Assembly Language for Beginners

1.19. FLOATING-POINT UNIT

• And ifa=b, then:ZF=1, PF=0, CF=0.

Depending on the CPU flags and conditions,SETNBEstores 1 or 0 to AL. It is almost the counterpart of

JNBE, with the exception thatSETcc^126 stores 1 or 0 inAL, butJccdoes actually jump or not. SETNBE

stores 1 only ifCF=0andZF=0. If it is not true, 0 is to be stored intoAL.

Only in one case bothCFandZFare 0: ifa>b.

Then 1 is to be stored toAL, the subsequentJZis not to be triggered and the function will return _a. In all

other cases, _b is to be returned.

Optimizing GCC 4.4.1

Listing 1.213: Optimizing GCC 4.4.1

public d_max

d_max proc near

arg_0 = qword ptr 8

arg_8 = qword ptr 10h

push ebp

mov ebp, esp

fld [ebp+arg_0] ; _a

fld [ebp+arg_8] ; _b

; stack state now: ST(0) = _b, ST(1) = _a

fxch st(1)

; stack state now: ST(0) = _a, ST(1) = _b

fucom st(1) ; compare _a and _b

fnstsw ax

sahf

ja short loc_8048448

; store ST(0) to ST(0) (idle operation),

; pop value at top of stack,

; leave _b at top

fstp st

jmp short loc_804844A

loc_8048448:

; store _a to ST(1), pop value at top of stack, leave _a at top

fstp st(1)

loc_804844A:

pop ebp

retn

d_max endp

It is almost the same except thatJAis used afterSAHF. Actually, conditional jump instructions that check
“larger”, “lesser”or“equal”forunsignednumbercomparison(theseareJA,JAE,JB,JBE,JE/JZ,JNA,JNAE,
JNB,JNBE,JNE/JNZ) check only flagsCFandZF.

Let’s recall where bitsC3/C2/C0are located in theAHregister after the execution ofFSTSW/FNSTSW:

6 2 1 0

C3 C2C1C0

Let’s also recall, how the bits fromAHare stored into the CPU flags after the execution ofSAHF:

7 6 4 2 0

SFZF AF PF CF

After the comparison, theC3andC0bits are moved intoZFandCF, so the conditional jumps are able work

after.JAis triggering if bothCFareZFzero.

(^126) cciscondition code