Stack and heap in real mode

Hello, I'm reading source code of arch/x86/boot/header.S and arch/x86/boot/main.c and can't understand is stack under heap in memory or vice versa. I'm reading code and see that heap (if CAN_USE_HEAP is set) is under the stack in memory but some comments in code and documentation says that heap is above stack. So am i right that stack is above heap? Thank you.