X86 Assembly/Control Flow

Almost all programming languages have the ability to change the order in which statements are evaluated, and assembly is no exception. The instruction pointer (EIP) register contains the address of the next instruction to be executed. To change the flow of control, the programmer must be able to modify the value of EIP. This is where control flow functions come in.

mov eip, label   ; wrong
jmp label        ; right

Comparison Instructions

Performs a bit-wise logical AND on arg1 and arg2 the result of which we will refer to as Temp and sets the ZF(zero), SF(sign) and PF(parity) flags based on Temp. Temp is then discarded.

Operands

arg1

• Register
• Immediate

arg2

• AL/AX/EAX (only if arg1 is immediate)
• Register
• Memory

Modified flags

• SF = MostSignificantBit(Temp)
• If (Temp == 0) ZF = 1 else ZF = 0
• PF = BitWiseXorNor(Temp[Max-1:0])
• CF = 0
• OF = 0
• AF is undefined

Performs a comparison operation between arg1 and arg2. The comparison is performed by a (signed) subtraction of arg2 from arg1, the results of which can be called Temp. Temp is then discarded. If arg2 is an immediate value it will be sign extended to the length of arg1. The EFLAGS register is set in the same manner as a sub instruction.

Note that the GAS/AT&T syntax can be rather confusing, as for example cmp $0, %rax followed by jl branch will branch if %rax < 0 (and not the opposite as might be expected from the order of the operands).

Operands

arg1

• AL/AX/EAX (only if arg2 is immediate)
• Register
• Memory

arg2

• Register
• Immediate
• Memory

Modified flags

• SF = MostSignificantBit(Temp)
• If (Temp == 0) ZF = 1 else ZF = 0
• PF = BitWiseXorNor(Temp[Max-1:0])
• CF, OF and AF

Jump Instructions

The jump instructions allow the programmer to (indirectly) set the value of the EIP register. The location passed as the argument is usually a label. The first instruction executed after the jump is the instruction immediately following the label. All of the jump instructions, with the exception of jmp, are conditional jumps, meaning that program flow is diverted only if a condition is true. These instructions are often used after a comparison instruction (see above), but since many other instructions set flags, this order is not required.

See X86_Assembly/X86_Architecture#EFLAGS_Register for more information about the flags and their meaning.

Unconditional Jumps

Loads EIP with the specified address (i.e. the next instruction executed will be the one specified by jmp).

Jump if Equal

ZF = 1

Loads EIP with the specified address, if operands of previous CMP instruction are equal. For example:

mov $5, ecx
mov $5, edx
cmp ecx, edx
je equal
; if it did not jump to the label equal, then this means ecx and edx are not equal.
equal:
; if it jumped here, then this means ecx and edx are equal

Jump if Not Equal

ZF = 0

Loads EIP with the specified address, if operands of previous CMP instruction are not equal.

Jump if Greater

SF = OF and ZF = 0

Loads EIP with the specified address, if first operand of previous CMP instruction is greater than the second (performs signed comparison).

Jump if Greater or Equal

SF = OF or ZF = 1

Loads EIP with the specified address, if first operand of previous CMP instruction is greater than or equal to the second (performs signed comparison).

Jump if Greater (unsigned comparison)

CF = 0 and ZF = 0

Loads EIP with the specified address, if first operand of previous CMP instruction is greater than the second. ja is the same as jg, except that it performs an unsigned comparison.

Jump if Greater or Equal (unsigned comparison)

CF = 0 or ZF = 1

Loads EIP with the specified address, if first operand of previous CMP instruction is greater than or equal to the second. jae is the same as jge, except that it performs an unsigned comparison.

Jump if Lesser

The criteria required for a JL is that SF <> OF, loads EIP with the specified address, if the criteria is meet. So either SF or OF can be set but not both in order to satisfy this criteria. If we take the SUB(which is basically what a CMP does) instruction as an example, we have:

arg2 - arg1

With respect to SUB and CMP there are several cases that fulfill this criteria:

1. arg2 < arg1 and the operation does not have overflow
2. arg2 > arg1 and the operation has an overflow

In case 1) SF will be set but not OF and in case 2) OF will be set but not SF since the overflow will reset the most significant bit to zero and thus preventing SF being set. The SF <> OF criteria avoids the cases where:

1. arg2 > arg1 and the operation does not have overflow
2. arg2 < arg1 and the operation has an overflow
3. arg2 == arg1

In case 1) neither SF nor OF are set, in case 2) OF will be set and SF will be set since the overflow will reset the most significant bit to one and in case 3) neither SF nor OF will be set.

Jump if Less or Equal

SF <> OF or ZF = 1.

Loads EIP with the specified address, if first operand of previous CMP instruction is lesser than or equal to the second. See the JL section for a more detailed description of the criteria.

Jump if Lesser (unsigned comparison)

CF = 1

Loads EIP with the specified address, if first operand of previous CMP instruction is lesser than the second. jb is the same as jl, except that it performs an unsigned comparison.

Jump if Lesser or Equal (unsigned comparison)

CF = 1 or ZF = 1

Loads EIP with the specified address, if first operand of previous CMP instruction is lesser than or equal to the second. jbe is the same as jle, except that it performs an unsigned comparison.

Jump if Overflow

OF = 1

Loads EIP with the specified address, if the overflow bit is set on a previous arithmetic expression.

Jump if Not Overflow

OF = 0

Loads EIP with the specified address, if the overflow bit is not set on a previous arithmetic expression.

Jump if Zero

ZF = 1

Loads EIP with the specified address, if the zero bit is set from a previous arithmetic expression. jz is identical to je.

Jump if Not Zero

ZF = 0

Loads EIP with the specified address, if the zero bit is not set from a previous arithmetic expression. jnz is identical to jne.

Jump if Signed

SF = 1

Loads EIP with the specified address, if the sign bit is set from a previous arithmetic expression.

Jump if Not Signed

SF = 0

Loads EIP with the specified address, if the sign bit is not set from a previous arithmetic expression.

Function Calls

Pushes the address of the next opcode onto the top of the stack, and jumps to the specified location. This is used mostly for subroutines.

Loads the next value on the stack into EIP, and then pops the specified number of bytes off the stack. If val is not supplied, the instruction will not pop any values off the stack after returning.

Loop Instructions

The loop instruction decrements ECX and jumps to the address specified by arg unless decrementing ECX caused its value to become zero. For example:

 mov ecx, 5
 start_loop:
 ; the code here would be executed 5 times
 loop start_loop

loop does not set any flags.

These loop instructions decrement ECX and jump to the address specified by arg if their condition is satisfied (that is, a specific flag is set), unless decrementing ECX caused its value to become zero.

• loope loop if equal
• loopne loop if not equal
• loopnz loop if not zero
• loopz loop if zero

Enter and Leave

Creates a stack frame with the specified amount of space allocated on the stack.

destroys the current stack frame, and restores the previous frame. Using Intel syntax this is equivalent to:

mov esp, ebp
pop ebp

This will set EBP and ESP to their respective value before the function prologue began therefore reversing any modification to the stack that took place during the prologue.

Other Control Instructions

Halts the processor. Execution will be resumed after processing next hardware interrupt, unless IF is cleared.

No operation. This instruction doesn't do anything, but wastes an instruction cycle in the processor. This instruction is often represented as an XCHG operation with the operands EAX and EAX.

Asserts #LOCK prefix on next instruction.

Waits for the FPU to finish its last calculation.