What could numeric rounding possibly have to do with MS-DOS?

After the previous episode, a reader pointed out to me that FormatNumber uses yet a different rounding algorithm. FormatNumber rounds numbers ending in .5 away from zero, not towards evens. The reader also asked whether FormatNumber, FormatCurrency, and so on, actually call into the VBA Format$ code.

It does not. The VBA runtime is freely redistributable, but it is also large. Back in 1996 we did not want to force people to download the large VBA runtime. IE was on our case for every kilobyte we added to the IE download.

However, FormatNumber, and the other related functions were written by the same developer who implemented Format$ in VBA. That was the legendary Tim Paterson, who you may recall wrote a little program called QDOS that was eventually purchased by Microsoft and called MS-DOS.

And let me tell you, when I was an intern in 1993 I had to debug Format$. There are very good reasons why we decided to go with a stripped-down let’s-solve-the-90%-usage-case version for VBScript! Format$ is one of the most enormously complicated hard-core-bit-twiddling functions that I’ve ever seen. That’s what happens to these “everything but the kitchen-sink” functions that tries to be all things to all callers — they get enormously bloated and complex. Porting that thing to a new codebase and maintaining it independently would have been nightmarish.

Here’s a comment that Tim wrote into the FormatNumber code on September 16th, 1996 which confirms the reader’s observation:

// We have more digits that we're not using. Round if needed.
// This is simplistic rounding that does not use IEEE
// rules (rounding even if exactly x.5). This simple
// way is how VBA Format$ works, so it's good enough for us.

When I was first an intern who barely knew how to program real-world code, Tim Paterson was very intimidating to me, and he did not suffer foolish questions gladly. But once I got to know him, it turned out that he’s a great guy. I had many enjoyable conversations with him about the early days of Microsoft. I particularly remember walking with him to the launch event for Windows 95, reminiscing about how much operating systems had changed since the QDOS days.

Incidentally, Format$ was about 5500 lines of C++ code (counting blank lines and comments). About 1300 of those lines are the parser that turns the “format picture” into an internal representation. It’s an entire programming language of its own, practically.

I cordially hate the “format picture” approach to formatting strings for this reason; we’ve created a programming language that follows none of the standards, rules or idioms of the “host” language. Much of the work of Format$ was parsing a language that never should have existed in the first place; instead we should have had an object model of combinators that could be combined programmatically. But we’re stuck with this 1970s era idiom now.


2 thoughts on “What could numeric rounding possibly have to do with MS-DOS?

  1. Pingback: Porting old posts, part 2 | Fabulous adventures in coding

  2. Pingback: New grad vs senior dev | Fabulous adventures in coding

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