ic.expert 发表于 2010-9-19 21:30:18

[GPU in Dreamcast] PowerVR DC

PowerVR DC  

History of the Dreamcast

In 1997, the Sega Saturn was struggling in North America, and Sega of America president Bernie Stolar was pressed by Sega's Japanese headquarters to develop a new platform. Two competing teams were tasked with developing the console–a skunkworks group headed by IBM researcher Tatsuo Yamamoto and another team led by Sega hardware engineer Hideki Sato.

Sato and his group chose the Hitachi SH4 processor architecture and the VideoLogic PowerVR2 graphics processor for their prototype. Yamamoto and his Skunkworks group also opted for the SH4, but with 3dfx hardware. Initially, Sega decided to use Yamamoto's design and suggested to 3Dfx that they would be using their hardware in the upcoming console, but Sega later opted to use the PowerVR hardware of Sato's design. This was attributed to 3Dfx leaking details and technical specifications of the then-secret Dreamcast project when declaring their Initial Public Offering in June 1997 a move which readers on Gamespy.com named one of the dumbest mistakes in video game history. Sega's shift in design prompted a lawsuit by 3dfx that was eventually settled.

With Sega's machine, no operating system resides in the device until it is loaded in on a disc with each game. The advantage, Sega executives say, is that developers can always ship products that use the version of an operating system with the newest features and performance enhancements. The operating system used by some Dreamcast titles was developed by Microsoft after 2 years of work with Sega. It was an optimized version of Windows CE supporting DirectX. According to Richard Doherty, president of Envisioneering Group, "Microsoft had initially wanted Windows CE to be Dreamcast's main operating system. It isn't." The Dreamcast's boot-up sequence was also composed by accomplished Japanese pianist, Ryuichi Sakamoto.


The Dreamcast was released in November 1998 in Japan; on September 9, 1999 in North America and on October 14, 1999 in Europe. Despite problems with the Japan launch, the system's launch in the United States was successful. In the United States alone, a record 300,000 units had been pre-ordered and Sega sold 500,000 consoles in just two weeks (including a record 225,132 sold during the first 24 hours). In fact, due to brisk sales and hardware shortages, Sega was unable to fulfill all of the advance orders. Sega confirmed that it made US$98.4 million on combined hardware and software sales with Dreamcast with its September 9, 1999 launch. Four days after its launch in the US, Sega stated 372,000 units were sold bringing in US$132 million in sales.

Launch titles such as Soul Calibur, Sonic Adventure, Power Stone, and Hydro Thunder helped Dreamcast succeed in the first year. Sega Sports titles helped fill the void left by a lack of Electronic Arts sports games on the system. Dreamcast sales grew 156.5% from July 23, 2000 to September 30, 2000 putting Sega ahead of the Nintendo 64 in that period. However, Sony's launch of the much-hyped PlayStation 2 that year marked the beginning of the end for the Dreamcast.

End of production

On January 31, 2001, Sega announced that it was discontinuing Dreamcast support by March of that year. The last North American release was NHL 2K2, which was released in February 2002. With the company announcing no plans to develop a next-generation successor to Dreamcast, this was Sega's last foray into the home console business.

Sega Europe continued to support the Dreamcast until 2003. Notable exclusives include Shenmue 2, Head Hunter and Rez, which are amongst the most critically acclaimed games released for the system.

During the following years, unreleased games like Propeller Arena, Hellgate and Half-Life were leaked on the Internet .

Although production of the Dreamcast ended in 2001, Sega of Japan continued selling refurbished systems and releasing new games till 2007. Many of the games were initially developed for Sega's NAOMI arcade hardware, including Sega's final first-party Dreamcast game, Sonic Team's Puyo Puyo Fever, released on February 24, 2004.

The last Dreamcast units were sold through the Sega Direct division of Japan in early 2006. These refurbished units were bundled with Radilgy, and a phone card. The last Dreamcast games published by Sega of Japan were the 2007 releases Trigger Heart Exelica and Karous.

3 other NAOMI games Exzeal, Illmatic Envelope: Illvelo and Mamonoro were supposed to be ported to the Dreamcast, when Sega abruptly decided to discontinue the production of GD-ROM's.

A method was found for playing Mil-Cd's without any hardware modification and through a free software development kit called KallistiOS, software support of the console continues with homebrew games, emulators for older systems and media players being released for the system. Independent commercial games such as Feet of Fury, Last Hope and DUX have also been released.


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The Dreamcast introduced many new things to home consoles. All models where shipped with modems this allowed owners to browse the net and play games online via a dedicated server, this was not seen again until the original Xbox. The Dreamcast was the first console to have DLC which was stored on the memory card, this allowed for extra content on games such as Phantasy Star Online and Skies of Arcadia, these would included such things as new items and missions. Jet Set Radio, a Dreamcast original, introduced cell shaded graphics which for a time became very popular.
Digital revival

Several Dreamcast emulation projects have also emerged including Chankast and nullDC.

