Flags and CMP

Condition Flags

ARM64 uses condition flags to make decisions. These four flags, stored in the NZCV register, are the foundation of all conditional logic -- from simple if/else to complex loops.

The NZCV Flags

Important: Regular ADD and SUB do not set flags. Only flag-setting instructions (CMP, ADDS, SUBS) modify NZCV. This means you can safely do arithmetic without accidentally changing flags set by a previous comparison.

Condition flags are the sensor readings of the CPU -- after every comparison, they report the status of your last operation, just like the Enterprise's sensors report conditions ahead.

CMP -- Compare

CMP is the primary comparison instruction. It subtracts its second operand from its first, sets the flags, and discards the subtraction result:

CMP X0, #10      // Computes X0 - 10, sets flags, result discarded
CMP X0, X1       // Computes X0 - X1, sets flags, result discarded

After CMP X0, X1:

• Z=1 if X0 == X1 (the subtraction is zero)
• N=1 if X0 < X1 (signed -- the result is negative)
• C=1 if X0 >= X1 (unsigned -- no borrow occurred)

Internally, CMP X0, X1 is encoded as SUBS XZR, X0, X1 -- a subtraction into the zero register with flag-setting.

CMN -- Compare Negative

CMN adds its operands and sets flags. It is useful when comparing against a negative value:

CMN X0, #5       // Computes X0 + 5, sets flags

ADDS and SUBS

These are flag-setting versions of ADD and SUB that keep the result:

ADDS X0, X1, X2   // X0 = X1 + X2, AND sets flags
SUBS X0, X1, X2   // X0 = X1 - X2, AND sets flags

Use these when you need both the result and the flags.

Your Task

Write a program that compares two values (15 and 15). If they are equal, print EQ\n. If not, print NE\n. Use CMP and a conditional branch (B.NE, which we will cover in the next lesson -- but use it here as a preview).

Hint: After CMP X0, X1, use B.NE not_equal to skip to a different label if they are not equal.