From: Ralf Jung <post@ralfj.de>
Date: Tue, 24 Jul 2018 21:35:23 +0000 (+0200)
Subject: clarify the problem in the last section
X-Git-Url: https://git.ralfj.de/web.git/commitdiff_plain/4234943c0d17581f213fdec15b023163b2b32a9d?ds=sidebyside;hp=4ff0d6f1866f9f6509c5a0abc70cfa1f92d76deb

clarify the problem in the last section
---

diff --git a/personal/_posts/2018-07-24-pointers-and-bytes.md b/personal/_posts/2018-07-24-pointers-and-bytes.md
index 29cc24e..5d7a40b 100644
--- a/personal/_posts/2018-07-24-pointers-and-bytes.md
+++ b/personal/_posts/2018-07-24-pointers-and-bytes.md
@@ -157,9 +157,14 @@ This is why pointers are not simple, either.
 
 I hope I made a convincing argument that integers are not the only data one has to consider when formally specifying low-level languages such as C++ or (the unsafe parts of) Rust.
 However, this means that a simple operation like loading a byte from memory cannot just return a `u8`.
+Imagine we [implement `memcpy`](https://github.com/alexcrichton/rlibc/blob/defb486e765846417a8e73329e8c5196f1dca49a/src/lib.rs#L39) by loading (in turn) every byte of the source into some local variable `v`, and then storing it to the target.
 What if that byte is part of a pointer?  When a pointer is a pair of allocation and offset, what is its first byte?
-We cannot represent this as a `u8`.
+We have to say what the value of `v` is, so we have to find some way to answer this question.
+(And this is an entirely separate issue from the problem with multiplication that came up in the last section. We just assume some abstract type `Pointer`.)
+
+We cannot represent a byte of a pointer as an element of `0..256`.
 Instead, we will remember both the pointer, and which byte of the pointer we got.
+So, a byte is now *either* an element of `0..256` ("raw bits"), *or* the n-th byte of some abstract pointer.
 If we were to implement our memory model in Rust, this might look as follows:
 {% highlight rust %}
 enum ByteV1 {
@@ -168,12 +173,12 @@ enum ByteV1 {
 }
 {% endhighlight %}
 For example, a `PtrFragment(ptr, 0)` represents the first byte of `ptr`.
-This way, we can "take apart" a pointer into the individual bytes that represent this pointer in memory, and assemble it back together.
+This way, `memcpy` can "take apart" a pointer into the individual bytes that represent this pointer in memory, and copy them separately.
 On a 32bit architecture, the full value representing `ptr` consists of the following 4 bytes:
 ```
 [PtrFragment(ptr, 0), PtrFragment(ptr, 1), PtrFragment(ptr, 2), PtrFragment(ptr, 3)]
 ```
-Such a representation supports performing all byte-level "data moving" operations on pointers, like implementing `memcpy` by copying one byte at a time.
+Such a representation supports performing all byte-level "data moving" operations on pointers, which is sufficient for `memcpy`.
 Arithmetic or bit-level operations are not fully supported; as already mentioned above, that requires a more sophisticated pointer representation.
 
 ## Uninitialized Memory