Question

I’ve been recently working towards learning a bit of assembly and I’m currently stumped on an exercise which requires me to find the maximum number of a list of long values.

The code is as follows:

.section .data
    data_items: .long 200, 201, 101, 10, 0
    min_val: .long 0x8000000000000000 # MIN_VALUE in long

.section .text
.global _start

    _start:
        movl $0, %edi   # init counter to 0
        movl min_val, %ebx
    start_loop:
        cmpl $0, %eax
        je loop_exit    # go to end if 0 encountered

        incl %edi
        movl data_items(,%edi,4), %eax
        cmpl %ebx, %eax
        jle start_loop  # if new value < max value in ebx, read next element

        movl %eax, %ebx
        jmp start_loop

    loop_exit:
        movl $1, %eax
        int $0x80

Two problems with this code:

• When trying to assemble the code, I get the message: Warning: value 0x8000000000000000 truncated to 0x0
• If I rewrite my code in an alternative logic (one which doesn’t require min_value variable), any value greater than 255 in the list of data_items is truncated or returned as value % 256 even though the range of .long should be much larger?

Can anyone help me understand what I’m doing wrong?

EDIT: After changes, the code looks like below. Note how maximum in this case turns out to be 145 instead of 401.

.section .data
    data_items: .long 401, 201, 101, 10, 0
    max_val: .long 0x80000000

.section .text
.global _start

    _start:
        movl $0, %edi   # init counter to 0
        movl max_val, %ebx
    start_loop:
        movl data_items(,%edi,4), %eax
        cmpl $0, %eax
        je loop_exit    # go to end if 0 encountered

        incl %edi

        cmpl %ebx, %eax
        jle start_loop  # if new value < max value in ebx, read next element

        movl %eax, %ebx
        jmp start_loop

    loop_exit:
        movl $1, %eax
        int $0x80

Answer 1

First of all, 0x8000000000000000 doesn’t even fit in a long, it’s a long long. A long -1 is 0xffffffff.

As for the other point, I can’t comment on code you haven’t posted.