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Asus's A8R-MVP motherboard

CrossFire on a budget

Price (Street)

ALTHOUGH ATI'S CROSSFIRE multi-GPU rendering technology has become a reasonable alternative to SLI, the Radeon Xpress 200 CrossFire Edition chipset is still saddled with a comparatively poor south bridge chip. ATI's SB450 is missing a number of handy features, such as support for Native Command Queuing and RAID 0+1 and 5 arrays. More importantly, though, it's plagued by I/O issues that hinder USB and PCI performance. That latter is particularly damning, as it can affect the performance of onboard Firewire, Gigabit Ethernet, and Serial ATA peripherals.

Fortunately, the CrossFire Edition chipset has an ace up its sleeve. Its north bridge is capable of interfacing with alternative south bridge chips, including those built by ULi. Asus has taken advantage of that capability with the A8R-MVP, a CrossFire-capable motherboard built with ULi's M1575 south bridge. The M1575 fills in many of the features missing from the SB450, and it shouldn't suffer from the I/O performance problems that afflict the SB450.

Of course, the A8R-MVP is more than just a showcase for a unique chipsets combo. At under $100 online, it's also one of the most affordable CrossFire boards around. Oh, and it overclocks like a mofo, too. Keep reading for more on what may be the most attractive CrossFire board on the market.

The specs
As usual, we'll get things started with a look at the A8R-MVP's spec sheet.

CPU supportSocket 939-based Athlon 64 processors
North bridgeATI Radeon Xpress 200 CrossFire Edition
South bridgeULi M1575
InterconnectPCI Express (1GB/sec)
Expansion slots2 PCI Express x16
1 PCI Express x1
3 32-bit/33MHz
Memory4 184-pin DIMM sockets
Maximum of 4GB of DDR266/333/400 SDRAM
Storage I/OFloppy disk
2 channels ATA/133
4 channels Serial ATA with RAID 0, 1, 0+1, 5 support
Audio6-channel HD audio via M1575 and Analog Devices AD1986A codec
Ports1 PS/2 keyboard
1 PS/2 mouse
1 Serial port
1 Parallel port
USB 2.0 with headers for 4  more
1 Firewire via VIA TSB43AB22A with header for 1 more
1 RJ45 10/100/1000 via Marvell 88E8001

1 analog front out
1 analog bass/center out (shared)
1 analog rear out (shared)
1 analog line in
1 analog mic in
1 coaxial digital S/PDIF output

Header for 1 game port
Bus speedsHT: 200-400MHz in 1MHz increments
PCI-E: 100-150MHz in 1MHz increments
DDR: 100, 133, 166, 183, 200, 216, 233, 250MHz
Bus dividersHT: 1x, 2x, 3x, 4x, 5x
VoltagesCPU: auto, 1.175-1.55V in 0.025V increments
DDR: auto, 2.65-3.2V in 0.05V increments
PCI-E: 1.2-1.5V in 0.1V increments
MonitoringVoltage, fan status, and temperature monitoring
Fan speed controlCPU, chassis

The secret behind the Radeon Xpress 200's south bridge flexibility is its use of PCI Express for the chipset interconnect. Two PCI Express lanes are normally used to link the two chips, providing 500MB/s of bandwidth in each direction. If that's not enough, the link can actually be boosted to four PCI Express lanes using a simple BIOS switch. Doing so monopolizes all the PCI-E lanes available on the M1575, but still leaves 18 lanes free on the north bridge.

PCI Express lanes not used for the chipset interconnect are shared by a pair of physical x16 slots and a single x1. Since there aren't enough available PCI-E lanes to run both of the x16 slots at full bandwidth, the motherboard routes eight lanes to each. This lane distribution is identical to that of NVIDIA's first SLI core logic, although more recent SLI chipsets have enough PCI Express lanes to provide a full 16 lanes of bandwidth to each x16 slot. Such a configuration doesn't actually do much to improve performance in current apps, so CrossFire should be just fine with eight lanes of PCI-E per card.

While most CrossFire boards share the A8R-MVP's PCI-E lane distribution, only the A8R-MVP taps ULi's latest M1575 south bridge chip. The chip has all the bells and whistles you'd expect from a cutting-edge south bridge, including a Serial ATA controller that supports Native Command Queuing, 300MB/s transfer rates, and RAID 0, 1, 0+1, and 5 arrays. Eight-channel High Definition Audio is supported, as well, although the A8R-MVP's Analog Devices codec chip only provides six output channels. (I suppose that's the price you pay for escaping assimilation by Realtek's crab collective.)

Despite strong storage and audio components, the M1575 falls a little flat in the networking department. The south bridge is only equipped with a 10/100 Fast Ethernet controller, which Asus forgoes in favor of a Marvell 88E8001 Gigabit Ethernet chip. That chip is stuck on the PCI bus, so throughput will be lower than PCI-E GigE implementations. You can only expect so much from a sub-$100 mobo, I guess.