On the Minix system, the memory for a process is divided in the following manner:
Instructions (e.g., MOV AX, BX) are stored in the text segment, initialized global variables are stored in the data segment, and uninitialized global variables are stored in the bss. Dynamically allocated memory is allocated from the heap (generally using malloc()), and automatic variables (among other things) are allocated from the stack.
The "break" is the boundary between the (data + bss + the space previously allocated from the heap) and the unallocated space from the heap. alloc(size_t size) increases the size of the (data + bss + already allocated space from the heap) (by calling sbrk()) and returns a pointer to the newly claimed area. If size, alloc()'s only parameter, is a multiple of 4, size bytes are claimed. If size is not a multiple of 4, the value is rounded up to the next multiple of 4 and this space is claimed.