Assembly Language

1
Assembly Language

• The inputs to a microprocessor to form its
  program are termed instructions and the set of
  instructions that a microprocessor recognizes is
  termed its instruction set.

2
Assembly to Binary Code

• Writing instruction in binary code is a tedious
  and difficult task.
• Instructions can be written in a mnemonic form
  termed assembly language and then translated into
  machine code by a computer program called an
  assembler.

3
Instructions

• Each instruction must contain two parts
• Operation - The code must detail the operation
  required, i.e. opcode.
• Operand - The operation will require some data to
  operate on and its destination after processing
  they are known as the operands.
• In principle all instructions require two
  operands, one to define the source of data prior
  to processing and one to define the destination
  for the processed data

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Instruction ..

• For example, in assembly language an instruction
  might be LD A,BThe opcode used is LD to
  specify the operation is to load the operand in
  register B to accumulator A.

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Instruction ..

• LD A,B
• LD - Operation code for load
• A Destination
• B - Source

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Instructions ..

• Another example of an instruction isJMP
  2000Hwhich has the operation jump with the
  destination being address 2000H

7
Operations

• In general, instructions can be classified as
  falling into four main groups1. Data
  Transfer2. Arithmetic3. Logical4. Program
  Control

8
Data Transfer

• Load/MoveThis instruction reads the contents of
  a specified memory location and copies it to a
  specified register location, e.g. Before
  instructionData in memory location 0010After
  instructionData still in memory location
  0010Data from 0010 in accumulator

9
Data Transfer ..

• Store This instruction copies the current
  contents of a specified register into a specified
  memory location, e.g.Before instructionData
  in accumulatorAfter instructionData still in
  accumulatorData copied to memory location 0010

10
Arithmetic

• ADDThis instruction adds the contents of a
  specified memory location to the data in some
  register, e.g.Before instructionAccumulator
  with data 0001Memory location with data
  0010After instructionAccumulator with data
  0011

11
Arithmetic

• SUBTRACTThis instruction subtracts the content
  of a specified memory location from data in some
  register.
• DECREMENTThis instruction subtracts 1 from the
  contents of a specified memory location
  (accumulator)

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Arithmetic

• INCREMENTThis instructions adds 1 to the
  contents of a specified location
• COMPAREThis instruction indicates whether the
  contents of a register are greater than, less
  than or the same as the contents of a specified
  memory location. The result appears in the status
  register as flags. If the two are equal we can
  have the zero flag set to 1 and the carry flag to
  0.If greater the zero flag and the carry flag
  are both 0If less than the zero flag is 0 and
  the carry flag is 1

13
Logical

• ANDThis instruction carries out the logical AND
  operation bit by bit with the contents of a
  specified memory location and the data in some
  register, e.g.Before instructionAccumulator
  with data 0011Memory location with data
  1001After instructionAccumulator with data
  0001

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AND ..

• One of the main uses of the AND instruction is to
  force bits in the accumulator to become 0,
  e.g.If accumulator content 1010 1111 the ANDing
  this with mask data 1111 0000 gives the result
  1010 0000 . Every 0 in the mask data has forced
  logic 0s into the accumulator. (Note Use 1111
  1101 used to switch certain bit 2 to 0, while
  all the other bits remain)

15
OR

• Like the AND operation the OR operation can be
  used for bit masking. E.g.If we have 0011 1000
  in the accumulator then ORing it with 1111 0000
  results in 1111 1000. Whenever a logic 1 appears
  in the mask data then the corresponding bit in
  the accumulator becomes 1 a logic 0 in the mask
  data does not change the data in the accumulator.

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EXCLUSIVE-OR

• The Exclusive-OR function gives a 1 if either of
  the inputs is 1 bit it gives 1 0 if both are
  1.If we have 0110 0101 in the accumulator and
  XOR it with 1111 0000 then the result is 1001
  0101. A logic 1 in the mask data will invert a
  bit in the accumulator, but a logic 0 will not
  change the data.

17
Logical Shift (Left or Right)

• Logical shift instructions involve moving the
  pattern of bits in the register one place to the
  left or right by moving a 0 into the LSB of the
  number and the overflow bit into the carry.For
  example logical shift rightBefore
  instructionAccumulator with data 0011After
  instructionAccumulator with data 0001Status
  register indicates Carry 1

18
Arithmetic Shift (left or right)

• Arithmetic shift instructions involve moving the
  pattern of bits in the register one place to the
  left or right but they preserve the sign bit at
  the left end of the number. The overflow goes
  into the carryFor example Arithmetic shift
  rightBefore instructionAccumulator with data
  1011After instructionAccumulator with data
  1001Status register indicates Carry 1

