> And if you were really fancy, you wrote a custom allocator that operated on that fixed-size buffer so people could “allocate” and “free” memory from the buffe

> And if you were really fancy, you wrote a custom allocator that operated on that fixed-size buffer so people could “allocate” and “free” memory from the buffer. That's still what current day allocators are. Think of RAM as just a byte array in your favorite systems programming language. It's size is that fixed-size buffer (or, when RAM size is unknown due to hardware modularity, its size is the maximum possible and you pinky-promise not to look beyond the end). The kernel sees all of the RAM as a fixed-size buffer, and an allocator manages what's free etc. These days, userspace processes see virtual addresses and you can think of them as all getting 2*64 bytes as their "fixed-size buffer". brk and mmap are really more hints to the kernel as to what part of the buffer you won't be needing.