format calling convention

README.md

@@ -1,3 +1,38 @@
-# symphony_stdlib
+# Symphony Stdlib
+
+This is a standard library for symphony.
+It is both intended as a practical toolkit to develop more complex software as well as a teaching resource.
+
+If you just want to use the standard library [[stdlib.asm]] is your main header, include it after your code.
+You'll also need to include globals.asm in the first line of your main assembly file.
+
+If you are using it as a learning resource have a look at the [teaching folder](teaching).
+
+If you are intersted in contributing have a look at [[CONTRIBUTING.md]]
+
+---
+
+## ABI
+
+### Calling Convention
+| class | registers |
+| ----- | --------- |
+| n.a. | zr |
+| preserved | sp, r8 - r12 |
+| scratch | flags, r1 - r7 |
+| arguments | r1 - r5, r6 = stack args, r7 = stack res |
+| result | r1 = low word, r2 = high word |
+| return address | r13 |
+
+### Stack
+grows downwards from top of memory
+arguments are passed in reverse order with the stack so:
+lowest address = 1st stack arg
+highest address = last stack arg
+
+### Types
+
+#### String
+Strings are stored in memory as null terminated sequences of bytes encoding ascii characters.
+They should be passed by reference.
 
-standard library for symphony