But the halting problem is specifically for any kind of program. Otherwise you can just say that every codebase is smaller than X petabytes anyway so it’s always decidable.
Architectures like x86 can only address a finite amount of RAM, since they have a fixed word size (e.g. 64 bits). However, their memory is still unlimited, since they can read and write to arbitrary IO devices (HDDs, SSDs, S3, etc.); though those operations aren't constant time, they're O(sqrt(n)), since they require more and more layers of indirection (e.g. using an SSD to store the S3 URL of the address of ....)