19
Arithmetic Shift Left

• For example Arithmetic shift leftBefore
  instructionAccumulator with data 1011After
  instructionAccumulator with data 1110Status
  register indicates Carry 0

20
Rotate (Left or Right)

• Rotate instructions involve moving the pattern of
  bits in the register one place to the left or
  right through the carry the bit that spills out
  is written back into the other end, e.g. for a
  rotate right Before instructionAccumulator
  with data 0011After instructionAccumulator
  with data 1001

21
Program Control

• JUMPThis instruction changes the sequence in
  which the program steps are carried out.
  Normally, the program counter causes the program
  to be carried out sequentially, one instruction
  after another, in the strict numerical sequence
  in which the instructions are written. However,
  the jump instruction causes the program counter
  to jump to some other specified location in the
  program.

22
Program Control

• BRANCHThis is a conditional instruction which
  might be branch if zero or branch if plus. This
  branch instruction is followed if the right
  conditions occur.
• ENDThis instruction stops all further
  microprocessor activity.

23
Program Control

• NO OPERATIONThis instruction does nothing except
  use an instruction cycle and increment the
  program counter. It can therefore be used to
  provide short delays.

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Numerical Values

• Numerical data may be binary, octal, hex or
  decimal.
• Generally in the absence of any indicator the
  assembler assumes the number is
  decimal.Intelnumerical values must be
  preceded by to indicate a number and B for
  binary, O or Q for octal, H or h for hex and D or
  nothing for decimal.

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Numerical Values ..

• Motorola/RockwellA number is indicated by the
  prefix A binary number is preceded by or
  followed by B An octal number is preceded by _at_
  or followed by O A hex number is preceded by
  or followed by H.

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Addressing

• When a mnemonic, such as LD, is used to specify
  an instruction it will be followed by additional
  information to specify the source and destination
  of the data required by the instruction.
• There are several different methods that are used
  for specifying data location, i.e. addressing
• Different microprocessors have different
  addressing modes.

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Addressing

• The Motorola 68HC11 has the six addressing modes
  of immediate, direct, extended, indexed, inherent
  and relative.
• The Intel 8051 has the five modes of immediate,
  direct, register, indirect and indexed.
• The PIC microcontroller has the three modes of
  immediate, direct and indirect and indirect mode
  allowing indexing.

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Immediate Addressing

• The data immediately following the mnemonic is
  the value to be operated on.
• This type of instruction with immediate
  addressing is used with the loading of a
  predetermined value into a register or memory
  location, e.g.Z80 CodeLD A, 25Hmeans
  load the A register with the number 25, the H
  indicating that it is a hex number

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Immediate Addressing ..

• Motorola/Rockwell CodeLDA B 25means load
  the number 25 into accumulator B. The signifies
  immediate mode and a number, the that the
  number is in hexadecimal
• Intel CodeMOV A,25Hmeans move the number 25
  to the accumulator A. The indicates a number
  and the H indicates a hex number.

30
Immediate Addressing ..

• PIC Codemovlw H25to load the number 25
  into the working register w, the H indicating it
  is a hex number.

31
Direct Addressing

• Also called absolute, extended or zero-page
  addressing.
• With this form of addressing the data byte that
  follows the opcode directly gives an address that
  defines the location of the data to be used in
  the instruction. For example
• Z80 codeLD A,(0400H)means load the
  accumulator with the data at address 0400

32
Direct Addressing ..

• Motorola codeLDAA 25means load the
  accumulator with the contents of memory location
  0025, the 00 is assumed.
• Rockwell codeLDA 25
• Intel codeMOV A, 20H
• PIC codemovwf Reg1to copy the contents of
  Reg1 into the working register, the address of
  Reg1 having been previously defined.

33
Inherent Addressing

• Also called Implied addressing
• With this mode of addressing, the address is
  implied in the instruction. For
  exampleZ80/Motorola/Intel codeCLR Ameans
  clear accumulator A.PIC codeclrwmeans clear
  the working register.

34
Register Addressing

• With this form of addressing, the operand is
  specified as the contents of one of the internal
  registers. For exampleZ80 codeADD A,Bto
  add register B to the accumulator.Intel
  codeADD R7,Ato add the contents of the
  accumulator to register R7.

35
Indirect Addressing

• This form of addressing means that the data is to
  be found in a memory location whose address is
  given by the instruction. For exampleZ80
  codeLD A,(HL)we might first load the HL
  register pair with the address of the location of
  data then use LD A,(HL) to load the accumulator
  with the data found in the memory location given
  by HL register pair.

36
Index Addressing

• Indexed addressing means that the data is in a
  memory location whose address is held in an index
  register. The first byte of the instruction
  contains the opcode and the second byte contains
  the offset the offset is added to the contents
  of the index register to determine the address of
  the operand.

