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--- a/zk/Operating_system_API.md
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@@ -7,9 +7,12 @@ created: Wednesday, July 17, 2024
 # Operating system API
 
 A user can interact with the operating system either via the GUI or the
-[shell](Intro_to_the_Unix_shell.md). Both ultimately access the OS via the shell
-since the UI is an abstraction layer of the shell. When _programs_ interact with
-the operating system they use the **operating system's API**.
+[shell](Intro_to_the_Unix_shell.md). (Both ultimately access the OS via the
+shell since the UI is an abstraction layer of the shell.)
+
+When _programs_ interact with the operating system they use the **operating
+system's API** directly. The GUI and shell also invoke the OS API but the access
+is mediated.
 
 Every OS has an API. This specification details how a program should interact
 with the OS, exposing functions, properties and data types.
@@ -38,3 +41,19 @@ open("foo.txt", 0_WRONLY|0_CREAT)
 This command makes the action write-only and creates the file because it does
 not yet exist. When the user opens a file in their File Explorer, the
 application translates their graphical actions into this API call.
+
+## Different OSs have different APIs
+
+Unix-like and Linux operating systems use variations on the **Portable Operating
+System Interface** (POSIX) standard. This defines the OS API and also the rules
+for the shell's behaviour and its included utilities.
+
+Variations on this include _Cocoa_, macOS's API and Android's _Android Platform_
+API.
+
+Windows has its own API called _The Windows API_. The original version was
+16-bit called _Win16_, then when it moved to 32-bits it became _Win32_. When it
+moved to an x86 [instruction_set_architecture](Instruction_set_architectures.md)
+it became _Win64_. Starting with Windows 10, it introduced the _Universal
+Windows Platform_ which aims to make app development consistent accross
+different devices that run Windows.