How does a computer turn a string of ASCII into a signed or unsigned number?

Question

For example if I type:

-6

Through what mechanism is that turned into:

1010

Would it be hardware based or somewhere in the kernel?

Answer 1

Would it be hardware based or somewhere in the kernel?

Usually no and no.

The kernel in a mainstream OS like Linux will usually just pass along bytes of text to user-space.

So a user-space program gets a string, i.e. a sequence of characters. (In simple cases, e.g. the ASCII subset of UTF-8, each character is a single byte.) A program would typically use a function like atoi() to convert a sequence of characters (representing ASCII codes for digits) to a binary integer. It's a standard library function because many programs need to deal with strings that represent integers, but it's a software function just like any other.

A simple implementation would have a loop like

int sum = 0;
for (auto d: digits) {   // look at digits in MSB-first order
    sum = 10*sum + d;
}
// the first digit ends up being multiplied by 10 n times
// the 2nd by 10 n-1 times, and so on. Each digit is multiplied by its place value.

This C++ source would be compiled to multiple asm instructions that implement it. Handling an optional - by negating is also a separate instruction. There's typically a neg instruction of some sort, or a way to subtract from zero, to get the 2's complement inverse. (Assuming 2's complement hardware).

---

You can speed this up by using fancier instructions that do more work per instruction / per clock cycle. On x86 for example you can convert a multi-digit string of digits to a binary integer with a few SIMD instructions, but that's still just using multiply and add instructions. See How to implement atoi using SIMD? for a nice use of pmaddwd to multiply by a vector of place-values and horizontally add. Also Fastest way to get IPv4 address from string is a cool examples of what you can do with packed-compare and looking up a pshufb shuffle-control vector from a table based on that compare result.

A function like scanf("%d", &num) that reads input as a number is implemented in user-space, but under the hood it uses a system call like read() to get data. (If the C stdio input buffer was empty.)

---

Some "toy" / teaching systems like the MARS and SPIM MIPS simulators have system calls that get get or print integers (with the input or result in an integer register). In that case, yes, the kernel does it in software.

Or depending on the implementation, there isn't actually a kernel at all, and the syscall instruction escapes to the emulator / simulator's input/output function, so from the POV of software running inside this virtual simulated machine, there really is hardware support for integer conversion. But no real hardware does the entire thing in microcode or actual hardware, at least not any mainstream architectures.