Talk:Java virtual machine

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Regarding this edit and subsequent reversion made by User:Comp.arch, I still don't see how it's relevant. There is no such thing as "64-bit Java". Java bytecode only understands 32-bit words, with longs and doubles being handled as double-words. That's what JVMs must interpret and execute. They can do so on any hardware they want, but it doesn't change the fact that the Java bytecode is still using 32-bit words.

If you could write a JVM for the PDP-10, you would not have "36-bit Java". You would have a JVM that runs 32-bit Java bytecode on a 36-bit machine.

It doesn't matter that the heap can be larger when you run on a 64-bit machine; the JVM doesn't require any particular implementation of the heap and references, so this is just an artifact of using different techniques to implement the heap and references in JVM implementations for 64-bit platforms. I'm certain it's possible to use techniques on 32-bit platforms to get a heap larger than 4GB, but it would just be unusably slow because it wouldn't all fit in RAM, so nobody has bothered to do it.

The quoted source even clearly explains all of this. "You can think of this environment as being just another platform to which we've ported the SDK." Exactly my point. And they are equally clear that the Java bytecode is unchanged. "We did not increase the size of Java integers from 32 to 64... Array indexes, which are defined in the Java Virtual Machine Specification, are not widened from 32 to 64."

This text just amounts to saying "You can also implement a JVM on a 64-bit platform, but it changes nothing about Java bytecode or the JVM's capabilities." Not interesting, not informative, and also not relevant in an article that's describing the characteristics of abstract JVMs rather than characteristics of JVM implementations. I maintain that this text is unnecessary and should be removed. --Bigpeteb (talk) 16:07, 4 April 2018 (UTC)[reply]

You say the text amounts to 64-bit data model "changes nothing about [..] the JVM's capabilities." I believe you're simply wrong there, as 64-bit addressing, means virtually unlimited heap size (only limited by RAM (or virtual memory, RAM+swap)). This is the most common reason people want and go for 64-bit CPUs. Saying the "64 bit data model" is "misleading" and dropping sourced information, as it's "misleading" is a WP:NPOV-violation, when I can also say NOT having the information is misleading". Unlimited heap size vs. much lower down to 1.4 GB:

"The maximum theoretical heap limit for the 32-bit JVM is 4G. Due to various additional constraints such as available swap, kernel address space usage, memory fragmentation, and VM overhead, in practice the limit can be much lower. On most modern 32-bit Windows systems the maximum heap size will range from 1.4G to 1.6G. On 32-bit Solaris kernels the address space is limited to 2G."

"I'm certain it's possible to use techniques on 32-bit platforms to get a heap larger than 4GB, but it would just be unusably slow because it wouldn't all fit in RAM, so nobody has bothered to do it." That's not (just) the reason. 32-bit x86 CPUs have long allowed up to 64 GB of RAM, with Physical Address Extension (PAE), "total physical address size increases from 32 bits to 36 bits", usable for the OS and all programs combined, but each one still uses 32-bit pointers, thus limited to 4 GB (not total RAM size). I.e. the pointer size matters (not just integers size), and the "[base] pointer" (or in Java "reference") is enlarged in the 64 bit data model. comp.arch (talk) 16:35, 15 July 2018 (UTC)[reply]

One more thing, note: "Writes to and reads of references are always atomic, regardless of whether they are implemented as 32-bit or 64-bit values."[1] I.e. 64-bit is supported that way (and is important), as opposed to for integer (or double) types what happen to be 64-bit in length. For such 64-bit values, it's implementation defined (but yes can be atomic, but not to be relied on). comp.arch (talk) 16:51, 15 July 2018 (UTC)[reply]

There are links to a section in this article about a list of C language compilers targeted for JVM. Where is that section? Bravo editors Boycotting Wikipedia! The pretext to erase that section is stupid Why should a section about C compilers targeting the JVM is a so strange subject deserving to be erased? I wont enter in and edition war, I just will search more reliable sources on the web. When I see thinks like this, I wonder if those Boycotting editors are paid to do that. Bravo, bravo, bravo!

oops concept

I'd glad if someone could point out, why there is V15.x.y stated as current version (info box, citing a JDK download page), whereas the online installer from Oracle gives me V1.8.

Somewhere, I noticed that "15.x" is short for "1.15.x", but even then -- why is there 15 stated and I get 8? --2001:4DD4:F333:1:99E0:6841:8281:A0CA (talk) 22:25, 7 February 2021 (UTC)[reply]

A Java virtual machine (JVM) is a virtual machine that enables a computer to run Java programs as well as programs written in other languages that are also compiled to Java bytecode. The JVM is detailed by a specification that formally describes what is required in a JVM implementation. Having a specification ensures interoperability of Java programs across different implementations so that program authors using the Java Development Kit (JDK) need not worry about idiosyncrasies of the underlying hardware platform. 2409:4042:2D01:A237:0:0:5D8B:4703 (talk) 11:27, 25 May 2022 (UTC)[reply]

its ok but what about other languages compilers targeting JVM not JS, that goes in the JS article

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Java virtual machine and keyboard 156.196.222.120 (talk) 17:30, 3 July 2024 (UTC)[reply]

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Paperwork factory 156.196.222.120 (talk) 17:32, 3 July 2024 (UTC)[reply]

There is a move discussion in progress on Talk:JVM (disambiguation) which affects this page. Please participate on that page and not in this talk page section. Thank you. —The Sky Bot 16:04, 17 July 2024 (UTC)[reply]