Arithmetic & logical operations in 8051

Subject Code :151001
Name Of Subject :Microcontroller & Interfacing
Name of Unit :Arithmetic& logical instructions
Topic :Arithmetic & logical operations
Name of Faculty : Mr. Haresh Suthar
Miss. Madhuri Thakkar
Name of Students : (i) Savalia Avani(100870111020)
(ii) Patel Jay (100870111021)

Arithmetic Instruction :
There are 24 arithmetic opcodes which are grouped
into the following types: ADD and ADDC
SUBB
MUL
DIV
INC
DEC
DA

Sub: MC Topic: Arithmetic & Logical operations

Arithmetic Flags
Flag: It is a 1-bit register that indicates the
status of the result from an operation

Flags are either at a flag-state of value 0 or 1

Arithmetic flags indicate the status of the
results from mathematical operations ( +, −,
*, / )


Arithmetic Flags (Conditional Flags)
There are 4 arithmetic flags in the 8051
Carry (C)
Auxiliary Carry (AC)
Overflow (OV)
Parity (P)

All the above flags are stored in the Program
Status Word (PSW)

CY AC -- RS1 RS0 0V -- P
PSW.7 PSW.6 PSW.5 PSW.4 PSW.3 PSW.2 PSW.1 PSW.0

Arithmetic Flags (Conditional Flags)

CY PSW.7 Carry flag
 AC PSW.6 Auxiliary carry flag
 -- PSW.5 Available to the user for general purpose
 RS1 PSW.4 Register Bank selector bit 1
 RS0 PSW.3 Register Bank selector bit 0
 0V PSW.2 Overflow flag
 -- PSW.1 User definable flag
 P PSW.0 Parity flag

The C flag is keeping track in unsigned operations
The OV flag is keeping track in signed operations


Instructions that Affecting Flags (1/2)

Instruction Mnemonic Flags Affected
ADD C AC OV
ADDC C AC OV
SUBB C AC OV
MUL C=0 OV
DIV C=0 OV
DA A C
SETB C C=1
MOV C, bit C

Instructions that Affecting Flags (2/2)

Instruction Mnemonic Flags Affected
ORL C, bit C
ANL C, bit C
RLC C
RRC C
CLR C C=0
CPL C C = /C
CJNE C


The ADD and ADDC Instructions

ADD A, source ; A = A + source
ADDC A, source ; A = A + source + C

A register must be involved in additions
The C flag is set to 1 if there is a carry out of bit 7
The AC flag is set to 1 if there is a carry out of bit 3
ADD is used for ordinary addition
ADDC is used to add a carry after the LSB addition in a
multi-byte process


Example -1
Show how the flag register is affected by the following instructions.
MOV A, #0F5h ; A = F5h
ADD A, #0Bh ; A = F5 + 0B = 00

Solution F5h 1111 0101
+ 0Bh + 0000 1011
100h 0000 0000
After the addition, register A (destination) contains 00
and the flags are as follows:
CY = 1 since there is a carry out from D7
P = 0 because the number of 1s is zero
AC = 1 since there is a carry from D3 to D4


Example -2
Assume that RAM locations 40h – 42h have the following
values. Write a program to find the sum of the values in these
locations. At the end of the program, register A should contain
the low byte and R7 contain the high byte.
RAM locations: 40h = (7Dh), 41h = (EBh), 42h = (C5h)
Solution:
MOV A, 40h ; set A = RAM location 40h
MOV R7, #0 ; set R7 = 0
ADD A, 41h ; add A with RAM location 41h
JNC NEXT ; if CY = 0 don’t accumulate carry
INC R7 ; keep track of carry
NEXT: ADD A, 42h ; add A with RAM location 42h
JNC NEXT1 ; if CY = 0 don’t accumulate carry
INC R7 ; keep track of carry
NEXT1:
END

Example -3
Write a program segment to add two 16-bit numbers. The numbers are
3CE7h and 3B8Dh. Place the sum in R7 and R6; R6 should store the lower
byte.

CLR C ; make C=0
MOV A, #0E7h ; load the low byte now A=E7h
ADD A, #8Dh ; add the low byte now A=74h and C=1
MOV R6, A; save the low byte of the sum in R6
MOV A, #3Ch ; load the high byte
ADDC A, #3Bh ; add with the carry
; 3B + 3C + 1 = 78 (all in hex)
MOV R7, A; save the high byte of the sum

ADDC ….. ADD …..
The DA Instruction DA A DA A

DA A
 The action is to “decimal adjust” the register A
 Used after the addition of two BCD numbers

Example 4 :
MOV A, #47h ; A=47h first BCD operand
MOV B, #25h ; B=25h second BCD operand
ADD A, B ; hex (binary) addition (A=6Ch)
DA A ; adjust for BCD addition (A=72h)


Example 4 of DA Instruction

Hex BCD
47 0100 0111
+ 25 + 0010 0101
6C 0110 1100
+ 6 + 0110
72 0111 0010

Offset decimal 6 !


