The emulator will give you instant access to two versions. Both are currently at v1.55. By two versions I mean they map into memory slightly differently or boot differently I think. Page 4 of the Altirra BASIC manual will explain it. All the commands and usages and conventions should otherwise be identical. Too lazy to summarize it at the moment.
The built-in BASIC the emulator uses is a "rom" and is 8,192 bytes in size.
The executable version of BASIC is on the ADDITIONS.ATR disk image and is 9,003 bytes in size.
You can dump the "rom" version by using Tools> Export Rom Set. You will then get ATBASIC.ROM, along with a few other files and a readme.html file.
You can extract the "executable" version by using Tools> Disk Explorer. Open the ADDITIONS.ATR disk image and drag and drop the ATBASICX.COM to wherever.
You can get the latest version of the BASIC Manual as well as the nicely done Hardware Reference Manual from Virtualdub.org
Edited by Keatah, Fri Sep 28, 2018 6:05 PM.