That looks very cool on the videos,
but unfortunately the Symstudio's webpage does not work for me ...
Anyway ... Is the API used by Symstudio the API of SymbOS, too ? So with Symstudio somebody calls SymbOS APIs directly, or has Symstudio its own API to its own library ?
No the website has been gone for some years. It hopefully will be back with a redesign of the symbos site very soon. And yes it 100% uses the Symbos API's where it can, but also has its own library of what I call SymBasic. Apps written in Symbasic are pretty quick. Not as quick as pure z80, but logic & loops etc are 5-10x faster than you'd get from a normal 8bit basic interpreter. I'm pretty confident that the final version will be as quick if not quicker than any C compiler you know on a z80. But you can mix and match between symbsic and Assembler anyway if you need 100% speed. And considering that the symbos API does most of the work the speed difference isnt that noticable anyway.
Currently any app created in Symbasic will carry the Basic code inside the app, meaning that a simple Hello world will be around 13k in size. I am working on a new compile system which actually only includes the used commands/functions and that with the highly optimized compiler will see that come down to around 4k for a simple App.
There actually is a version of C