Understanding the 8085 CMP Instruction and Flags

The question asks about the status of the Carry flag (CY) and Zero flag (Z) after executing the 8085 instruction CMP B when the content of the Accumulator (A) is less than the content of Register B.

The CMP r instruction in the 8085 microprocessor is used to compare the content of the Accumulator with the content of the specified register 'r' (in this case, Register B). The comparison is performed by subtracting the content of register 'r' from the content of the Accumulator (A - r). However, the result of this subtraction is not stored anywhere; it is only used to update the status flags in the Flag Register.

How CMP Affects Flags

The main flags affected by the CMP instruction are the Zero flag (Z) and the Carry flag (CY). The behavior of these flags depends on the result of the implicit subtraction A - B:

• If A = B: The result of A - B is 0. The Zero flag (Z) is set (1). No borrow is needed, so the Carry flag (CY) is reset (0).
• If A > B: The result of A - B is positive and non-zero. The Zero flag (Z) is reset (0). No borrow is needed, so the Carry flag (CY) is reset (0).
• If A < B: The result of A - B is negative, meaning a borrow is needed for the subtraction. The Zero flag (Z) is reset (0) because the result is not zero. A borrow is required, so the Carry flag (CY) is set (1).

Analyzing the Given Condition: A < B

The question specifies the condition where the content of the Accumulator (A) is less than the content of Register B (A < B). As explained above, when A < B, the subtraction A - B requires a borrow. In the 8085, the Carry flag (CY) acts as a borrow flag for subtraction. Therefore:

• The Zero flag (Z) is reset (0) because A - B is not equal to zero.
• The Carry flag (CY) is set (1) because a borrow is generated during the subtraction A - B when A is less than B.

So, for the condition A < B after executing CMP B, the Carry flag will be Set, and the Zero flag will be Reset.

Based on this analysis, when the content of the Accumulator is less than that of Register B, the Carry flag is Set, and the Zero flag is Reset.

Revision Table: 8085 Flag Behavior

Additional Information: 8085 Comparison Logic

The CMP instruction is a fundamental comparison instruction in the 8085. While it performs a subtraction internally, it's non-destructive, meaning it doesn't change the contents of the Accumulator or the source register. Its primary purpose is to set the flags, which can then be tested by subsequent conditional jump instructions (like JZ, JNZ, JC, JNC) to alter the program flow based on the comparison result.

For example:

• To jump if A = B, use JZ after CMP B.
• To jump if A < B, use JC after CMP B (since CY=1 when A < B).
• To jump if A > B, use JNC after CMP B (since CY=0 when A > B).

Understanding how CMP affects flags, especially the Carry and Zero flags, is crucial for writing conditional logic in 8085 assembly language programs.