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That's half the definitional difference between Harvard and Von Neumann architectures, though - That never the twain shall meet. There's a second insight on th

That's half the definitional difference between Harvard and Von Neumann architectures, though - That never the twain shall meet. There's a second insight on the tip of my tongue, here: Harvard architectures can emulate Von Neumann ones, and vice versa. And problems, specific problems, can be solved by either. But it seems to me that there's something interesting about emulation too - That Harvard architectures can open themselves up to precisely the same vectors of attacks if you enable "Copy this data block to code memory".

______-_-______ 5y ago
If you couldn't transfer data memory to code memory at all, how would it be possible to write an ordinary compiler? If it's possible to run a compiler on the system, just have your JIT do whatever the compiler does.
- gumby 5y ago
  Well you write a bytecode compiler and run an interpreter for it out of code space. It could be a JIT compiler, just not one that generated native code. This approach was pretty common in the 80s Lisp days IIRC.
  - samus 5y ago
    Java already compiles to bytecode, which executes on the JVM. But this is too slow for most practical applications. JIT compilation to native code is crucial for execution of Java programs with competitive speed. It would be possible to compile everything to machine code ahead of time (pretty much what GraalVM does), but that would compile everything even if it would never end up being executed*. Without JIT compiler, it would not be possible to recompile code to take advantage of tracing information gathered at runtime. Also, newly loaded Java code would not be able to be optimized at all. There are workarounds for all of these. For example, the JIT compiler could generate a new executable with the optimized code and migrate execution state to a new process. But it seems very clunky. *: Not really an issue with microservices and stripped-down binaries. A huge deal for IDEs and environments with plugin architecture though.
- bronson 5y ago
  Write it to the filesystem (ELF format presumably) and execute the file. Some Harvard architecture implementations provide ways of shoving data into executable space and back (like special copy instructions) but it's not general purpose. Generally you compile on Von Neumann and flash to Harvard.
compressedgas 5y ago
But not if instead of copying, they support a "Run this data as a program in virtual machine with access to these segments of memory" operation. The behavior would be functionally identical to emulating the architecture and interpreting the data, except that it wouldn't be slow and could be done recursively with no cumulative overhead.

______-_-______ 5y ago

If you couldn't transfer data memory to code memory at all, how would it be possible to write an ordinary compiler? If it's possible to run a compiler on the system, just have your JIT do whatever the compiler does.

gumby 5y ago
Well you write a bytecode compiler and run an interpreter for it out of code space. It could be a JIT compiler, just not one that generated native code. This approach was pretty common in the 80s Lisp days IIRC.
- samus 5y ago
  Java already compiles to bytecode, which executes on the JVM. But this is too slow for most practical applications. JIT compilation to native code is crucial for execution of Java programs with competitive speed. It would be possible to compile everything to machine code ahead of time (pretty much what GraalVM does), but that would compile everything even if it would never end up being executed*. Without JIT compiler, it would not be possible to recompile code to take advantage of tracing information gathered at runtime. Also, newly loaded Java code would not be able to be optimized at all. There are workarounds for all of these. For example, the JIT compiler could generate a new executable with the optimized code and migrate execution state to a new process. But it seems very clunky. *: Not really an issue with microservices and stripped-down binaries. A huge deal for IDEs and environments with plugin architecture though.
bronson 5y ago
Write it to the filesystem (ELF format presumably) and execute the file. Some Harvard architecture implementations provide ways of shoving data into executable space and back (like special copy instructions) but it's not general purpose. Generally you compile on Von Neumann and flash to Harvard.

compressedgas 5y ago

But not if instead of copying, they support a "Run this data as a program in virtual machine with access to these segments of memory" operation. The behavior would be functionally identical to emulating the architecture and interpreting the data, except that it wouldn't be slow and could be done recursively with no cumulative overhead.