Control Flow

All Write Ups

Low Level Architecture

Control Flow

Polished on 8 September 2025 (First written in May 2025)

Control flow helps in breaking the linearity of execution in assembly.

These three high level constructs in C are all implemented using jump statements.

If else ladder: if, else if and else.
Iteration: for, while and do-while loops.
Switch case.

Jump Statements

Unconditional jumps are the jumps that are certain to happen. Nothing can stop them. jmp mnemonic is used for this.
Conditional jumps are based on a conditions. The cmp mnemonic helps in comparing two entities.
The comparison instruction affects certain CPU Flags which the jump statements use to make their decision of jumping.
The jump is made to a code symbol.

Comparison Operation

cmp compares two operands by subtracting the second operand from the first operand.

cmp op1, op2
=> op1 - op2

The result is used to set certain CPU flags. There is a special purpose register called RFLAGS in x64 and EFLAGS in x86 which holds these flags. And other architectures have similar mechanisms.

Note: cmp is just one instruction that changed RFLAGS; there are other operations that set them as well.
Although RFLAGS is a 64-bit wide register, the total CPU flags aren’t 64. Most of the bits are reserved by the CPU for internal things. The most common ones include: CF ZF OF PF AF SF TF .

Flag	Bit Place	Meaning (Set When)	How it’s Set	Assumption
CF(Carry Flag)	0	Unsigned overflow (carry/borrow out of MSB)	From final carry out of the adder/subtractor
PF(Parity Flag)	2	Result has even parity in low 8 bits	1 if least-significant byte of result has even number of 1s
AF(Auxiliary Carry)	4	Carry out from bit 3 → bit 4	Set if lower nibble overflowed
ZF(Zero Flag)	6	Result is zero	Set if result == 0	Both the operands are same.
SF(Sign Flag)	7	Sign of result (MSB)	Set to MSB of result
OF(Overflow Flag)	11	Signed overflow occurred	Set if (carry into MSB) ≠ (carry out of MSB)

Based on these CPU flags the conditional jump statements make an assumption about the result of comparison.

Unsigned Comparison

Instruction	High-level Equivalent	Meaning	Flags Tested	Condition (Flag State)
JE / JZ	==	Jump if equal / zero	ZF	ZF = 1
JNE / JNZ	!=	Jump if not equal / not zero	ZF	ZF = 0
JB / JC / JNAE	<	Jump if below / carry / not above or equal	CF	CF = 1
JAE / JNB / JNC	>=	Jump if above or equal / not below / not carry	CF	CF = 0
JA	>	Jump if above (strictly greater, unsigned)	CF, ZF	(CF = 0) AND (ZF = 0)
JBE	<=	Jump if below or equal (unsigned)	CF, ZF	(CF = 1) OR (ZF = 1)

Since unsigned integers uses full set of bits available to store the magnitude of the value, we don’t require other flags.

Signed Comparison

Instruction	High-level Equivalent	Meaning	Flags Tested	Condition (Flag State)
JE / JZ	==	Jump if equal / zero	ZF	ZF = 1
JNE / JNZ	!=	Jump if not equal / not zero	ZF	ZF = 0
JL / JNGE	<	Jump if less (signed)	SF, OF	SF ≠ OF
JGE / JNL	>=	Jump if greater or equal (signed)	SF, OF	SF = OF
JG / JNLE	>	Jump if greater (signed)	ZF, SF, OF	(ZF = 0) AND (SF = OF)
JLE / JNG	<=	Jump if less or equal (signed)	ZF, SF, OF	(ZF = 1) OR (SF ≠ OF)

Last updated on September 30, 2025