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Subject:	Re: Request for an example x68 assembler portable Hello World script
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From:	Eliot Moss <moss AT cs DOT umass DOT edu>
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Date:	Fri, 26 Apr 2019 07:16:38 -0400
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On 4/26/2019 3:25 AM, Jesse Thompson wrote:
> I would like to learn how to write assembly programs for the command line
> that with as little alteration as is feasable will compile both in Cygwin
> and in other flavors of Unix like Linux and/or FreeBSD.
> 
> I am targeting only x64 CPUs and I'm perfectly happy to use libc calls
> instead of direct syscalls or interrupts. I'm hoping to use nasm+gcc, or
> perhaps fasm to do the deed. Crosspiling is not a concern, I'll build
> cygwin binaries in cygwin and unix binaries in unix.
> 
> But I'm confused by the differences in calling convention/ABI between
> Windows and/or Cygwin and Linux?
> 
> For example, I can get this to compile and run in Cygwin:
> 
> ```
>          global  main
>          extern  puts
>          section .text
> main:
>          sub     rsp, 20h                        ; Reserve the shadow space
>          mov     rcx, message                    ; First argument is address
> of message
>          call    puts                            ; puts(message)
>          add     rsp, 20h                        ; Remove shadow space
>          ret
> message:
>          db      'Hello', 0                      ; C strings need a zero
> byte at the end
> ```
> 
> 
> but it segfaults in Linux (and complains about "Symbol `puts' causes
> overflow in R_X86_64_PC32 relocation")
> 
> and I can get the following to compile and run in Linux:
> ```
>      extern puts
>      global main
> 
> section .text
> main:
>      mov rdi,message
>      call puts
>      ret
> 
> message:
>      db  "Hello World",0
> ```
> 
> but *that* segfaults in cygwin.
> 
> TL;DR: I think I could get a lot more done if I could start from a single
> Hello World asm file that can compile and run in both places, calling out
> to puts or something simple like that.
> 
> Any help would be appreciated, I hope everything about my question makes
> sense. :)

Der Jesse -- Someone else may be able to speak to the specifics, but
register use and calling conventions, and to some extent stack layout,
very from platform to platform.  Roughly, platform = processor + OS.
So (to me anyway) it would not be at all surprising if you have to
write code different for each of Windows, Cygwin, and Linux.  Cygwin
tries to offer library level compatibility for program designed to
run under Posix (there are some seams here and there, where Windows
differences are hard to hide).  But the programs have to be recompiled
to the Cygwin ABI.

Another thing you may be encountering is the difference between the
32-bit and 64-bit worlds.  Recent x86 processor support the x86_64
version as well.  Cygwin offers both 32 and 64 bit versions, but they
are distinct, and a program needs to be compiled to the one under
which you wish to run it.  (I have both 32 and 64 bit Cygwin on my
computer, and the programs can invoke one another, but the installations
need to be in separate file hierarchies.)  The same would tend to hold
under Linux and Windows, though the OS can determine automatically
for a given program whether it is 32 or 64 bit from details of the
first bytes of the executable file.

Regards - EM

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