Assembly Language for Beginners

3.6. NETWORK ADDRESS CALCULATION EXAMPLE

uint32_t netmask=form_netmask(netmask_bits);

uint32_t ip=form_IP(ip1, ip2, ip3, ip4);

uint32_t netw_adr;

printf ("netmask=");

print_as_IP (netmask);

netw_adr=ip&netmask;

printf ("network address=");

print_as_IP (netw_adr);

};

int main()

{

calc_network_address (10, 1, 2, 4, 24); // 10.1.2.4, /24

calc_network_address (10, 1, 2, 4, 8); // 10.1.2.4, /8

calc_network_address (10, 1, 2, 4, 25); // 10.1.2.4, /25

calc_network_address (10, 1, 2, 64, 26); // 10.1.2.4, /26

};

3.6.1 calc_network_address()

calc_network_address()function is simplest one: it just ANDs the host address with the network mask,

resulting in the network address.

Listing 3.7: Optimizing MSVC 2012 /Ob0
1 _ip1$ = 8 ; size = 1
2 _ip2$ = 12 ; size = 1
3 _ip3$ = 16 ; size = 1
4 _ip4$ = 20 ; size = 1
5 _netmask_bits$ = 24 ; size = 1
6 _calc_network_address PROC
7 push edi
8 push DWORD PTR _netmask_bits$[esp]
9 call _form_netmask
10 push OFFSET $SG3045 ; 'netmask='
11 mov edi, eax
12 call DWORD PTR impprintf
13 push edi
14 call _print_as_IP
15 push OFFSET $SG3046 ; 'network address='
16 call DWORD PTR impprintf
17 push DWORD PTR _ip4$[esp+16]
18 push DWORD PTR _ip3$[esp+20]
19 push DWORD PTR _ip2$[esp+24]
20 push DWORD PTR _ip1$[esp+28]
21 call _form_IP
22 and eax, edi ; network address = host address & netmask
23 push eax
24 call _print_as_IP
25 add esp, 36
26 pop edi
27 ret 0
28 _calc_network_address ENDP

At line 22 we see the most importantAND—here the network address is calculated.

3.6.2 form_IP().

Theform_IP()function just puts all 4 bytes into a 32-bit value.

Here is how it is usually done:

• Allocate a variable for the return value. Set it to 0.