assembly - Did I translate the very short C code correctly into assembler? -

i'm learning assembly x86 , have made little task myself.

the c code:

if (a == 4711) { = + 2 } else                { = - 2 } 

assembler code (eax register, cmp compare, jne jump if not equal , jmp jump if equal):

         mov eax,          cmp eax, 4711          jmp equal equal:   add eax, 2          jne unequal unequal: sub eax, 2 

i think little more efficient be:

         mov eax,          cmp eax, 4711          jne unequal          add eax, 2 unequal: sub eax, 2 

edit:

         mov eax,          cmp eax, 4711          jne unequal equal:   add eax, 2          jmp continue unequal: sub eax, 2 continue: ... 

did translate correctly?

let's first code:

         mov eax,          cmp eax, 4711          jmp equal equal:   add eax, 2          jne unequal unequal: sub eax, 2 

let's pretend first instruction load eax "a" (it in tasm/masm, rather stick explicit , accurate [a], it's easier read source , works in nasm).

second instruction cmp, subtract 4711 eax, throws result away (not storing anywhere), , flag register affected. if "a" 4711, result of subtraction zero, zf=1 then. otherwise zf=0. (for other flags affected cmp see documentation).

so on line 3 eax still contains value "a", , flag register contains result of cmp eax,4711. , jmp. unconditional jump, happening no matter what, directly continue instruction @ "equal" address, add eax,2. => add 2 "a" in every case.

also add affects flags, "a" == -2 zf=1, otherwise zf=0!

then comes first conditional jump, branching code, based on current flag register content. jne abbreviation of "jump not equal", , "equal" in context means set 0 flag (zf=1).

so when "a" -2, zf 1 ("is equal") ahead of jne, jne not jump "unequal" address, continue next instruction (which @ "unequal" address anyway, jne meaningless).

for "a" different -2 zf 0 ("is not equal"), jne execute jump on provided label, continuing instruction @ address "unequal".

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so have navigate cpu away instructions don't want execute.

    xor eax,eax   ; sets eax 0, , zf=1     jz  label_1   ; zf 1, jump executed, cpu goes "label_1"     inc eax       ; instruction skipped , not executed label_1:     ; eax being still 0, , zf being still set on     ; whatever instruction here, cpu execute after "jz" 

slightly modified example show case when condition false

    xor eax,eax   ; sets eax 0, , cf=0, zf=1, ...     jc  label_1   ; cf 0, "jump carry" not executed     inc eax       ; instruction executed after "jc" label_1:     ; here eax 1     ; cf still 0 (not affected inc)     ; zf 0 (affected inc) 

summary: should have pretty idea instructions affect flags, , in way. when unsure, keep cmp + jcc pair (to not affect flag results cmp accidentally). jcc stands "conditional jump" instruction. when condition met, jump provided label executed. otherwise jcc instruction ignored, , execution continue instruction right after it.

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btw, write c code:

if (a == 4711) { = + 2 } else                { = - 2 } 

as:

    cmp [a],dword 4711     mov eax,2     je a_is_4711     neg eax  ; -2 non 4711 value a_is_4711:     add [a],eax