be more explicit about the abstract machine earlier

[web.git] / ralf / _posts / 2019-07-14-uninit.md

diff --git a/ralf/_posts/2019-07-14-uninit.md b/ralf/_posts/2019-07-14-uninit.md
index c5043ff1bf7925ba584fcfe0cd3092634c1e866a..f925b796096e0b537569be1361a4bc49ce64a626 100644 (file)
--- a/ralf/_posts/2019-07-14-uninit.md
+++ b/ralf/_posts/2019-07-14-uninit.md
@@ -54,7 +54,7 @@ However, if you [run the example](https://play.rust-lang.org/?version=stable&mod
 ## 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.
@@ -69,7 +69,8 @@ So, one time we "look" at `x` it can be at least 150, and then when we look at i
 `x` was just uninitialized all the time.
 That explains why our compiled example program behaves the way it does.
 
-When thinking about Rust (or C, or C++), you have to imagine that every byte in memory is either initialized to some value in `0..256`, or *uninitialized*.
+When thinking about Rust (or C, or C++), you have to think in terms of an "abstract machine", not the real hardware you are using.
+Imagine that every byte in memory is either initialized to some value in `0..256`, or *uninitialized*.
 You can think of memory as storing an `Option<u8>` at every location.[^pointers]
 When new memory gets allocated for a local variable (on the stack) or on the heap, there is actually nothing random happening, everything is completely deterministic: every single byte of this memory is marked as *uninitialized*.
 Every location stores a `None`.