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Logical Group Instructions (Part-IV) - CMP and CPI

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CMP R:
  • This instruction compares the contents of the specified register with the contents of the accumulator.
  • This comparison is performed by subtracting the contents of the specified register from the contents of the accumulator.
  • Register R can be any 8-bit general purpose register like A, B, C, D, E, H and L.
  • If A=R, then CY=0 and Z=1.
  • If A>R, then CY=0 and Z=0.
  • If A<R, then CY=1 and Z=0.
  • S, AC and P are modified to reflect the status of subtraction.
  • Z and CY are used to indicate the result of comparison.
  • It is a one byte instruction.
  • Register addressing mode is used.
Example:
If A = 12H and B = 14H 
CMP B ; This instruction will compare the contents of A and B and since A<B, carry flag will set.

CMP M:
  • This instruction compares the contents of the memory location pointed by HL register pair with the contents of the accumulator.
  • This comparison is performed by subtracting the contents of the memory location from the contents of the accumulator.
  • If A=M, then CY=0 and Z=1.
  • If A>M, then CY=0 and Z=0.
  • If A<M, then CY=1 and Z=0.
  • S, AC and P are modified to reflect the status of subtraction.
  • Z and CY are used to indicate the result of comparison.
  • It is a one byte instruction.
  • Register indirect addressing mode is used.
Example:
If HL = 4320H, (4320H) = 33H and A = 33H
CMP M ; This instruction will compare the contents of A with the contents of location 4320H and since A = M, carry flag will reset and zero flag will set.

CPI 8-bit data:
  • This instruction compares the 8-bit data given in the instruction itself with the contents of the accumulator.
  • This comparison is performed by subtracting the 8-bit data from the contents of the accumulator.
  • If A=data, then CY=0 and Z=1.
  • If A>data, then CY=0 and Z=0.
  • If A<data, then CY=1 and Z=0.
  • S, AC and P are modified to reflect the status of subtraction.
  • Z and CY are used to indicate the result of comparison.
  • It is a two byte instruction.
  • Immediate addressing mode is used.
Example:
If A = 50H
CPI 30H ; This instruction will compare the contents of A with 30H and since A>30H, carry flag and zero flag both will reset.
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