Introduction to Assembly Language Programming

Introduction to Assembly Language Programming

Registers:

Register are used to store information temporarily, while the information could be

• a byte of data to be processed, or
• an address pointing to the data to be fetched

The 8 bits of a register are shown from MSB D7 to the LSB D0

• With an 8-bit data type, any data larger than 8 bits must be broken into 8-bit chunks before it is processed

The most widely used registers are

• A (Accumulator)
• For all arithmetic and logic instructions
• B, R0, R1, R2, R3, R4, R5, R6, R7
• DPTR (data pointer), and PC (program counter)

MOV Command

MOV destination, soruce

purpose of this command copy source to destination.

symbol "#" shows that it is value (number)

Example: 

MOV A,#55H ;load value 55H into reg. A

                        ;NOTE: Method use to transfer value is known as

                        ;Direct Addressing mode

MOV R0,A ;copy contents of A into R0

;(now A=R0=55H)

                        ;NOTE: Method use to transfer value is known as

                        ;Register Addressing mode

MOV R1,A ;copy contents of A into R1

;(now A=R0=R1=55H)

MOV R2,A ;copy contents of A into R2

;(now A=R0=R1=R2=55H)

MOV R3,#95H ;load value 95H into R3

;(now R3=95H)

MOV A,R3 ;copy contents of R3 into A

;now A=R3=95H

important points 

• MOV A, #23H
• if you forgot to add symbol "#" then it will consider it as location. With this symbol it will consider it as value.
• MOV R5, #0F9H
• Add an extra 0 before HEX number A to F to indicate that it isis a hex number and not a letter as done in example; MOV R5, #0F9H
• If values 0 to F moved into an 8-bit register, the rest of the bits are assumed all zeros
• “MOV A, #5”, the result will be A=05; i.e., A= 00000101 in binary
• Moving a value that is too large into a register will cause an error 
• MOV A, #7F2H ; ILLEGAL: 7F2H>(FFH) 8 bits 

ADD command

ADD A, source

The ADD instruction tells the CPU to add the source byte to accumulator register A and put the result in register A

• Source operand can be either a register or immediate data, but the destination must always be register A
• Remember: “ADD R4, A” and “ADD R2, #12H” are invalid since A must be the destination of any arithmetic operation

Example:

MOV A, #25H ;load 25H into A

MOV R2, #34H ;load 34H into R2

ADD A, R2            ;add R2 to Accumulator

                              ;(A = A + R2)

There are always many ways to write the same program, depending on the registers used

MOV A, #25H ;load one operand

                              ;into A (A=25H)

ADD A, #34H        ;add the second

                               ;operand 34H to A