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Explain flag manipulation instruction of 8086 in detail.



Explain flag manipulation instruction of 8086 in detail.

Flag Instructions (Flag Transfer) : These instructions are related to movement of flag register to/from a register and memory.

Flag Instructions (Flag transfer) :

  1. LAHF (Load AH register from flags)
  2. SAHF (Store AH register flags)
  3. PUSHF (Push flags onto stack)
  4. POPF (Pop flags off stack)
=> LAHF - Load AH register from flags : Copy lower byte of flag register to AH.

Mnemonic       LAHF       Flags       No flags are affected
Algorithm        AH = flag register's lower byte
Addr. Mode     Implied Addressing mode
Operation        AH <- Lower byte of flag register 
                        The lower byte of 8086 flag register is copied to the AH register.
=> SAHF - Store AH register in Flags : Copy contents of AH to lower byte of flag register

Mnemonic       SAHF          Flags      No flags are affected.
Algorithm         AH = flag register
Addr. Mode      Implied Addressing mode
Operation         AH -> Lower byte of flag register.
  • This instruction copies the contents of AH register to the lower byte of flag register.
  • It is included for 8085 compatibility.
  • The OF, DF, IF and TF are not affected.
=> PUSHF - Push flags onto stack : PUSH flag register on the stack

Mnemonic       PUSHF           Flags        No flags are changed.
Algorithm        SP = SP - 2
                       SS : [SP] (top of stack) = operand.
Addr. Mode     Register Addressing mode
Operation        SP -> SP -2        SS -> data from flag register
  • This instruction decrements the stack pointer by 2 and copies word in the flag register to the memory location pointed by stack pointer.
  • The stack segment register is not affected.
=> POPF - Pop flags off stack : 

Mnemonic     POPF                 Flags         All flags are affected.
Algorithm       SS = data to flag register    SP = SP + 2
Addr. Mode    Register Addressing mode
Operation       SS : [SP] -> Copy data to flag register        SP = SP +  2.
  1. This instruction copies a word from the two memory locations at the top of the stack to flag register and increment the stack pointer by 2.
  2. The stack segment register and word on the stack are not affected.
 

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