[sillywalk]

The IBM System/38 and its descendants (AS/400 which was renamed to iSeries, then i5, now just the horribly unsearchable IBM i) sort of did/does this, except 'newer things' ported over like modern web stuff generally use the Unix portability layer, so it's not enforced. Also, it has a SQL database built-in at the OS level..

The original/legacy OS is quite interesting and high level. It's Object Based. You can only use the objects built-in methods e.g. you can't WRITE arbitrary bytes to an Application object, or a User Object. They are accessed as a giant Single address space, and whether they are on disk or in RAM is transparent to higher layers, i.e. if an object isn't in ram it will page it in..

The designer of System/38 etc. Frank Soltis wrote a couple of interesting books on it - Inside The AS/400 and its 2nd edition Fortress Rochester: Inside the iSeries. They're out of print and expensive unfortunately, I wish I hadn't gave my copy away.

[pjmlp]

IBM Redbooks are an alternative source of information for that.