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12/11/00 Monday's News: Story DetailReturn to News Page
Connectix Comments on Virtual PC 4 Altivec Support & Why Voodoo Card Support Was Removed:
Last Updated: 12/11/2000, 2:36 PM
Connectix sent a reply to my questions on why VPC v4 removed 3D hardware acceleration support for the 3dfx Voodoo1 and Voodoo2 cards which were supported in VPC versions 2 and 3. Their comments also cover the requirements of a G3 or G4 CPU, and how Altivec is utilized for better performance.
" ...the support for Voodoo1 and Voodoo2 cards that was in VPC3 has been removed. Let me explain why.

One of the challenges in emulating a Pentium processor is that Intel chips store multi-byte data in reverse order from the PowerPC. This difference in "endianness" must be taken into account when executing Pentium code. The PowerPC has a special mode which allows it to simulate álittle endianî mode (the native mode of Intel-compatible processors). However, on PowerPC processors prior to the G3, this mode came at a high performance cost - especially when the data in memory was "misaligned" (i.e. not stored to memory locations that are divisible by the size of the data). The G3 and G4 processors removed these performance bottlenecks, allowing Connectix engineers to finally take advantage of this previously ignored feature.

Because of Virtual PC 4.0's reliance on this "little endian" mode, support for processors older than G3's had to be dropped. Users who own Macs based on 601, 603 or 604 will have to either upgrade their processors to G3's or continue to use Virtual PC 3.0.

For those Mac users who are lucky enough to own a G4 machine, Virtual PC 4.0 also makes more extensive use of AltiVec to speed up certain operations. In particular, MMX emulation is now up to three times faster on G4 processors. AltiVec is also used within Virtual PC 4.0 to speed up the transfer of pixel data to the Mac video display, resulting in a "snappier" feel when running PC software.

With specific regard to video, there are three primary changes in Virtual PC 4.0: the VRAM is now always set to 4MB (it used to be configurable), video updates are much faster (resulting in a snappier feel), and the emulated video card now has support for wide aspect resolutions found on the Apple cinema display (1600x1024 and 800x512).

Virtual PC 4.0 does not contain support for 3D hardware acceleration. To date, we have avoided adding 3D support because of the complex issues involved. There are many technical obstacles to providing 3D acceleration support from within an emulator - most of them are not obvious at first glance. We will be exploring the feasibility of 3D emulation in the future, but we felt that faster core processor emulation was of greater importance to most of our current Virtual PC users.

Virtual PC 4.0 also removes support for Voodoo 1 and Voodoo 2 video cards. As mentioned above, the new Pentium emulation core runs in the PowerPC's "little endian" mode. This mode is incompatible with Voodoo frame buffers, so it is no longer feasible to support Voodoo cards within the emulator. The decision to drop Voodoo support was supported by marketing research showing that the vast majority of Virtual PC customers use the product for business and education applications where 3D support is of limited use. Users who want Voodoo compatibility should stick with Virtual PC 3.0.

Kurt Schmucker
Director, Macintosh Products
Connectix Corporation"

For those that missed last week's news pages or the weekend summary, here are some stories/articles related to Virtual PC 4.

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