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## Assembly Language - Conditional Judgment Conditional judgment is the cornerstone of program control flow. Assembly language implements if-else, switch and other judgment logic through comparison instructions and conditional jump instructions. * * * ## CMP - Compare Instruction `CMP` performs subtraction (dest - src) on two operands, but does not save the result, only updates the flag bits. Essentially `CMP dest, src` is equivalent to `SUB dest, src` but discards the calculation result. ## Example ; CMP compare instruction example section.text global _start _start: mov eax,10 cmp eax,10; eax == 10? ; ZF = 1 (equal, result is zero) ; CF = 0 (no borrow) ; SF = 0 (result is non-negative) mov eax,5 cmp eax,10; eax == 10? ; ZF = 0 (not equal, result is non-zero) ; CF = 1 (borrow: 5 < 10) mov eax,20 cmp eax,10; eax == 10? ; ZF = 0 (not equal) ; CF = 0 (no borrow: 20 >= 10) ; SF = 0 (result is non-negative: 10 > 0) mov eax,1 mov ebx,0 int 0x80 * * * ## Conditional Jump Instructions Conditional jump instructions determine whether to jump based on the status of flag bits. If the condition is met, jump to the target label; otherwise, continue executing the next instruction. | Instruction | Meaning | Flag Bits Checked | Applicable Scenario | | --- | --- | --- | --- | | JE / JZ | Jump if equal / zero | ZF = 1 | Check a == b after `cmp a, b` | | JNE / JNZ | Jump if not equal / not zero | ZF = 0 | Check a != b after `cmp a, b` | | JG / JNLE | Jump if greater (signed) | ZF=0 and SF=OF | Signed a > b | | JGE / JNL | Jump if greater or equal (signed) | SF = OF | Signed a >= b | | JL / JNGE | Jump if less (signed) | SF != OF | Signed a < b | | JLE / JNG | Jump if less or equal (signed) | ZF=1 or SF!=OF | Signed a <= b | | JA / JNBE | Jump if greater (unsigned) | CF=0 and ZF=0 | Unsigned a > b | | JAE / JNB | Jump if greater or equal (unsigned) | CF = 0 | Unsigned a >= b | | JB / JNAE | Jump if less (unsigned) | CF = 1 | Unsigned a < b | | JBE / JNA | Jump if less or equal (unsigned) | CF=1 or ZF=1 | Unsigned a <= b | > Many instructions have two aliases (such as JE and JZ), which are exactly the same at the machine code level. Which one to use depends on the context: use JE/JNE after comparisons, and JZ/JNZ after checking arithmetic results. This makes the code more readable. * * * ## Single-branch IF Structure ## Example ; File path: if_demo.asm ; Implementation: if (x > 10) x = 10; section.data x dd 15; Test value limit dd 10 section.text global _start _start: mov eax,; Load x into eax cmp eax,; x > 10 ? jle skip_update ; If x <= 10, skip update mov dword,10; x = 10 skip_update: ; Program continues... (here x is already 10) mov eax,1 mov ebx,0 int 0x80 * * * ## IF-ELSE Dual-branch Structure ## Example ; File path: if_else_demo.asm