added short overview of the library modules

docs/source/index.rst

@@ -171,6 +171,7 @@ If you're targeting the CommanderX16, there's the `x16emu <https://github.com/co
     building.rst
     programming.rst
     syntaxreference.rst
+    libraries.rst
     todo.rst
 
 

docs/source/libraries.rst

@@ -0,0 +1,94 @@
+************************
+Compiler library modules
+************************
+
+The compiler provides several "built-in" library modules with useful subroutine and variables.
+
+Some of these may be specific for a certain compilation target, or work slightly different,
+but some effort is put into making them available across compilation targets.
+
+This means that as long as your program is only using the subroutines from these
+libraries and not using hardware- and/or system dependent code, and isn't hardcoding certain
+assumptions like the screen size, the exact same source program can
+be compiled for multiple different target platforms. Many of the example programs that come
+with Prog8 are written like this.
+
+You can ``%import`` and use these modules explicitly, but the compiler may also import one or more
+of these library modules automatically as required.
+
+.. note::
+    The resulting compiled binary program *only works on the target machine it was compiled for*.
+    You must recompile the program for every target you want to run it on.
+
+
+
+syslib
+------
+The "system library" for your target machine. It contains many system-specific definitions such
+as ROM/kernal subroutine definitions, memory location constants, and utility subroutines.
+
+Many of these definitions overlap for the C64 and Commander X16 targets so it is still possible
+to write programs that work on both targets without modifications.
+
+conv
+----
+Routines to convert strings to numbers or vice versa.
+
+- numbers to strings, in various formats (binary, hex, decimal)
+- strings in decimal format into numbers
+- (TODO: strings in hex and binary format into numbers)
+
+textio (txt.*)
+--------------
+This will probably be the most used library module. It contains a whole lot of routines
+dealing with text-based input and output (to the screen). Such as
+
+- printing strings and numbers
+- reading text input from the user via the keyboard
+- filling or clearing the screen and colors
+- scrolling the text on the screen
+- placing individual characters on the screen
+
+
+diskio
+------
+Provides several routines that deal with disk drive I/O, such as:
+
+- show directory
+- display disk drive status
+- load and save data from and to the disk
+- delete and rename files on the disk
+
+
+floats
+------
+Provides definitions for the ROM/kernel subroutines and utility routines dealing with floating
+point variables.  This includes ``print_f``, the routine used to print floating point numbers.
+
+
+graphics
+--------
+High-res monochrome bitmap graphics routines:
+
+- clearing the screen
+- drawing lines
+- drawing circles and discs (filled circles)
+- plotting individual pixels
+
+
+math
+----
+Low level math routines. You should not normally have to bother with this directly.
+The compiler needs it to implement most of the math operations in your programs.
+
+
+cx16logo
+--------
+A 'fun' module that contains the Commander X16 logo and that allows you
+to print it anywhere on the screen.
+
+
+prog8_lib
+---------
+Low level language support. You should not normally have to bother with this directly.
+The compiler needs it for verious built-in system routines.