Quick introduction to reverse engineering for beginners

sub esp, 10h
; put a and b to local stack:
mov eax, [ebp+a_first_half]
mov dword ptr [ebp+a], eax
mov eax, [ebp+a_second_half]
mov dword ptr [ebp+a+4], eax
mov eax, [ebp+b_first_half]
mov dword ptr [ebp+b], eax
mov eax, [ebp+b_second_half]
mov dword ptr [ebp+b+4], eax
; load a and b to FPU stack:
fld [ebp+a]
fld [ebp+b]
; current stack state: ST(0) - b; ST(1) - a
fxch st(1) ; this instruction swapping ST(1) and ST(0)
; current stack state: ST(0) - a; ST(1) - b
fucompp ; compare a and b and pop two values from stack, i.e., a and b
fnstsw ax ; store FPU status to AX
sahf ; load SF, ZF, AF, PF, and CF flags state from AH
setnbe al ; store 1 to AL if CF=0 and ZF=0
test al, al ; AL==0 ?
jz short loc_8048453 ; yes
fld [ebp+a]
jmp short locret_8048456
loc_8048453:
fld [ebp+b]
locret_8048456:
leave
retn
d_max endp
FUCOMPP — is almost like FCOM, but popping both values from stack and handling “not-a-numbers” differently.
More about not-a-numbers:
FPU is able to work with special values which are not-a-numbers or NaNs 37 . These are infinity, result of
dividing by zero, etc. Not-a-numbers can be “quiet” and “signalling”. It is possible to continue to work with
“quiet” NaNs, but if one try to do some operation with “signalling” NaNs — an exception will be raised.
FCOM will raise exception if any operand — NaN. FUCOM will raise exception only if any operand —
signalling NaN (SNaN).
The following instruction is SAHF — this is rare instruction in the code which is not use FPU. 8 bits from
AH is movinto into lower 8 bits of CPU flags in the following order: SF:ZF:-:AF:-:PF:-:CF