How do programs allocate large amounts of memory?

I have 3 questions concerning memory allocation that I thought better to put into one question than 3.

1. When memory is allocated as I understand, it is allocated on the heap, which is just 16mb. How hen do programs such as video games or modern browsers manage to use over 1GB?

2. Since it is obviously possible for this much memory to be used, why can it not be allocated at the start? I have found the most I can allocate in High Level Assembly language is around 100MB. This is a lot more than 16MB, and far less than I have 3, so where does this limitation come from?

3. Why allocate memory in the first place, rather than allocat开发者_JAVA技巧ing variables and letting the compiler/system handle it?

When memory is allocated as I understand, it is allocated on the heap, which is just 16mb. How hen do programs such as video games or modern browsers manage to use over 1GB?

The heap can grow. It isn't limited to any value and certainly not 16MB. You can easily allocate 1GB of heap, just make a program test and you'll see.

Since it is obviously possible for this much memory to be used, why can it not be allocated at the start? I have found the most I can allocate in High Level Assembly language is around 100MB. This is a lot more than 16MB, and far less than I have 3, so where does this limitation come from?

I'm not sure why your OS isn't filling larger allocation requests. Perhaps due to memory fragmentation? It's going to be a problem specific to your setup, which you didn't share. I can allocation much more memory than that without an issue.

You can try to use the mmap system call if malloc (which uses the brk system call) is having some sort of issue. Note that for GNU libc, malloc actually uses mmap instead of brk when the allocation is large enough (over 128k I think).

Why allocate memory in the first place, rather than allocating variables and letting the compiler/system handle it?

Variable must live in memory somewhere. What you are saying is "why manually manage memory? Why can't some algorithm do that for me?". It is actually very common for the compiler and a runtime component to handle allocation/freeing - it's called garbage collection.