The simplest allocator ever is this:

    void *malloc(int size) {
        static long nextAddress = BASE_ADDRESS;
        void *ptr = (void *)nextAddress;
        nextAddress += size;
        return ptr;
    }

    void free(void *ptr) {
        return;
    }
    

Using that will last you a long time in writing a toy operating system kernel. Of course, it will eventually run out of memory pages, but that shouldn't happen until your system has been running for a long time.