8086 Assembly Language - Microprocessor And Interfacing

• Theory
• DataType
• Operations
• Instructions
• Others

Theory

• Basic
  • 
  • Conversion of Binary & Hexadecimal
    • 
    • 
• Registers
  • General purpose registers
    • All are 16-bit
    • 
  • Segment registers
    • CS, DS, ES, SS
    • 
• Memory
  • Represented in square brackets -=> [BX]
  • Combination of BX,SI,DI,BP Registers
  • MOV AX, @DATA -=> Loads starting address of data segment into AX register
• Immediate
  • Constant value
• Declaration
  • variable dataType value

DataType

• Basic
  • DB (DEFINE BYTE)
    • 1 byte memory allocation
    • Example
      • A DB 9
      • MESSAGE DB 'HELLOWORLD'
  • DW (DEFINE WORD)
    • Memory allocation based on system architecture
    • Example
      • VAR DW 1122H
  • DD (DEFINE DOUBLEWORD)
  • DQ (DEFINE QUADWORD)
• String
  • STR DB "HELLO WORLD"
  • STR DB "HELLO WORLD", '$'
  • STR DB 10, 13, "HELLO WORLD", '$'
• Array
  • Basic
    • a DB 1h,2h,3Fh,7Fh
    • a DW 1111h,2222h,3333h
  • Declaring using Duplicate
    • X DB 3DUP(7) ≈ X DB 7,7,7
    • X DB 3DUP(5,6) ≈ X DB 5,6,5,6,5,6
    • X DB 10DUP(?) -=> Equivalent to int var[10]={0}
  • Accessing values
    • MOV AL, a[N]
    • MOV SI, N > MOV AL, a[SI]

Operations

• MOV
  • Copy second operand/source to first operand/destination
  • 
• Processor status Register or Flags
  • 
  • Carry Flag -=> Set to 1 when there is unsigned overflow
  • Parity Flag
  • Auxillary Flag
  • Zero Flag -=> Set to 1 when result is 0
  • Sign Flag -=> Set to 1 when result is negative
  • Trap Flag
  • Interrupt Enable Flag
  • Direction Flag
  • Overflow

Instructions

• Arithmetic
  • Addition
    • ADD destination, source
  • Subtraction
    • SUB destination, source
  • Multiplication
    • MUL source
    • Assumes one of the operand in AX (16-bit) or AL (8-bit) for storing result
    • Immediate multiplication is not allowed
    • Two 8-bit numbers multiplied generate 16-bit product
    • Two 16-bit numbers multiplied generate 32-bit product which appears in DX-AX
    • IMUL source
      • Used for signed numbers, Preserves sign
  • Division
    • DIV source
    • Immediate division is not available
    • Interrupt is generated when an error occurs
      • Divide by zero
      • Divide overflow
    • IDIV source
• Logical Instructions
  • AND
    • AND destination, source
    • Clear higher order bits to zero
    • Identify even/odd numbers
  • OR
    • OR destination, source
    • Set lower bits as 1
  • XOR
    • XOR destination, source
    • Clear a register by performing XOR with itself
  • TEST
    • TEST destination, source
    • Performs bitwise AND operation, doesn't change the first operand but only affects the Flag
    • Used to check even/odd numbers
  • NOT
    • NOT source
• JUMP
  • Jump
    • Unconditional
      • It transfers control to another point in the program
      • JMP Label
      • Label is any specific set of code -=> Label:
    • Conditional
      • Jump only when some condition is satisfied
      • JE/JZ Label
        • Jump when zero flag is equal to 1
      • JNE/JNZ Label
        • Jump when zero flag is equal to 0
        • "JE" & "JNE" are mostly used after "CMP" instruction
        • "JZ" & "JNZ" are used when you just check whether something is zero or not
      • JA/JG Label
        • "JA" -=> Jump if above, if CF=0 & ZF=0
        • "JG" -=> Jump is greater, if SF=O & ZF=0
      • JB/JC Label
        • "JC" jumps if CF=1
      • JCXZ Label
        • "JCXZ" jumps if CX=0
• Others
  • Comparison
    • CMP source, destination
    • Subtract source from destination and set the flags
  • Increment/Decrement
    • INC source
      • Adds 1 to any register
    • DEC source
      • Subtract 1 from any register
  • Shift
    • SHL source, N
      • Shifts bits of bytes to left
      • 0 comes in LSB & MSB goes into CarryFlag
      • Use "CL" if "N" is more than 1otherwise use "CX"
      • Value multiplied by 2
    • SHR
      • It puts zero in the MSB and LSB is shifted into "CF"
      • Value divided by zero
  • Rotate
    • ROL
      • Shift MSB to LSB, LSB will also go into "CF"
    • ROR
  • Negation

Others

• Interrupt
  • Basic
    • It is an input signal to the processor indicating an event which needs immediate attention
    • There are 256 Interrupts
    • Interrupts are handles using Interrupt Vector Table
      • It is a data structure that associates a list of interrupt handler with a list of Interrupt requests in a table of Interrupt vectors
    • Input Will Be stored in AL/AX register, Output to be stored in DL/DX
  • Types
    • Software Interrupt
    • Hardware Interrupt
• Macros
  • Copy and Pasts exactly the same defined code wherever used, takes more memory space
• Procedure
  • A group of instructions that usually performs one task, it is reusable section that is stored in memory once, but used often
  • Uses less memory but takes more time as compared to macros
  • "CALL" instruction links to the procedure and "RET" instruction returns from the procedure