Question

I’m currently working on an assignment in AT&T Assembly and now I have to append two strings:

message: .asciz "String 1"
before: .asciz "String 2"

I have really no idea how to do this or how to begin. I’ve already searched on internet but I couldn’t find any helpful information. I think I have to manually copy the characters of the second string to the end of the first string but I’m not sure about that.

Could anyone please explain to me how to do this? 🙂

Answer 1

This is not a trivial matter. Strings are variable in length and occupy different spaces in memory, and there has to be some way to know how long they are or where they end. With C or C++, a nul bytes (bytes of zero value) indicates the end of the string. With some other
program languages, you have a pointer to the start of the string and the length of the string stored separately, which has the advantage of letting you store binary (including bytes of
zero value) in the string. Even with C and the rest, you have to have a pointer to where the string starts.

What generally has to happen is that you have to use asm to contact the operating system and request a block of memory that is currently free that is big enough to contain the contents of the two strings once they are attached. This would be memory separate from either of the
two strings to start with, and it comes from what is referred to as the Memory Heap, Once you are given the beginning point of that memory block, you copy the contents of the first
string into it, then you continue on while copying the contents of the second string in
there right behind the first. Then you release the memory that had been assigned to the
first string and reassign the block to that string by changing its pointer, and possibly
its length. The released memory is returned to the Memory Heap by the Operating System for reuse elsewhere.

Actually, the operating system is not the only source of freed up memory. Some compilers, even assemblers, either handle memory management on their own, or provide suitable tools
to the programmer to do it as the need arises.

In other words, this can be a very ambitious undertaking, and you have to know quite a
bit about what is going on to do it right. You do it wrong, you can expect consequences
like crashing your system and needing to reboot.