37
Index Addressing ..

• For example(Motorola code)LDA A FF, Xthis
  means load accumulator A the data at the address
  given by adding the content of the index register
  and FF.

38
Relative Addressing

• This is used with branch instructions. The opcode
  is followed with a byte called the relative
  address. This indicates the displacement in
  address that has to be added to the program
  counter if the branch occurs. For
  example,(Motorola code)BEQ F1indicates if
  the data is equal to zero then the next address
  in the program is F1 further on. The relative
  address of F1 is added to the address of the next
  instruction.

39
Separate Addressing Mode

• Source and destination operands may each have
  their own separate addressing modes and so an
  instruction might involve two different
  addressing modes. For example,(Z80 code)ADD
  A,(HL)uses indirect addressing to specify the
  data source and register addressing to specify
  the location of the data to which the source data
  must be added.

40
Separate Addressing Mode ..

• (Intel code)MOV A, 7EHhere the data source
  is specified using immediate addressing and
  register addressing to specify where the data is
  to be moved.Note number in Intel

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Motorola Addressing Modes

• Load accumulator A with data F0LDAA F0
  (Immediate)
• Load accumulator A with data at address 0050LDAA
  50 (Direct)
• Load accumulator with data at the address given
  by the index register plus CFLDAA CF, X
  (Indexed)
• Add the hexadecimal value 16 to the
  accumulatorADDA 16 (Immediate)

42
Motorola Addressing Modes ..

• Branches to the address indicated by the label
  THERE if equal, i.e. the Z bit in the CCR
  register is 1BEQ THERE (Relative)
• Clear accumulator ACLR (Inherent)
• Clear the address given by the index register
  plus 10, i.e. store all 0s at that addressCLR
  10, X (Indexed)

43
Examples of 8051 Assembly Codes

• Load accumulator A with the data contained in
  memory address 22HMOV A, 22H
• Add the contents of register 5 to accumulator
  AADD A, R5
• Subtract hex 30 from the accumulatorSUBB A,
  30
• Set the carry bit to 0CLR C

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Examples of 8051 Assembly Codes

• Increment register R2INC R2
• Logical AND the contents of accumulator with the
  data at address 25HANL A, 25H
• Change al the 0s to 1s and all the 1s to 0s in
  the accumulator, i.e. obtain the complementCPL
  A
• Jump to address 0200 if the accumulator is not
  zeroJNZ 0200H

45
8051 Instruction Sequence

• Example 1 Add the hexadecimal numbers 1234 and
  4142 and store the result in register R6 and
  R7.This requires the low byte of the sum to be
  added, with no carry, and the result stored in
  R6. Then the high byte is added is added, with
  any carry set by the first addition, and the
  result stored in register R7.

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8051 Instruction Sequence ..

• MOV A, 34H move 34H into accumulator
• ADD A, 42H add 42H, with no carry to A
• MOV R6, A store result in R6
• MOV A, 12H move 12H into accumulator
• ADDC A, 41H add 41H, with carry to A
• MOV R7, A store result in R7
• END

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More 8051 Instruction Sequence

• Example 2 Subtract the number stored at address
  22H from the number stored at address 23H and
  stored the result at address 24HSolutionThe
  accumulator is where the 8051 accumulates the
  results of arithmetic operation. Thus we have to
  copy data to the accumulator before we can carry
  out the arithmetic. We must clear carry flag
  since this acts as the borrow bit

48
Solution for Example 2

• MOV A, 22H move 22H into accumulator
• CLC C clear the carry flag
• SUBB A, 23H subtract with borrow
• MOV 24H, A move result to 24H
• END

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Interpreting Instruction Codes

• MOV A, direct
• Description Move direct byte to A
• Byte 2
• Hex Code 74 (Decimal 116)
• Mnemonic MOV
• Operands A, direct
• Operation/Explanation (A) ? data
• Encoding 0 1 1 1 0 1 0 0 (Decimal 116)

50
Interpreting Instruction Codes ..

• MOV Rn, A
• Description Move Accumulator to Register
• Byte 1
• Hex Code F8 to FF
• Mnemonic MOV
• Operands Rn, A
• Operation/Explanation (Rn) ? (A)
• Encoding 1 1 1 1 1 r r r

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Interpreting Instruction Codes ..

• ADDC A, direct
• Description Add with carry from direct to A
• Byte 2
• Hex Code 35 (decimal53)
• Mnemonic ADDC
• Operands A, direct
• Operation/Explanation (A) ? (direct) (C) (A)
• Encoding 0 0 1 1 0 1 0 1 direct address