> Algol on the Alto, like Smalltalk, had a VM

Was there really an Algol compiler on the Alto? Do the authors rather mix that up with BCPL (which isn't mentioned otherwise in the paper)?

"The Alto software included four different programming environments: BCPL, Mesa, Smalltalk, and Lisp."

https://computerhistory.org/blog/xerox-alto-source-code/

Seems likely given the BCPL implementation was based on a VM.

Was it really a VM, or wasn't just the O-code translated to microcode by the compiler?

VM enough to be discussed in a book about Virtual Machines :-)

https://www.google.com/books/edition/Virtual_Machines/vIB-np...

There was indeed an interpterer for O-code; but according to http://www.bitsavers.org/pdf/xerox/alto/bcpl/AltoBCPLdoc.pdf the Alto BCPL compiler generated relocatable binary files.

"We have demonstrated an implementation of Smalltalk that has no virtual machine, just native code (JIT or statically compiled)."