Vintage code, common hardware and value of backwards compatibility


A data point on the value of backwards compatibility. This is an area where MSFT is frequently slammed, for its insistence on favoring compatibility with past mistakes instead of throwing everything overboard to start over again– the way Apple has done with OS-X and later switching to Intel x86 chips.

Imagine having to demonstrate an application running on Windows Mobile to a crowded room full of security professionals. This code will not run on the emulator— and even if it did, emulation hardly makes for a compelling demo. “Code always wins” as the old MSFT adage goes and working code on an actual device is the golden standard. This is the predicament confronting the blogger next week. Slide decks are not a problem; they can run off a locally installed PowerPoint or OpenOffice instance (the import process still loses some details if the latest eye-candy from Office 2007 is used) or better yet run from the cloud-hosted Google Presently. Showing UI from the device is a different challenge.

Most phones can not project the view of their own display to an external monitor using a standard VGA, DVI or HDMI output. (Oddly enough a few can project other video over Bluetooth to specialized devices and there is nascent efforst to give phones projectors of their own.) Having a dedicated, fixed camera pointing at the phone was not an option in this setting. And since it is not possible for dozens of people to cluster around a single handset trying to get a peek at the tiny screen, one option is to capture static screenshots at relevant points and project these as part of the slide-deck.

Even that is non-trivial: there is no “print screen” functionality on Windows Mobile out of the box. One quick Google search finds several third-party substitutes, including a freeware version from Ilium software. Luckily the search results also unearthed a better solution. An entry on the Windows Embedded Blog dated November 2004 references a Remote Display application included as part of the Windows Mobile Power Toys. This is good news; power toys are officially unsupported applications typically written by MSFT developers on the side. According to the description Remote Display shows real-time view of the phone display mirrored on the desktop.  Much better than static screen-shots and the audience can now follow along with the exact flow.

One problem: the Power Toys are dated December 2003. Supported operating systems on the download page include W2K SP3– long defunct– as well as “Windows Mobile 2002 based smart-phones.” In other words, this code is archaic. The MSI installed without a problem on Vista, even bearing a proper Authenticode signature to keep the inane UAC prompt happy about reporting the author. But the first attempt to run it with an HTC Diamond failed with an error message about unknown CPU type on the device. Not surprisingly 5 years after the code was written, the architecture of mobile devices, as well as the operating system MSFT is shipping to run on these devices had become unrecognizable under the assumptions the original author had made. Vista does not even have a separate Active Sync component, instead Mobile Device Center handles synchronization with phones.

But the README file provided plan B: the instructions described what to do for that particular error. The recommended fix was to manually find the correct binary over for the mobile device and copy it over. There were not exactly many choices either: Windows CE 3.0, CE 4.0 and smartphones based on CE 4.0. Each OS SKU had a corresponding array of architecture choices, including x86, MIPS and ARM4. As for the device? It is running Windows Mobile 6 on ARMv5i.

So it was a surprise that copying the ARM4 binary built for CE4 worked: Vista could mirror the device screen in real time. Even more impressively, Remote Display works both ways: clicking on the phone UI from the desktop PC actually sends the mouse clicks to the phone, allowing the phone to be driven by the full-size mouse and keyboard combination.

This is one of the rare cases where the insistence on backwards compatibility paid dividends. Not only did the ARM processor have to remain backwards compatible and run binaries compiled for an earlier version, but Windows Mobile itself had to evolve such that code written for earlier CE variants could run without any changes.

cemp

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