diff --git a/content/blog/writing-portable-arm64-assembly.md b/content/blog/writing-portable-arm64-assembly.md
index 6a02804..6815161 100644
--- a/content/blog/writing-portable-arm64-assembly.md
+++ b/content/blog/writing-portable-arm64-assembly.md
@@ -47,6 +47,14 @@ data, for example `STT_FUNC` and `STT_OBJECT`.  There is no equivalence
 in Mach-O, and thus the `.type` directive that you would use when writing
 assembly for ELF targets is not supported.
 
+### A brief note on Platform ABIs
+
+You will also need to be aware of minor differences between the Darwin
+ABI and other platform ABIs.  A notable example is that the `x18`
+register is reserved by the Darwin ABI and is explicitly zeroed on
+context switches in some cases.  This register is also reserved on
+Android, but not on GNU/Linux or Alpine.
+
 ## Apple-specific vector mnemonics
 
 The other main thing to watch out for is Apple's custom mnemonics for
@@ -82,7 +90,7 @@ The first macro allows you to deal with the underscore requirement of the
 Darwin ABI:
 
 ```c
-#ifdef __MACH__
+#ifdef __APPLE__
 # define PROC_NAME(__proc) _ ## __proc
 #else
 # define PROC_NAME(__proc) __proc