diff --git a/doc/syscalls.md b/doc/syscalls.md
index aaf092c..d3434de 100644
--- a/doc/syscalls.md
+++ b/doc/syscalls.md
@@ -11,6 +11,10 @@ register arguments they take, and what if any values they return.
 The system API is based loosely on, but not compatible with, that of
 CP/M-3 and MSX-DOS 2.
 
+Most calls return a status code in C. This code has one value, STATUS_OK,
+which applies to all such calls; this value indicates that the call was
+successful. Any other value will indicate some specific error.
+
 ## 0x00: Terminate with Error Code
 
 Arguments: