[Pgubook-readers] Type .long

[George Nyoro]

Hey everyone,

     I'm enjoying learning more about the under-hood of the computer.

     There is something I don't quite understand.

Here is my program to find  maximum using an end address.

.section .data

#eax holds current value and exit status
#ebx holds the highest value
#edx holds the current index
#edi  holds the current address

data_items:
    .long 3,204,5,6,0,1,55,30
eol:
.long 105

.section .text
.globl _start

_start:
    movl $0, %edx
    movl data_items(,%edx,4), %eax
    movl %eax, %ebx

loop:
    leal data_items(,%edx,4), %edi   #move the current address to edi
    cmpl $eol, %edi                         #compare the current address to the eo
        jg end

    incl %edx
    movl data_items(,%edx,4), %eax
    cmpl %eax, %ebx
        jge loop

    movl %eax, %ebx
        jmp loop


end:
    movl $1, %eax
    int $0x80

At the offending line, if I replace it with je end, the program works. If I leave it as is, the exit status is 0, showing that the program did not run even once. The reason I came to understand or diagnose is that the value of eol is some value greater than 255 while %edi is, for the start of the program after moving the address to it, less than 255. eol is then seen as smaller than edi by the value that it overshoots 255 by hence the program never runs  through the rest of loop.

Now, my problem is, why? The only part in the book that referenced anything like this said that only the exit status is allowed to be less than 256. But why do longs overshoot? How then can I ever compute large numbers? Unless something is wrong with my code.

Please help, Much appreciated.