Example for darkcoding mem allocation post. http://www.darkcoding.net/software/how-memory-is-allocated/

alloc.s

.text

.global _start

_start:
        mov $12, %rax   # brk syscall number
        mov $0, %rdi    # 0 is invalid, want to get current position
        syscall

        mov %rax, %rsi  # rsi now points to start of heap mem we'll allocate

        mov %rax, %rdi  # move top of heap to here ...
        add $4, %rdi    # .. plus 4 bytes we allocate
        mov $12, %rax   # brk, again
        syscall

        # Move "HI\n" to our memory
        movb $72, (%rsi)        # 'H'
        inc %rsi
        movb $73, (%rsi)        # 'I'
        inc %rsi
        movb $10, (%rsi)        # \n
        inc %rsi
        movb $0, (%rsi) # \0, C string terminator

        # Print it
        sub $3, %rsi    # Move rsi back to start of string
        mov $3, %rdx    # String length 3
        mov $1, %rax    # write system is number 1
        mov $1, %rdi    # destinatiout is fd 1, stdout
        syscall

        # Exit
        mov $60, %eax   # exit syscall is number 60
        mov $0, %edi    # exit code 0
        syscall