title: 32-bit assembly code running on 64-bit platform
body: <p>I am studying x86 assembly language via the book called Programming from the Ground Up by Jonathan Barlett. Recently I updated my system to 64-bits platform and problems with the syntax of the assembly code appeared, the pushl instruction to be more specific. I spent some time looking for info on x86_64 isa, but I thought would be better to finish learning the basics of x86. In order to do that I was wondering if  there's a way of assembling the older syntax into a 64-bits object, or something like that. Or there were major changes which makes that impossible? 
  I am using Ubuntu 10.10 and GNU portable assembler.</p>
  
  <p>Anyway. Would be nice if you indicate a good book, or any source of info, on the x86_64 or in the differences between it and his predecessor.</p>
  
  <p>edit: Thanks! I got what I needed. Both answers were quite useful. And don't worry, I'm looking forward to let go of the X86. </p>