SUBB A, #data
The SUBB Instruction SUBB A, direct
SUBB A, @Ri , where i =0 or 1
SUBB A, source SUBB A, Rn, where n =0,1,,7

No borrow: A = A – source
With borrow:
A = A – source – carry (i.e. borrow)
Note that the 8051 uses the 2’s complement method to do
subtraction
After execution:
The C flag is set to 1 if a borrow is needed into bit 7
The AC flag is set to 1 if a borrow is needed into bit 3


The MUL Instruction
MUL AB

Uses registers A and B as both source and destination
registers
Numbers in A and B are multiplied, then put the
lower-order byte of the product in A and the high-
order byte in B
The OV flag is set to 1 if the product > FFh
Note that the C flag is 0 at all times


The DIV Instruction
DIV AB

Similarly, it uses registers A and B as both source and
destination registers
The number in A is divided by B. The quotient is put
in A and the remainder (if any) is put in B
The OV flag is set to 1 if B has the number 00h
(divide-by-zero error)
Note that the C flag is 0 at all times


The INC and DEC Instructions
To increment (INC) or decrement (DEC) the internal
memory location specified by the operand

No change with all the arithmetic flags in this
operation

e.g. INC 7Fh ; content in 7Fh increased by 1
DEC R1 ; content in R1 decreased by 1
INC A
INC direct
INC @Ri where i=0,or 1
INC Rn where n=0,,7


Logic Operation in 8051

Logical operations

Rotate and swap operations

Comparison operations


Logical Instructions
The source operand can be any of the 4 addressing modes (i.e.
immediate/register/ direct/indirect)

ANL can be used to clear (0) certain bits

ORL can be used to set (1) certain bits
Examples

Instruction ANL A,R0 ORL A,R0 XRL A,R0

A before: 10010111 10010111 10010111
R0 before: 11110010 11110010 11110010
A afterwards: 10010010 11110111 01100101


The CLR and CPL Instructions

CLR A
All bits in register A are cleared
CPLA
All bits in register A are complemented (inverted)
Note that CLR and CPL instructions operate on
register A only


RL A
The Rotate Instructions RR A

Contents in register A is rotated one bit position to the
left or to the right (operated in A only)

The bit shifted out is used as the new bit shifted in

May include the C flag in the operation

Useful in inspecting the bits in a byte one by one

Also useful for multiplication and division in powers of
2


The Rotate Instructions
RL A
Rotates A one bit position to the left
RLC A
Rotates A and the carry flag one bit position to the left
RR A
Rotates A one bit position to the right
RRC A
Rotates A and the carry flag one bit position to the right
Note that for RLC and RRC, you have to know the C flag
first


The Rotate Instructions
7 6 5 4 3 2 1 0
Before: 10011100
After: 00111001
RL A

C 7 6 5 4 3 2 1 0 Before: 10011100 CY = 0
After: 00111000 CY = 1

Carry Flag RLC A

7 6 5 4 3 2 1 0 Before: 10011100
After: 01001110

RR A

7 6 5 4 3 2 1 0 C Before: 10011100 CY = 1
After: 11001110 CY = 0

RRC A Carry Flag

The SWAP Instruction
Swapping the lower-nibble (lower 4 bits) and the
higher-nibble (upper 4 bits) of register A.

7 6 5 4 3 2 1 0

High Nibble Low Nibble

SWAP A

Register A = 5Eh (original value) after SWAP Register A =
E5h

Comparison Operation

CJNE destination, source, relative address

Compare the source and destination operands first

Jump to the relative address (subroutine) if they are
not equal

Carry flag = 1, if destination-byte is less than the source-byte,
Otherwise, the carry flag is cleared.


Example -5
Write a program segment to monitor P1 continuously
for the value of 63h. It should get out of the
monitoring only if P1 = 63h.

Solution :
MOV P1, #0FFh ; make P1 an input port
HERE: MOV A, P1 ; get P1
CJNE A, #63h, HERE ; keep monitoring
unless ; P1=63h


Arithmetic & logical operations in 8051

Arithmetic & logical operations in 8051

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