carry flag and the given address 2 byte 24 clock cycles Affected Unaffected Unaffected
C, /Address Performs OR operation between the data stored in the carry flag and the complement of the data at any given address 2 bytes 24 clock cycles Unaffected Unaffected Unaffected

Examples

ORL C, 09H
ORL C, /07H

MOV Operation

Opcode
Operand
Description
Size
Execution Time
Flags affected
Carry Overflow Auxilary carry
MOV
C, Address Moves data from a particular address to the carry flag 2 byte 12 clock cycles Unaffected Unaffected Unaffected
Address,C Moves data from the carry flag to a particular address 2 bytes 24 clock cycles Unaffected Unaffected Unaffected

Examples

MOV C, 09H
ORL 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.

Opcode
Operand
Description
Size
Execution Time
Flags affected
Carry Overflow Auxilary carry
JC Address/label This instruction transfers the control to the specified address/label if the carry flag has the value 1. 2 bytes 24 clock cycles Unaffected Unaffected Unaffected
JNC Address/label This instruction transfers the control to the specified address/label if the carry flag has the value 0. 2 bytes 24 clock cycles Unaffected Unaffected Unaffected
JNB Address, Address This instruction transfers the control to the destination address if the source address has the bit 0. 3 bytes 24 clock cycles Unaffected Unaffected Unaffected
JB Address, Address This instruction transfers the control to the destination address if the source address has the bit 1. 3 bytes 24 clock cycles Unaffected Unaffected Unaffected
JBC Address, Address This instruction transfers the control to the destination address if the source address has the bit 1. This instruction clears the carry flag as well 3 bytes 24 clock cycles Unaffected Unaffected Unaffected

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 seconds
AGAIN : 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 timer
BACK:   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 reset
DJNZ    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

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