Question: There are a lot of retro-computers with BASIC as an integral part of the operating system. The IBM 5100 was one of the first computers with a separate memory space for BASIC, followed by the Commodore PET, Apple II and TRS-80.

But why was BASIC built into their operating systems, using memory? From [this memory map](http://sta.c64.org/cbm64mem.html), the Commodore 64 had memory completely reserved for BASIC. There were computers, like the Altair 8800, that provided BASIC as software that could be loaded into memory when necessary, and these computers both worked and sold well.

So why was BASIC built into so many computers' operating systems?

Answer: 
Having BASIC available for the machine was a selling point so early adopters wouldn't have to wait for software to become available--they could write what they need themselves, and they wouldn't need to learn machine language to do it.

> 
> [Bill] Gates [believed](https://en.wikipedia.org/wiki/Altair_BASIC#Origin_and_development) that, by providing a BASIC interpreter for
>  the new [MITS Altair] computer, they could make it more attractive to
>  hobbyists.
> 
> 
>
A lot of great information has been given.

One more thing to consider is what the machines were like. Most had no decent non-volatile storage - no hard drives, many didn't have diskette drives either. 

Even the original IBM-PC came with a cassette tape interface - they used a standard audio cassette recorder/player. Even when they had a diskette drive, the drive was often painfully slow. (Looking at you, Commodore 64.)

Having a programming language built in meant that you only needed to save/load the very small program, and it meant that every instance of a given computer (ie, C64, TRS-80 Model III, TI/99, etc) had the exact same language available on it.

This had value because computer magazines actually had programs printed in them, and the readers would laboriously type them in, debug the typos, and run them.