Documentation of kernel API

doc/syscalls.md

@@ -286,3 +286,29 @@ the buffer pointed to by `dest`. On success, the C accumulator will contain STAT
 and the X register will contain the number of bytes actually read. On error, the C
 accumulator will contain a negative value indicating the specific error, and the X
 register will contain the number of bytes actually read.
+
+## 0x11: Write to File Handle
+
+Arguments:
+    C: 0x11
+    X: Bank of argument block
+    Y: Address of argument block
+
+Return Values:
+    C: Status code
+    X: Number of bytes actually written
+
+Argument block structure:
+
+    struct {
+        void *src; // Pointer to the buffer to use.
+        uint16_t file; // File handle.
+        size_t length; // Maximum number of bytes to write.
+    };
+
+
+This call attempts to write up to `length` bytes to the file handle `file`, from
+the buffer pointed to by `src`. On success, the C accumulator will contain STATUS_OK,
+and the X register will contain the number of bytes actually written. On error, the C
+accumulator will contain a negative value indicating the specific error, and the X
+register will contain the number of bytes actually written.