3.6. NETWORK ADDRESS CALCULATION EXAMPLE
; ECX=000000bb
shl eax, 8
; EAX=00ddcc00
or eax, ecx
; EAX=00ddccbb
movzx ecx, BYTE PTR _ip4$[esp-4]
; ECX=000000aa
shl eax, 8
; EAX=ddccbb00
or eax, ecx
; EAX=ddccbbaa
ret 0
_form_IP ENDP
We could say that each byte is written to the lowest 8 bits of the return value, and then the return value
is shifted left by one byte at each step.
Repeat 4 times for each input byte.
That’s it! Unfortunately, there are probably no other ways to do it.
There are no popularCPUs orISAs which has instruction for composing a value from bits or bytes.
It’s all usually done by bit shifting and ORing.
3.6.3 print_as_IP().
print_as_IP()does the inverse: splitting a 32-bit value into 4 bytes.
Slicing works somewhat simpler: just shift input value by 24, 16, 8 or 0 bits, take the bits from zeroth to
seventh (lowest byte), and that’s it:
Listing 3.10: Non-optimizing MSVC 2012_a$ = 8 ; size = 4
_print_as_IP PROC
push ebp
mov ebp, esp
mov eax, DWORD PTR _a$[ebp]
; EAX=ddccbbaa
and eax, 255
; EAX=000000aa
push eax
mov ecx, DWORD PTR _a$[ebp]
; ECX=ddccbbaa
shr ecx, 8
; ECX=00ddccbb
and ecx, 255
; ECX=000000bb
push ecx
mov edx, DWORD PTR _a$[ebp]
; EDX=ddccbbaa
shr edx, 16
; EDX=0000ddcc
and edx, 255
; EDX=000000cc
push edx
mov eax, DWORD PTR _a$[ebp]
; EAX=ddccbbaa
shr eax, 24
; EAX=000000dd
and eax, 255 ; probably redundant instruction
; EAX=000000dd
push eax
push OFFSET $SG2973 ; '%d.%d.%d.%d'
call DWORD PTR impprintf
add esp, 20
pop ebp
ret 0
_print_as_IP ENDP