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    <p>You are right, when you compile code for 32 bit architectures, you have to simulate 64 bit operands and operations using 32 bit operands.</p> <p>An 8-byte variable (<code>uint64_t</code> which is just a typedef for <code>long long</code>) is stored in 2 4-byte registers.</p> <p>For adding (and subtracting), you have to first add the lower 4 bytes and then perform a second add <strong>with carry</strong> (or a subtract <strong>with borrow</strong>) on the higher 4 bytes. Since the second add also adds the carry from the first add, the result is correct. The overhead for adding and subtracting is not much.</p> <p>For multiplying and division however, things aren't that simple. Usually a routine is called to perform these kind of operations, and the overhead is significantly bigger.</p> <hr> <p>Lets take this simple c code:</p> <pre><code>int main() { long long a = 0x0102030405060708; long long b = 0xA1A2A3A4A5A6A7A8; long long c = 0xB1B2B3B4B5B6B7B8; c = a + b; c = a - b; c = a * b; c = a / b; return 0; } </code></pre> <p>Analyzing the assembly generated by MSVC we can see:</p> <hr> <pre><code> 2: long long a = 0x0102030405060708; 012D13DE mov dword ptr [a],5060708h 012D13E5 mov dword ptr [ebp-8],1020304h 3: long long b = 0xA1A2A3A4A5A6A7A8; 012D13EC mov dword ptr [b],0A5A6A7A8h 012D13F3 mov dword ptr [ebp-18h],0A1A2A3A4h 4: long long c = 0xB1B2B3B4B5B6B7B8; 012D13FA mov dword ptr [c],0B5B6B7B8h 012D1401 mov dword ptr [ebp-28h],0B1B2B3B4h </code></pre> <p>a 64 bit variable is split in 2 32-bit locations.</p> <hr> <pre><code> 6: c = a + b; 012D1408 mov eax,dword ptr [a] 012D140B add eax,dword ptr [b] 012D140E mov ecx,dword ptr [ebp-8] 012D1411 adc ecx,dword ptr [ebp-18h] 012D1414 mov dword ptr [c],eax 012D1417 mov dword ptr [ebp-28h],ecx 7: c = a - b; 012D141A mov eax,dword ptr [a] 012D141D sub eax,dword ptr [b] 012D1420 mov ecx,dword ptr [ebp-8] 012D1423 sbb ecx,dword ptr [ebp-18h] 012D1426 mov dword ptr [c],eax 012D1429 mov dword ptr [ebp-28h],ecx </code></pre> <p>a sum is performed with an <code>add</code> instruction on the lower 32 bits and then with an <code>adc</code> (add with carry) for the higher 32 bits. Subtraction is similar: the second operation is <code>sbb</code> (subtract with borrow).</p> <hr> <pre><code> 8: c = a * b; 012D142C mov eax,dword ptr [ebp-18h] 012D142F push eax 012D1430 mov ecx,dword ptr [b] 012D1433 push ecx 012D1434 mov edx,dword ptr [ebp-8] 012D1437 push edx 012D1438 mov eax,dword ptr [a] 012D143B push eax 012D143C call __allmul (012D105Ah) 012D1441 mov dword ptr [c],eax 012D1444 mov dword ptr [ebp-28h],edx 9: c = a / b; 012D1447 mov eax,dword ptr [ebp-18h] 012D144A push eax 012D144B mov ecx,dword ptr [b] 012D144E push ecx 012D144F mov edx,dword ptr [ebp-8] 012D1452 push edx 012D1453 mov eax,dword ptr [a] 012D1456 push eax 012D1457 call __alldiv (012D1078h) 012D145C mov dword ptr [c],eax 012D145F mov dword ptr [ebp-28h],edx </code></pre> <p>The product and division are performed by calling special routines.</p>
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    1. POHow are the 64-bit types and operations implemented on the lowest level on a 32-bit architecture?
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    1. POHow are the 64-bit types and operations implemented on the lowest level on a 32-bit architecture?
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    1. COThank you for the extensive answer.
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    2. CO@user2820379 You are welcome.
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