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

test arg1, arg2	GAS Syntax
test arg2, arg1	Intel syntax

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

cmp arg1, arg2	GAS Syntax
cmp arg2, arg1	Intel syntax

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

Operands

arg1

• Register
• Immediate
• Memory

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, 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

jmp loc

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

Jump on Equality

je loc

ZF = 1

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

mov ecx, 5
mov edx, 5
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 on Inequality

jne loc

ZF = 0

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

Jump if Greater

jg loc

ZF = 0 and SF = OF

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

jge loc

SF = OF

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

ja loc

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.

jae loc

CF = 0

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 Less

jl loc

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.

Example The example code below runs the five cases outlined above and prints out whether SF and OF are equal or not:

;
; nasm -felf32 -g jlFlagsCheck.asm
; gcc -o jlFlagsCheck jlFlagsCheck.o
;
global main 
 
extern printf
 
section .data
        sfneofStr: db 'SF <> OF', 0xA, 0
        sfeqofStr: db 'SF == OF', 0xA, 0
 
section .bss
 
section .text
        main:
 
;
; Functions will follow the cdecl call convention
;
 
        ;
        ; arg2 < arg1 and no overflow
        ;
        mov     eax, 1
        cmp     eax, 2
        call    checkSFNEOF
 
        ;
        ; arg2 < arg1 and overflow
        ;
        mov     al, -2
        cmp     al, 127
        call    checkSFNEOF
 
        ;
        ; arg2 > arg1 and no overflow
        ;
        mov     eax, 2
        cmp     eax, 1 
        call    checkSFNEOF
 
        ;
        ; arg2 > arg1 and overflow
        ;
        mov     al, 127
        cmp     al, -1 
        call    checkSFNEOF
 
        ;
        ; arg2 == arg1
        ;
        mov     eax, 2
        cmp     eax, 2 
        call    checkSFNEOF
 
        call    exit
 
;
; Check if SF <> OF, which means the condition for jump less would be meet. 
;
checkSFNEOF:
        push    ebp
        mov     ebp, esp
 
        jl      SFNEOF
        jmp     SFEQOF  
 
SFNEOF:
        push    dword sfneofStr 
        call    printf
        jmp     checkSFNEOFDone
 
SFEQOF:
        push    dword sfeqofStr 
        call    printf
 
checkSFNEOFDone:
 
        leave
        ret
 
exit:
                                ;
                                ; Call exit(3) syscall
                                ;       void exit(int status)
                                ;
        mov     ebx, 0          ; Arg one: the status
        mov     eax, 1          ; Syscall number:
        int     0x80

The expected output is as follows:

SF <> OF
SF <> OF
SF == OF
SF == OF
SF == OF

jle loc

The criteria required for a JLE is that SF <> OF or ZF == 1, loads EIP with the specified address if the criteria is meet. See the JL section for a more detailed description of the criteria, the one point of departure would be that arg2 == arg1 fulfills the criteria for JLE since ZF == 1.

Example The example code below runs the five cases outlined previously and prints out whether SF and OF are equal or not and whether ZF == 1 or not:

;
; nasm -felf32 -g jleFlagsCheck.asm
; gcc -o jleFlagsCheck jleFlagsCheck.o
;
global main 
 
extern printf
 
section .data
        sfneofStr:      db 'SF <> OF and ZF %s 1', 0xA, 0
        sfeqofStr:      db 'SF == OF and ZF %s 1', 0xA, 0
        ne:             db '<>', 0
        eq:             db '==', 0
section .bss
 
section .text
        main:
 
;
; Functions will follow the cdecl call convention
;
        ;
        ; arg2 < arg1 and no overflow
        ;
        mov     eax, 1
        cmp     eax, 2
        call    checkJLECriteria
 
        ;
        ; arg2 < arg1 and overflow
        ;
        mov     al, -2
        cmp     al, 127
        call    checkJLECriteria
 
        ;
        ; arg2 == arg1
        ;
        mov     eax, 2
        cmp     eax, 2 
        call    checkJLECriteria
 
        ;
        ; arg2 > arg1 and no overflow
        ;
        mov     eax, 2
        cmp     eax, 1 
        call    checkJLECriteria
 
        ;
        ; arg2 > arg1 and overflow
        ;
        mov     al, 127
        cmp     al, -1 
        call    checkJLECriteria
 
        call    exit
 
;
; Check the criteria for JLE, either SF <> OF or ZF == 1
;
checkJLECriteria:
        push    ebp
        mov     ebp, esp
 
        jz      ZFOne
        jmp     ZFZero
 
ZFOne:
        push    dword eq
        jmp     SFOFCheck
 
ZFZero:
        push    dword ne
 
SFOFCheck:
 
        jl      SFNEOF
        jmp     SFEQOF  
 
SFNEOF:
        push    dword sfneofStr 
        call    printf
        jmp     checkJLECriteriaDone
 
SFEQOF:
        push    dword sfeqofStr 
        call    printf
 
checkJLECriteriaDone:
 
        leave
        ret
 
exit:
                                ;
                                ; Call exit(3) syscall
                                ;       void exit(int status)
                                ;
        mov     ebx, 0          ; Arg one: the status
        mov     eax, 1          ; Syscall number:
        int     0x80

The expected output is as follows:

SF <> OF and ZF <> 1
SF <> OF and ZF <> 1
SF == OF and ZF == 1
SF == OF and ZF <> 1
SF == OF and ZF <> 1

jb loc

CF = 1

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

jbe loc

CF = 1 or ZF = 1

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

Jump on Overflow

jo loc

OF = 1

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

jno loc

OF = 0

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

Jump on Zero

jz loc

ZF = 1

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

jnz loc

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.

Function Calls

call proc

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.

ret [val]

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

loop arg

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.

loopx arg

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

enter arg

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

leave

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

hlt

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

nop

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.

lock

Asserts #LOCK prefix on next instruction.

wait

Waits for the FPU to finish its last calculation.