The x86 Compare instruction (CMP) subtracts the source operand from the destination operand; it updates the status flags (C, P, A, Z, S, O) but does not alter either of the operands. The CMP instruction can be used to determine if the destination operand is greater than, equal to, or less than the source operand.
a. Suppose the two operands are treated as unsigned integers. Show which status flags are relevant to determine the relative size of the two integer and what values of the flags correspond to greater than, equal to, or less than.
b. Suppose the two operands are treated as twos complement signed integers. Show which status flags are relevant to determine the relative size of the two integer and what values of the flags correspond to greater than, equal to, or less than.
c. The CMP instruction may be followed by a conditional Jump (Jcc) or Set Condition (SETcc) instruction, where cc refers to one of the 16 conditions listed in Table 10.10. Demonstrate that the conditions tested for a signed number comparison are correct.
The Correct Answer and Explanation is :
The x86 CMP (compare) instruction performs a subtraction of the source operand from the destination operand without storing the result; instead, it updates specific status flags in the EFLAGS register to reflect the outcome of the comparison. These flags are then utilized by subsequent conditional instructions, such as Jcc (conditional jump) and SETcc (set byte on condition), to determine the program’s control flow based on the comparison.
a. Unsigned Integer Comparison:
When treating operands as unsigned integers, the relevant status flags are:
- Carry Flag (CF): Indicates a borrow in subtraction; set if the destination operand is less than the source operand.
- Zero Flag (ZF): Set if the operands are equal.
The interpretation of these flags is as follows:
- Equal:
ZF = 1 - Not Equal:
ZF = 0 - Less Than:
CF = 1 - Greater Than or Equal:
CF = 0
For unsigned comparisons, the CF flag is crucial in determining the ordering of the operands.
b. Signed Integer Comparison:
When treating operands as two’s complement signed integers, the relevant status flags are:
- Sign Flag (SF): Indicates the sign of the result; set if the result is negative.
- Zero Flag (ZF): Set if the operands are equal.
- Overflow Flag (OF): Set if there is a signed overflow, meaning the result is too large to be represented in the destination operand’s number of bits.
The interpretation of these flags is as follows:
- Equal:
ZF = 1 - Not Equal:
ZF = 0 - Less Than:
(SF ≠ OF) - Greater Than or Equal:
(SF = OF)
In signed comparisons, both the SF and OF flags are essential to determine the relative ordering of the operands.
c. Conditional Jumps and Set Conditions:
The CMP instruction is often followed by conditional jump (Jcc) or set condition (SETcc) instructions, which alter the program’s flow or set a byte based on the state of the status flags. These instructions interpret the flags to perform signed or unsigned comparisons appropriately.
For example, after executing cmp eax, ebx, the following instructions can be used:
- Unsigned Comparisons:
ja(jump if above): Jumps ifCF = 0andZF = 0(i.e., destination > source).jb(jump if below): Jumps ifCF = 1(i.e., destination < source).je(jump if equal): Jumps ifZF = 1(i.e., destination == source).- Signed Comparisons:
jg(jump if greater): Jumps ifZF = 0andSF = OF(i.e., destination > source).jl(jump if less): Jumps ifSF ≠ OF(i.e., destination < source).je(jump if equal): Jumps ifZF = 1(i.e., destination == source).
These conditional instructions rely on the status flags set by the CMP instruction to determine the appropriate control flow based on the comparison of the operands. This mechanism allows for efficient implementation of decision-making processes in assembly language programming.