Allocating Executable Memory

[Randall Mercer]

I would like to allocate a chunk of executable memory, write code into
it and then execute it.  What versions of what vendors Unix OS's support
this feature?  I know Sun and Convex have mmap system functions for
this and for allocating shared memory, etc.  Is this a common feature?
Is mmap likely to be supported by a growing number of vendors?

I'll be glad to post a summary of the replies.