Windows system calls [duplicate]

The Nt* set of functions are undocumented with good reason: it's internal to Windows and changes between versions, meaning that programs that target it directly are at high-risk of breaking between Windows versions.

Really, there is not that big an overhead with targeting public, documented functions, and you get Microsoft's guarantee that your program will work with future versions of Windows provided you use the API correctly.

For that reason, I won't provide you with the answer you want. I strongly advise you to use the public console API: http://msdn.microsoft.com/en-us/library/windows/desktop/ms682073%28v=vs.85%29.aspx

Update

I assume that is for experimentation or fun, indeed the OP wrote:

Sometimes, however, I want to use low-level system calls -- mostly to keep my executables as small as possible.

...I find this line of reasoning problematic and I don't want to act as an enabler for these kinds of development practices, especially as there is no practical benefit to using syscalls for console tasks.

If someone wants to know how to use low-level syscalls in Windows then please post a new question, suitably framed, and I'll gladly answer it.

But as a starting point, see https://j00ru.vexillium.org/syscalls/nt/64/ for a reverse-engineered table of x64 NT system-call numbers broken down by Windows kernel version. (Do not use in portable code, only for experiments to satisfy your curiosity about how Windows and/or asm works.)