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README.md

@@ -479,4 +479,66 @@ Allows a line of input to be read from the keyboard and to be stored to an array
     pr.str buffer
     pr.nl
 
+## Line Editor
+Eightball includes a simple line editor for editing program text.  Programs are saved to disk in plain text format (ASCII on Apple II, PETSCII on CBM).
 
+Be warned that the line editor is rather primitive.  However we are trying to save memory.
+
+Editor commands start with the colon character (:).
+
+### Load from Disk
+To load a new source file from disk, use the `:r` 'read' command:
+
+    :r "myfile.8b"
+
+### Save to Disk
+To save the current editor buffer to disk, use the :w 'write' command:
+
+    :w "myfile.8b"
+
+On Commodore systems, this must be a new (non-existing) file, or a drive error will result.
+
+### Insert Line(s)
+
+Start inserting text before the specified line.  The editor switches to insert mode, indicated by the '>' character (in inverse green on CBM).  The following command will start inserting text at the beginning of an empty buffer:
+
+    :i0
+    >
+
+One or more lines of code may then be entered.  When you are done, enter a period '.' on a line on its own to return to EightBall immediate mode prompt.
+
+### Append Line(s)
+
+Append is identical to the insert command described above, except that it starts inserting /after/ the specified line.  This is often useful to adding lines following the end of an existing program.
+
+### Delete Line(s)
+
+This command allows one or more lines to be deleted.  To delete one line:
+
+    :d33
+
+or to delete a range of lines:
+
+    :d10,12
+
+### Change Line
+
+This command allows an individual line to be replaced (like inserting a new line the deleting the old line).  It is different to the insert and append commands in that the text is entered immediately following the command (not on a new line).  For example:
+
+    :c21:word var1=12
+
+will replace line 21 with `word var1=12`.  Note the colon terminator following the line number.
+
+Note that the syntax of this command is contrived to allow the CBM screen editor to work on listed output in a similar way to CBM BASIC.  Code may be listed using the `:l` command and the screen may then be interactively edited using the cursor keys and return, just as in BASIC.
+
+### List Line(s)
+
+This allows the program text to be listed to the console.  Either the whole program may be displayed or just a range of lines.  To show everything:
+
+    :l
+
+To show a range of lines:
+
+    :l0-20
+
+(The command is the letter Ell, not the number 1!)