Depending on how low-level you're going, you don't even need to call brk.

My toy stop/copy GC and allocation routines just have this:

    %define HEAP_SIZE (4 * 1024 * 1024 * 1024)

    .section bss
    ;;
    ;; Two heaps so we can flip-flop between them
    ;;
    heap_a:
        resb HEAP_SIZE
    heap_a_end:
    heap_b:
        resb HEAP_SIZE
    heap_b_end: