Comunicaci

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Comunicación y Gerencia
Fundamentals of Assembly Language

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Lesson plan

• Review of existing concepts
• Data transfer instructions
• Practice
• Basic arithmetic instructions and
• Repetitive move operations
• Practice

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Review of existing concepts

• Comments
• Directives (page, segment, title)
• Data type (Byte (DB), Word(DW), Doubleword(DD),
  String
• Some arithmetic operations ADD,SUB,MUL,DIV

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Data transfer instructions

• MOV instruction
• Transfers data referenced by the address of the
  second operand to the address of the first
  operand
• Destination has to have the same length as source
• label MOV register/memory
  register/memory/immediate
• Example
• MOV F, AX // Move content of AX to the
  variable F
• MOV CX, D // Move value of D to CX
• MOV ES, AX
• MOV AX, 215

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Note

• MOV instruction cant
• set the value of the CS and IP registers.
• copy value of one segment register to another
  segment register (should copy to general register
  first).
• MOV ES, DS
• copy immediate value to segment register (should
  copy to general register first).
• MOV DS, 100

• MOV instruction cant
• set the value of the CS and IP registers.
• copy value of one segment register to another
  segment register (should copy to general register
  first).
• MOV ES, DS
• copy immediate value to segment register (should
  copy to general register first).
• MOV DS, 100

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MOVSB and MOVSW

• MOVSB
• Copy byte at DSSI to ESDI. Update SI and
  DI.Algorithm ESDI DSSI
• if DF 0 then
• SI SI 1
• DI DI 1
• else
• SI SI - 1
• DI DI - 1
• DF direction flag from the flag register

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MOVSB and MOVSW

• MOVSW
• Copy word at DSSI to ESDI. Update SI and
  DI. ESDI DSSI
• if DF 0 then
• SI SI 2
• DI DI 2
• else
• SI SI - 2
• DI DI - 2
• DF direction flag from the flag register

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XCHG instruction

• XCHG swap the two data items
• label XCHG register/memory, register/memory
• Example
• MOV AL, 5
• MOV AH, 2
• XCHG AL, AH AL 2, AH 5
• XCHG AL, AH AL 5, AH 2

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LEA instruction

• Load Effective Address. REG address of memory
  (offset)
• label LEA register/memory
• Example
• LEA AX, m load offset address of m to AX

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Arithmetic instructions

• INC and DEC instruction
• Increasing or decreasing the contents of register
  or memory location by 1
• label INC/DEC register/memory
• Flag OF, SF and ZF
• OFis set when an instruction resulted in a carry
  into the sign bit of the result.
• SF is set if the sign bit of a result is set
• ZF is set if the result is equal to 0.

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Arithmetic instructions

• ADD
• label ADD/SUB operand1, operand 2
• operand1 operand 1 operand 2
• Operand 1 register/memory
• Operand 2 register/memory/immediate

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Arithmetic instructions

• SUB
• label SUB operand1, operand 2
• operand1 operand 1 - operand 2
• operand 1 register/memory
• operand 2 register/memory/immediate

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Arithmetic instructions

• MUL operand
• Unsigned multiply. Operand register/memory

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Arithmetic instructions

• IMUL operand
• Signed multiply. Operand register/memory
• Example
• MOV AX, -2
• MOV CX, -3
• IMUL CX AX 6
• CF 0

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Arithmetic instructions

• DIV operand
• Unsigned multiply. Operand register/memory
• when operand is a byteAL AX / operandAH
  remainder (modulus)
• when operand is a wordDX remainder (modulus)

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Arithmetic instructions

• IDIV operand
• Signed multiply. Operand register/memory
• when operand is a byteAL AX / operandAH
  remainder (modulus)
• when operand is a wordDX remainder (modulus)

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Practice

• Write a program to convert from Celsius to
  Fahrenheit and vice versa
• Tc (5/9)(Tf-32)
• Tc censius
• Tf fahrenheit
• (The result may not be accurate due to the
  integer division but that is fine)

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Repetitive move instructions
TITLE A04ASM1 (EXE) Move and add operations
--------------------------------------------- STAC
K SEGMENT PARA STACK 'Stack' DW 32
DUP(0) STACK ENDS ------------------------------
---------------- DATASEG SEGMENT PARA 'Data'
STRING1 DB "12345678","" STRING2 DB ?
DATASEG ENDS
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Repetitive move instructions

• CODESEG SEGMENT PARA 'Code'
• MAIN PROC FAR
• MOV AX, dataseg
• MOV DS, AX
• MOV ES, AX
• MOV CX, 09 Initialize to move
  9 characters
• LEA SI, STRING1 Initialize source
  index register to offset of string 1
• LEA DI, STRING2 Initialize
  destination index register to offset of string 2
• BEGINLOOP
• MOV AL,SI Get a current
  character from string 1 to AL
• MOV DI, AL Move it to the
  current character in string 2
• INC SI Move to the next
  character in string 1
• INC DI Move to the next
  character in string 2
• DEC CX Decrease the count
  for loop
• JNZ BEGINLOOP Continue to loop
  if count is not 0
• MOV AH, 09H

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Result
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Repetitive move instructions

• DEC CX
• ZF 1 if CX 0
• JNZ LABEL
• if ZF 0 then jump to the label

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Practice

• Develop an assembly program to
• Define byte items BYTE1 and BYTE2 (Assign any
  values for these two variables)
• Define a word item WORD3 and WORD30
• Move content of Byte1 to AL
• Add content of Byte2 to AL
• Set DL 42H
• Exchange the content of AL and DL
• Multiply the contents of AL by DL
• Transfer product from AX to WORD3

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Addressing mode

• Register addressing
• E.g ADD AX, BX
• fastest type of operations
• Immediate addressing
• Immediate contains a constant value or an
  expression
• E.g MOV AX, 0245H
• Direct memory addressing
• One of operand references a memory location and
  the other operand references a register
• E.G MOV FLDF, AX

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Addressing mode

• Direct-Offset addressing
• use arithmetic instruction to modify an address
• e.g MOV CX, DATAZ2
• Indirect memory addressing
• Use BX and BP, DI and SI within
• e.g. MOV BX, CL

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Addressing mode

• Base Displacement Addressing
• Uses BX, BP and DI, SI and combine with a
  displacement to form an effective address
• E.g MOV AL,SI2
• Base-Index Addressing
• Combine BX,BP with DI,SI to form effective
  address
• E.G MOV AL,BXSI

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Addressing mode

• Base-Index Displacement Addressing
• Combine BX, BP and DI, SI and a displacement to
  form an effective address
• E.g MOV AL,BXSI2

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NEAR and FAR address

• NEAR address
• consists of 16 bit offset portion of an address
• used in real mode
• FAR address
• consists of both the segment and offset portions
  in the form of 32 bit segmentoffset

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SEGMENT OVERRIDE PREFIX

• Example
• MOV ESSI2, CL move CL to ESSI2