Branching

Branching instruction are general instructions that allow to jump any point in program.

Unconditional branching

The JMP instruction is a basic control flow instruction in assembly that performs an unconditional jump to a specified location in code. If a program's execution is directed to another location it will continue processing from that point.

The basic jmp instruction is unconditional, which means that it will always jump to the specified location, regardless of the conditions.

Unconditional branching will always jump is not suitable for loops as it will loop forever.

Conditional branching

Conditional Branching is where the flow of execution is directed based on certain conditions, based on the Destination and Source operands. A conditional jump instruction has multiple varieties as Jcc, where cc represents the Condition Code.

Instruction	Condition	Description
jz	D = 0	Destination equal to 0
jnz	D != 0	Destnation not equal to 0
js	D < 0	Destination is negative
jsn	D >=	Destination is Not Negative (i.e. 0 or positive)
jg	D > S	Destination Greater than Source
jge	D >=	Destination Greater than or Equal Source
jl	D < S	Destination Less than Source
jle	D <= S	Destination Less than or Equal Source

The cmovcc (conditional move) instruction in assembly allows us to conditionally move data from one register to another only if a specific condition is met. This is different from regular mov instructions, which always move data without any condition.

cmovz rax, rbx  ; Move rbx into rax if the condition (zero flag) is set, meaning the last result was zero.

RFLAG Register

When executing conditional instructions like jmp. The processor needs to know whether conditions are true or false. It uses the RFLAGS register which consists of 64-bits and it holds flag bits instead of values. Each bit 'or set of bits' turns to 1 or 0 depending on the value of the last instruction.

mov eax, 1      ; eax = 1
dec eax         ; eax = 0
; ZF is now 1 (on) because result was zero
; This is called the "Zero" (ZR) condition

mov eax, 2      ; eax = 2
dec eax         ; eax = 1
; ZF is now 0 (off) because result was not zero
; This is called the "Not Zero" (NZ) condition

There are more flag to work with: https://www.geeksforgeeks.org/flag-register-8086-microprocessor/

CMP

If we want our program to stop at a certain value like 20 we can use js loopExample which will jump back took loopExample as long as the last instruction was a negative number. The compare instruction cmp compares 2 operands. by subtracting the second operand from first operand (i.e. D1 - S2) and sets flag in RFLAGS.

So as example:

• When we have first number 1: 1 - 10 = -9

  • Since -9 < 0, the jump condition is met

  • Program continues generating Fibonacci numbers

• When we reach 13: 13 - 10 = 3

  • Since 3 > 0, the jump condition is not met

  • Program stops as we found first Fibonacci > 10