Basic XL was a further, but much larger, expansion on the Basic A+ model. As it was also based on the original source code, Basic XL remains a strict superset of Atari BASIC, and will run any Atari BASIC program without modification. In contrast to A+, however, XL added many new commands to work with player/missile graphics, simplified handling of joysticks and light pens, autogenerated line numbers and renumbering, and other extensions.
Much more notable, however, was Basic XL's FAST command. Atari BASIC was notoriously slow, due largely to two problems. One was that the floating point routines in the Atari OS ROMs were very slow, and every number in BASIC, even line numbers and array indexes, were sent through this code. The other issue stemmed from the way Atari BASIC handled GOTO and FOR/NEXT commands. FAST fixed the later of these problems; when this command was encountered within a program, Basic XL cached the location line numbers pointed to by the GOTO and FOR/NEXT loops. This made the jumps dramatically faster than Atari BASIC, which had to scan through the entire source code every time through a loop.
Another notable feature of Basic XL was the AUTORUN system. This was a small runtime library that could be added to a disk with Basic XL programs, allowing them to run on machines that did not have the Basic XL system. This made it far more practical than A+. On top of this, Basic XL was shipped in the form of a 16k "supercartridge" that used bank switching so it only took up 8k of memory, making its footprint the same as Atari BASIC, as opposed to A+'s much larger size.
Basic XL was widely lauded in the press, and some stated it should be renamed Atari BASIC and built-into every machine. However, the release of TURBO-BASIC XL a year later largely ended the market for Basic XL. A further expansion, Basic XE, never became very popular.
On Side B the suffix BXE is used. Don't get confused, it is Basic XL, OSS just used the suffix BXE for that disk.
In October of 2019 robdaemon from AtariAge made a strange discovery the community didn't had a clue of, please see the discussion on AtariAge. Due to some discussions in the backgroud and again with the great help from 'kind soul', we could receive the toolkit in version 1.02. After a binary compare of each file on both disks, we could find, that 2 files only are different. These are AUTORUN.SYS and EXTEND.COM. The investigations still continue as of March 2020. If you have the 1.02 cartridge, then please use the toolkit 1.02 and if you have the 1.03 cartridge, then please use the toolkit 1.03. AtariWiki further highly recommends the reading of the fall 1986 OSS Newsletter from page 6 on for this case. In the newsletter the reader will even find a 6 lines code fix for problems regarding with the toolkit.
According to Alfred from AtariAge, there is a problem with EXTEND: Wrong is that the MOVE $0570,$C4,4 statement doesn't work. $0570 is loaded with the proper values, but the cart command doesn't actually move the four bytes. If you deposit the proper values in $C4 and $C6 then the extension works. Therefore please use: MOVE $0570,$C4,-4 to install the extension. Thank you very much Alfred, we really appreciate your help.
Some example programs ; thanks to Charlie Chaplin from AtariAge. :-)