|
Originally Posted by Arqentus
|
1. Floating point is done by the 2700g not the PXA270. So you are correct but you forgot the gpu detail 
|
Um, no, I disagree with most of your points. Sorry :)
First of all, graphics accelerators are not instruction processors. The 2700G does not perform floating point operations in the same way that that the MIPS R4000 does with its VFPU. The 2700G can only perform relatively course-grained 2D and 3D graphics acceleration functions through API calls, it does not perform specific floating point operations like a traditional instruction processor does. Yet, the floating point operations ON THE CPU are what many games use, e.g. on PC games. In other words, even though a typical PC may contain a powerful graphics card, the floating point unit on a Pentium or Athlon is still used heavily, and the performance of some games are CPU bound. Graphics chip floating point != CPU floating point, and in many cases you cannot substitute one for the other.
|
Originally Posted by Arqentus
|
2. The second Media Chip is to be compared to the 2700g, not the CPU. You are refering how the CPU get's it's data, and compare it to how the so called Media Chip ( a GPU in reality ) handle's data.
3. The PSP has 2 R4000 Microporcessor's. One act's as a CPU, while the other one as a Media Chip. It doesent have 2 R4000 & 2 Graphics's accelerator's ... no no ... Compare one as a cpu, and the other one as a gpu. Same idear.
|
The PSP *does* have two graphics accelerators, and they are *different* than the two R4000 processors. MIPS does not currently offer any sort of 2D or 3D graphics acceleration features as part of their instruction set (not really anyway). I know, I have written articles on embedded processor architectures and recently spoke to MIPS on their product plans.
These are the specifications of the *separate* graphics accelerators on the PSP:
PSP Graphics Core 1
3D Curved Surface + 3D Polygon
Compressed Texture
Hardware Clipping, Morphing, Bone(8)
Hardware Tessellator
Bezier, B-Spline(NURBS)
ex 4x4, 16x16, 64x64 sub-division
PSP Graphics Core 2
'Rendering Engine' + 'Surface Engine'
256bit Bus, 1-166 MHz @ 1.2V
(**Changed to 512bit Bus in final version)
VRAM :2MB(eDRAM)
Bus Bandwidth :5.3GB/sec
Pixel Fill Rate :664 M pixels/sec
max 33 M polygon /sec(T&L)
24bit Full Color:RGBA
These ARE DEFINITELY NOT part of the two MIPS CPUs. Since when do CPUs have color space conversion and 512 bit memory buses =). Doesn't exist. It even includes hardware MPEG acceleration. These separate "graphics cores" are what puts the PSP ahead of the X50V's graphics. Remember, the PowerVR architecture of the 2700G is comparable to the sega dreamcast, whereas several analysts have claimed the PSP's graphics power to rival (or beat) the playstation 2. In fact, I have heard some developers say that what must be done in software on the playstation 2 can now be done in the graphics accelerators of the PSP. You don't get this kind of graphics performance from two relatively slow MIPS processors!!!
|
Originally Posted by Arqentus
|
|
True, the PSP has 2.6 GB/sec embedded DRAM ( 2MB ). But this is where the shoe doesent fit. It still need's to get it's data from the main memory ( the 32 mb shared memory ). So, even if it's able to do 2.6GB/Sec internally, it still need's to grab data externaly. You pointed out that it's able to have 1.8GB of data on the card ... now that mean's it still need's to grab a lot of data all the time. A system is only as fast as it's slowest part.
|
It is simply untrue that a "system is only as fast as it's slowest part." If this were true, then a Pentium's performance would be bounded by hard drive access times, and L1 caches would be pointless. They don't put several megabytes of 2.6 GByte/sec memory in there just for the hell of it, it CAN and WILL be used! All processor-based systems must intelligently use the fastest part of its memory subsystem through prefetching. A CPU does not need all data immediately. In many cases it can be performing some work on data in its fastest memory (e.g. performing AI functions or graphics calculations in its FPU) while simultaneously prefetching the data it needs NEXT. This hides the latency of the slower second-level memories, whether it's slower DRAM or even the hard drive. But, it needs to have the data that it's currently working on in the
fastest local memory possible. The PXA270 has fast local SRAM too, just far less of it (this mem is also used for LCD frame buffers in non-accelerated PDAs and phones). It's somewhat irrelevant how long it takes for the data to get into local memory. For example, a Pentium 4 can waste hundreds or thousands of cycles just waiting for cache lines to fill from external DDR SDRAM, but once its there, it flies.
Stated another way, imagine that all of the data a CPU needs to use to render the next scene (or play the next level etc.) fits into the local 2-8 megabytes of eDRAM. I bet this is the case for most games on the PSP. It can spend a comparatively long time loading this data into eDRAM, but then it has everything locally and can perform compute-intensive tasks FAR better than the PXA270 could, because the PXA270 is ALWAYS limited by its external memory access times. The PSP pays the access penalty once (or infrequently), whereas a PXA270 pays penalties continuously unless someone is really good at working around the smallish 256 kbytes of memory alongside the XScale core caches (about 64 kbytes).
About the 1.8GB UMD media that the PSP will use... The access times don't matter at all here! The PSP will be able to hold almost all the data it needs in faster local memory. It might take a while to load levels into memory, but once it's loaded it will be fine. The is no different than XBox, playstation 2, etc. with their relatively slow CDROM drives. And once again, 32 megabytes of local DRAM is plenty for most games, and the faster eDRAM is probably plenty for the most important and frequently-accessed data.
The bottom line is that there is virtually nothing that the PSP can't do better than the x50v, based on the specifications i've seen, and it's certainly the most powerful handheld 3D gaming platform that has ever been produced (correct me if I'm wrong).