## What *is* uninitialized memory?
How is this possible?
-The answer is that every byte in memory cannot just have a value in `0..256` (this is Rust/Ruby syntax for a left-inclusive right-exclusive range), it can also be "uninitialized".
+The answer is that, in the "abstract machine" that is used to specify the behavior of our program, every byte in memory cannot just have a value in `0..256` (this is Rust/Ruby syntax for a left-inclusive right-exclusive range), it can also be "uninitialized".
Memory *remembers* if you initialized it.
The `x` that is passed to `always_return_true` is *not* the 8-bit representation of some number, it is an uninitialized byte.
Performing operations such as comparison on uninitialized bytes is undefined behavior.
The real, physical hardware that we end up running the compiled program on is a very efficient *but imprecise* implementation of this abstract machine, and all the rules that Rust has for undefined behavior work together to make sure that this imprecision is not visible for *well-behaved* (UB-free) programs.
But for programs that do have UB, this "illusion" breaks down, and [anything is possible](https://raphlinus.github.io/programming/rust/2018/08/17/undefined-behavior.html).
-UB-free programs can be made sense of by looking at their assembly, but *whether* a program has UB is impossible to tell on that level.
+*Only* UB-free programs can be made sense of by looking at their assembly, but *whether* a program has UB is impossible to tell on that level.
For that, you need to think in terms of the abstract machine.[^sanitizer]
[^sanitizer]: This does imply that tools like valgrind, that work on the final assembly, can never reliably detect *all* UB.
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