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Registers

A

B

R0

R1

R3

R4

R2

R5

R7

R6

DPH DPL

PC

DPTR

PC

Some 8051 16-bit Register

Some 8-bitt Registers of the 8051


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Description of SFR.. Accumulator (E0h) – It is an 8-bit register. It is the main Input

register of ALU(Arithmatic & Logical unit) & used in all arithmatic &

logical operations. The Bits of this register can be accessed by either

name of bits or by address of bits.

B (F0h) – It is also an 8-bit register. It is also the input register of

ALU(Arithmatic & Logical unit) & used in multiplication & divide

operations. The Bits of this register can be accessed by either name of

bits or by address of bits.

ACC.7 ACC.6 ACC.5 ACC.4 ACC.3 ACC.2 ACC.1 ACC.0

E7h E6h E5h E4h E3h E2h E1h E0h

B.7 B.6 B.5 B.4 B.3 B.2 B.1 B.0

F7h F6h F5h F4h F3h F2h F1h F0h


Description of SFR.. Program Counter (PC – Not Addressable) – It is an 16 bit not

addressable register that contain the address of next instruction of

program that is to be executed.

DPTR (DPH,DPL – 83h,82h) - The Data Pointer (DPTR) consists of a high

byte (DPH) and a low byte (DPL). Its intended function is to hold a 16-bit

address. It may be manipulated as a 16-bit register or as two

independent 8-bit registers.


Instructions

• Mov- mov destination,source ;copy source to

destination ex-mov a,#55h mov r0,a r0=a=55h

• add- add a,source ex-mov a,#25h mov r1,#22h add a,r1 a=a+r1(contents)• Db(define byte)- db directive is the most

widely used data directive in the assembler.it is used to define 8 bit data.

Org 00hData1: db 01010101b ;binary 35 in hex


• Org- The org directive is used to indicate the beginning of the address.

org 00h org 50h• Equ- This is used to define a constant without

occupying a memory location. count equ 25 -------------- ------------- mov r3,#count



LOOP & JUMP InstructionsConditional Jump instructions


jz

• Mov a,ro jz over mov a,r1 jz over1 ----- -----Over: ---------Over1: ----------


Unconditional Jump instructions

• LJMP(long jump)-it is a 3 byte instruction.it allows a jump to any memory location from 0000 to ffffh.

• SJMP(short jump)-it is a 2 byte instruction.it allows jump from oo to ffh.


CALL instructions

• Call instruction is used to call a subroutine.it saves the memory space.

• Types- LCALL and ACALL• The difference between ACALL and LCALL is

the target address for lcall may be anywhere within the 64 kbyte address while the target address of acall must be within 2kbyte range.


• DELAY: MOV R3,#255 HERE: DJNZ R3,HERE RET

• DELAY: MOV R1,#05H LOOP1: MOV R2,#55H LOOP2: DJNZ R2,LOOP2 DJNZ R1,LOOP1 RET


Arithmetic/Logical instruction

• Arithmetic instructions perform several basic operations such as addition, subtraction, division, multiplication etc.

• ADD INSTRUCTION• ADD and ADDC both add the value operand to

the value of the Accumulator, leaving the resulting value in the Accumulator.

Examples:-» ADD A,R0 :add register to accumulator» ADDC A, #data :add immediate data to Acc./carry


Add with carry

Mov r0,#00hMov a,#30hAdd a,#20hJc next mov r1,aNext:inc r0 mov a,r0 mov r2,a


Add 16 bit no.

• Add two 16 bit no,fc45h and 02ech• Mov a,#45h add a,#0ech mov r0,a mov a,#02h addc a,#0fch ; 02+fc+1=ff mov r1,a


• SUBTRACT instruction– SUBB subtract the value of operand from the value of the

Accumulator, leaving the resulting value in the Accumulator.

Examples:-» SUBB A,R0 :subtract register from accumulator/Borrow» SUBB A, #data :subtract immediate data from Acc./Borrow

MULTIPLY instructionMUL multiply the unsigned value of the Accumulator

by the unsigned value of the register.Examples:-

MUL AB :multiply A and B.


• Division instruction Examples:-

DIV AB :divide A by B.A=Quotient(A/B)B=Remainder(A/B)

– Increment & DecrementExamples:-

INC A :increment accumulator.INC @Ri :increment indirect RAMDEC Rn :decrement registerDEC @Ri : increment indirect RAM


Logical Operation

• AND OperationExamples:

ANL A,Rn :And register to accumulatorANL A, Direct :And direct byte to accumulator

• OR OperationExamples:

ORL A,Rn :Or register to accumulatorORL A, Direct :Or direct byte to accumulator


• XoR OperationExamples:

XRL A,Rn :Exclusive OR register to accumulatorXRL A, Direct : Exclusive OR direct byte to accumulator

• Complement OperationExample: CPL A : complement accumulator.

• Rotate instruction Example: RL A :rotate accumulator left

RR A :rotate accumulator right

• Swap instruction– SWAP A :swap nibbles within the accumulator• Exchange- xch a,r1


Addressing Mode

• The way by which the address of the operand (source or destination) are specified in the instruction is known as Addressing mode.


Addressing modes

• Immediate addressing mode• Register addressing mode• Direct addressing mode• Register indirect addressing mode• Indexed addressing mode


Immediate addressing mode

• Mov b,#40h• Mov r5,#39h• Mov a,#10101010b


Register addressing mode

• Mov a,r0• Mov r5,a• Mov r0,b

Data between Rn register is not allowed i.e, MOV R4,R7


Direct Addressing Mode

• Using this mode one can access internal data RAM and SFR directly.– Internal RAM uses addresses from 00H to 7FH.– The SFR addresses exist from 80H to FFH.

Examples:

MOV R0,40H :move content of RAM location 40H in R0

MOV 56H,A :save content of A in RAM location 56H

MOV 90H,A :save content of A in P1 (P1=90H)


Register indirect addressing mode

• Mov a,@r0 ; move contents of ram location whose address is held by r0 into A.

mov @r1,b ; move contents of B into ram location whose address is held by r1.

Note- only r0 and r1 are used,r2 –r7 are not used for this purpose.


Index Addressing Mode

• In the indexed addressing mode, only program memory can be accessed. The program memory can only be read.

• Either the DPTR or PC can be used as Index register.

Examples:MOVC A, @A+DPTR :copy the code byte, found

at the ROM address formed by adding A and DPTR, to A.

MOVC A, @A+PC : copy the code byte, found at the ROM address formed by adding A

and pc, to A.


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