Op de website van GGmania vonden we wat opvallend nieuws dat betrekking heeft op de specificaties van de Xbox 360 console. Het betreft het volgende:
– Among 3 cores in Xbox 360 CPU (codenamed “PX”), Core 0 is primary and Core 1/2 are secondary. Core 0 is fully usable by a game program. Core 1 and Core 2 are shared by a game program and the Xbox 360 system. Network stacks, services, drivers such as a USB driver run on those secondary cores. 5% computation usage of both core 1 and core 2 are reserved by the system.
– The XMA (modified WMAPro) decoder in the southbridge chip can decode 256 XMA channels at the same time. Though the compression rate is variable, 1/8 is just enough for typical usage. After decoding, all software sound processing (multi-channel mixing, 3D surround sound, Dolby Pro Logic II/Dolby Digital 5.1 encoding) are done on CPU Core 2. When it processes 256 channels at the same time it costs 25% load of Core 2.
– A hardware-assisted tile-rendering method called ‘Predicated Tiling’ is supplied as a library for Xbox 360 for the case when 10MB eDRAM is not sufficient, for example 64bit (FP16 * RGBA) HDR rendering + Z buffer + MRT in 720p. While it affects geometry processing with 1.2 – 1.3 times load, it doesn’t affect pixel processing as there’s no overlapping unlike geometry. As the result, it doesn’t affect the total performance as pixel processing load is inherently larger than that of geometry.
– The hardware tesselator in Xenos supports both adaptive and sequential, and adaptive tesselation requires 2-pass. If the tesselator is used the vertex output from it is limited to 1 vertex per clock though the performance impact can be mitigated as output vertices from the tesselator have higher locality for better caching.
– In a double-layer DVD for Xbox 360, 7GB is usable by a game. The transfer rate of the DVD drive is 15MB/sec max, 10-12MB/sec average. The seek time is 115ms, switching layers takes 75ms. Loading 512MB data takes 34 seconds.
– 2GB in the HDD is used for a temporary cache area for games. Its average transfer rate is 17MB/sec and the average seek time is 13ms.
– Game data is managed per user account and saved in HDD, but 64MB Memory Unit is also usable for checkout/backup. The Memory Unit slot is 2.5MB/sec write, 8MB/sec read. In its 512MB RAM, 32MB is allocated for the system. The RAM is GDDR3 SDRAM @ 700MHz (22.4GB/sec).
Opvallend is de toch ietwat lage transfer rate van de harde schijf van de Xbox 360 die slechts 17MB/s kan verzenden. Op zich zou dit wel voldoende zijn voor de games die zullen gaan verschijnen. De schijf wordt meer gebruikt als tijdelijke opslagplaats in games. Ook het systeemgebruik zelf is opvallend. 5% van de werkzaamheden van de cores zouden namelijk voor het systeem zelf gereserveerd worden. De overige kracht kan aan de games geschonken worden.
