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by James Robertson.
Original Post: Virtual Machines: a reprise
Feed Title: Richard Demers Blog
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James Robertson, Blaine Buxton and others have been talking about virtural machines (VMs) and their relationship to the features of specific languages -- specifically how the JVM's support for static typing makes it a dog for dynamic languages like Smalltalk. This got me thinking about virtual machines in general.
VMs provide a layer of abstraction for programming languages, making the languages portable to many platforms and providing them with a variety of higher level services, such as memory and process management. Further, VMs make it possible for multiple users to share the limited physical resources of the underlying system, such as printers and communication ports.
Most VMs are tailored to the needs of specific programming languages, but there have been attempts at VMs that support multiple languages. One such was the "universal" VM attempted by IBM to support Smalltalk, Java and other languages. I haven't heard anything about it recently.
But there is at least one multi-language VM that succeeded extremely well; the "system-level" VM of IBM OS/400 -- "system-level" because it was as an integral part of the operating system. The key difference is that this VM was designed to support a variety of well-known procedural languages; namely RPG, COBOL, PL/I, C++ and its own high-level Assembler Language. While there are many syntactic and semantic differences among them, these languages shared common views of how data is to be mapped to memory and how procedures are handled in a call-stack. And most important, a new suite of compilers was developed specifically to run on the VM. As far as I know, this VM has also been extended to support Java and Smalltalk, but I suspect they had their own VMs that were run on top of the system VM. (This is not as bad as it sounds because the VM "translates" intermediate programs to a lower-level machine interface.) We would have to get someone who is more current on OS/400 (than I am) to comment further.
I should also mention that the OS/400 VM is multi-user and multi-processor. All distinctions between main memory and auxiliary memory (on disk) are hidden from programmers and other users. This large (64-bit) memory consists of encapsulated objects with a high level of security and sharing provided by VM supported "capabilities." My dream of an ideal VM is a Smalltalk VM tightly integrated with the OS/400 VM. For example, a Smalltalk image could be built as a collection of "system objects" (each a separate paged address space) to handle transient vs persistent objects and small vs large objects.
Meanwhile, OS/400 remains IBM's best kept secret! In the 10+ years since I left IBM, I have hardly heard or seen a word about it. But yes, it is still being used.
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