Porting 32-bit NASM code to 64-bit

It’s terrible…yeah you heard me. Assembly language is hard even at the best of times and it is all so hardware dependent that porting is a super pain in the neck. Even on the same processor, the change from 32-bit code to 64-bit code is annoying.

First rule of business, change BITS 32 to BITS 64. It seems obvious but when you have a bunch of *.asm files and you are doing them one by one, forgetting one can cause “error: impossible combination of address sizes” which will proceed to befuddle you for the next 10 minutes. Or not as it seems I’m the only one on Google has actually gotten this error.

I also found my first use for Wolfram Alpha: taking the modulus of 16-byte hex numbers to determine if they are 16-bit aligned. Yeah, I couldn’t even find a quick way to do that in Matlab, which is surprising. But then I realized I was being really braindead because it is really simple to see if a pointer address is 16-byte aligned: the last digit should be zero! Oops…I’m being silly again.

The errror “error LNK2017: ‘ADDR32’ relocation to ‘.rdata’ invalid without /LARGEADDRESSAWARE:NO” means you forgot to add “default rel” to the top of your assembly language file.

Apparently you also have to preserve some of the new registers across function calls. None of the NASM/YASM manuals or anything I read mentioned this! Code after I ran one of my functions was crashing and through turning off bits of the code, I was able to narrow it down to mov r12,ecx to store the first parameter. Of course then the thought struck me: maybe I need to preserve r12 so I finally had to Google “preserve r12 assembly.” I found some 64-bit sample assembly code from Mark William’s blog which had some comments about preserving r12-r15 and that seemed to fix the problem.