xex(5): add pseudo-EBNF.

1 files changed, 19 insertions, 1 deletions

diff --git a/xex.rst b/xex.rst
index 41ecb0e..8f77f6e 100644
--- a/xex.rst
+++ b/xex.rst
@@ -21,7 +21,19 @@ The file format consists of one or more segments, each having its own
 load address. This is an absolute address in the Atari's memory map,
 and there's no facility for relocating the code.
 
-Segments have a 4- or 6-byte header. The first segment in the file
+File Structure
+--------------
+
+In pseudo-EBNF, the file structure looks like:
+
+- ffff_header = "$FF", "$FF"
+- initial_segment = ffff_header, segment
+- segment = start_address, end_address, data
+- file = initial_segment, [ (initial_segment | segment) ... ]
+
+In English:
+
+Each segment has a 4- or 6-byte header. The first segment in the file
 must use the 6-byte header. Further segments can use either the 4-byte
 or 6-byte header.
 
@@ -40,6 +52,9 @@ initial **$FF**, **$FF** bytes.
 The rest of each segment just consists of the data to be loaded,
 exactly *(end_address - load_address) + 1* bytes.
 
+Initialization
+--------------
+
 It's possible to run code in the middle of loading the file. Such code
 is usually referred to as an an "init routine". A XEX file can have
 multiple init routines, or none at all.
@@ -51,6 +66,9 @@ to this address, which should exit back to DOS with an **RTS**
 instruction. After the routine exits, DOS will continue loading the
 file normally.
 
+Run Address
+-----------
+
 A .xex file can (and usually does) also have a run address. Unlike
 init addresses, there can be only one run address. If the file
 contains more than one run address, only the last one loaded has any