16-bit Assembly on 64-bit Windows?

Question

I decided to start learning assembly a while ago, and so I started with 16-bit assembly, using FASMW. HOwever, I recently got a really new computer running Windows 7 64-bit, and now none of the compiled .COM files that the program assembles work anymore. they give an error message saying that the .COM is not compatible with 64-bit windows. 32-bit assemblies still work, however i'd rather start with 16 and work my way up... Is it possible to run a 16-bit program on windows 7? or is there a specific way to compile them? or should i give up and skip to 32-bit instead?

Answer 1

The reason you can't use 16-bit assembly is because the 16-bit subsystem has been removed from all 64-bit versions of Windows.

The only way to remedy this is to install something like DOSBox, or a virtual machine package such as VirtualBox and then install FreeDOS into that. That way, you get true DOS anyway. (NTVDM is not true DOS)

Personally, would I encourage writing 16-bit assembly for DOS? No. I'd use 32- or even 64-bit assembly - the reason being there are a different set of function calls for different operating systems (called the ABI). So, the ABI for 64-bit linux apps is different to 32-bit ones. Not sure if that's the case with Windows. However, I guarantee that the meaning of interrupts is probably different.

Also, you've got all sorts of things to consider with 16-bit assembly, like the memory-model in use. I might be wrong, but I believe DOS gives you 64K memory to play with "and that's it". Everything, your entire heap and stack along with code must fit into this space, as I understand it, which makes you wonder how anything ever worked, really.