Hardware Behind the Consoles - Part I: Microsoft's Xbox

Memory Bandwidth Galore

The Xbox features a 64MB memory subsystem made up of four 4x32Mb DDR SDRAM chips.  Our unit featured Samsung chips but as time goes on Microsoft may choose to explore other DRAM manufacturers for memory.  The memory used on the Xbox is very fast by PC memory standards but only decent by video memory standards.  The Samsung chips used on our unit are 5ns parts that run at 200MHz DDR offering the effective bandwidth of a 400MHz solution.  When combined with NVIDIA's 128-bit TwinBank memory architecture this offers a total of 6.4GB/s of memory bandwidth that is to be shared between the IGP and the CPU.

The 133MHz FSB path to the CPU limits the maximum amount that the CPU can ever use of that bandwidth to 1.06GB/s, leaving a minimum of 5.34GB/s to be used by the rest of the system.  However you also must realize that these are peak theoretical bandwidth numbers which will really never be reached in a real-world scenario.  In the end you're looking at a graphics solution that falls between a GeForce2 GTS and a GeForce3 Ti 200 in terms of memory bandwidth.  However as we showed in our original review of the GeForce3, that is more than enough memory bandwidth when running at 640 x 480 x 32 which is what the vast majority of Xbox titles (at least for now) will be rendered at internally.  Only when AA is enabled will memory bandwidth constraints have to be taken into account and as you'll see later on, there is unfortunately very little use of the GPU's AA capabilities in current games. 

With NVIDIA's GeForce3 Ti 500 shipping with 250MHz DDR memory thus offering 25% more memory bandwidth we would have preferred to see that memory used on the Xbox; after all, this system is expected to last for the next couple of years so it would have been nice for it to feature a little more time-enduring memory.

Then there is the issue of memory size; if the 64MB the Xbox is equipped with was only to be used as the framebuffer for the GPU then it would be more than enough, but this memory is also used for keeping a running copy of the game code to be executed as well as any other pertinent data.  For this reason we felt that it would also make sense for the Xbox to have a full 128MB of memory since it would make the job of the developers somewhat easier.  While the 64MB the system does have is much more than any competing console (Sony's PS2 has 32MB and Nintendo's GameCube has 40MB), we're always wanting more and having to use that 64MB as a frame buffer and as execution data storage can force developers to make some sacrifices going forward.

There are 8 solder pads for DDR SDRAM chips, only 4 are populated.

Looking at the Xbox motherboard itself you'll see solder pads for the additional 64MB of DDR SDRAM.  Although you could theoretically solder on an additional 64MB of memory, with games already written with 64MB of memory in mind there wouldn't be any performance improvement.