On June 10, 2010, at E3 Sega announced that Dreamcast titles would soon be available on Xbox Live Arcade and PlayStation Network. The first two titles to be released are Sonic Adventure and Crazy Taxi.

The Dreamcast was the first 128-bit console on the market and came from Sega. Stories of this console started appearing on several Internet sites on March 12th 1997. It was originally rumoured to be a 64-bit upgrade for the Sega Saturn, code-named Eclipse, but by March 31st, this story had changed and it was now believed that Sega was planning a completely new, separate console.

By June 1997 it was known that Sega had two different design specs for consideration to become the new console, one code-named Black Belt, and the other code-named Dural. They were almost identical apart from processors (Black Belt having an IBM/Motorola PowerPC 603e CPU with 3Dfx Voodoo2 graphics chipset and Dural having a Hitachi SH-4 CPU with NEC/Videologic PowerVR2 graphics chipset) and matched well with the specs of Sega's newest arcade board code-named NAOMI.

It soon came time to decide on which one was to be used as the final console. Sega chairman, Isao Okawa, ordered for both designs to be made so they could better evaluate the two. Sega of America went with the Black Belt design, but Sega of Japan went with the Dural design. They finally decided on the Dural design (renamed Katana and given 16MB of Video RAM rather than 8MB, allowing better graphics rendering capabilities). The console boasted that it was easy to produce games for and so it quickly gained a lot of third party support.

Sega worked with many other companies to produce this machine including Microsoft, Hitachi, NEC, Video Logic and Yamaha. The console is Windows CE compatible and can run the Internet with its built-in 56K modem allowing you to play games over the Internet or browse the net using the optional keyboard/mouse. In Australia, this Internet access was provided exclusively through Telstra Big Pond. The Dreamcast can also run emulators.

The controller is excellent with its two trigger buttons and compatibility with Sega VMU. No DVD drive was included as this would have been far too expensive to produce. Sega would later be criticised for this. Instead, they used a CD-ROM drive which runs at 12x. The machine can handle 3 million polygons per second.

The console was announced to the public as the Sega Katana on September 7th 1997 but this name had changed to Dreamcast by the time they were ready to release it a year later. When it came time to release the Dreamcast, many delays prevented it from running on schedule. Final touches had to be made on the console and first-release games. There were many pre-orders taken leading up to its release in Japan, which would seem like a good thing for Sega, but they could not fill these orders (both because they could not make that many consoles in such a short time and also because of a lack of parts) and so they asked for orders to be halted.

The release date was re-scheduled a number of times because of these problems, but the Dreamcast was finally released on November 25th 1998 in Japan. All 150 000 consoles that Sega had managed to produce by this time were sold out on the first day. They remained sold out until the next shipment arrived in mid December.

By 16th July 1999, the Dreamcast was outselling the Nintendo 64 by a 3 to 1 ratio. Meanwhile in America, advertising for the Dreamcast was taking place and by August 1999 it had broken the advanced sales record of the PlayStation with 200 000 pre-orders placed. The official North American release was September 9th 1999 at a cost of US$199.99.

Unfortunately, a number of problems occurred, which may have made a small contribution to the consoles failure later down the track because of angry consumers. Some Japanese games were shipped to USA and, of course, didn't work on the US consoles. Ready 2 Rumble was released with the wrong drivers, also making the game not work. The lack of a Sega-made light gun, some people criticising the controllers and lack of VMUs available also didn't help. But other than these problems, the US release was a big success for Sega.

The European release (14th October 1999) was also quite successful, but the Australian/New Zealand release (30th November 1999) was a failure with shortage of consoles and games. Because of this, no further interest was taken in the console and stores quickly stopped supporting the Dreamcast in Australia as it was just not worth it. Only a few shops that specialised in selling games continued to sell Dreamcasts and games.

By October 1999, Sega of America announced that it had sold 518 000 consoles in 1 month in the US. By the beginning of November, this had increased to 750 000 and by the end of the year, 1 million had been sold. At that rate, Sega expected to break 2 million by March 2000.

Sega had promised that the Dreamcast would be both expandable and upgradeable. By the end of the year, they had announcements of a DC Zip Drive and cable modem compatibility. Of course, the Internet access planned for the Dreamcast had not even begun yet because of various delays, and this is perhaps another contributing factor to the console's failure in the end.

The largest factor that contributed to the failure of the Dreamcast, however, was the announcement of the Sony PlayStation 2 (and later the Microsoft Xbox and Nintendo GameCube). Sony's console was superior to the Dreamcast in many ways (but the Dreamcast still had some things better than the Playstation 2) and Sony even dropped the price of the PlayStation 2 to compete with Sega. But there was no way Sega could drop the price of their console. To the public, it seemed that Sega was greedy, but if they dropped the price of the console, they would never meet the break-even point after losses from previous failures.

After delays, SegaNet finally began, allowing Dreamcast users to play games over the Internet. In January 2000, 30% of Japanese Dreamcast owners were using Dricas and by 17th February this had risen to almost 50%. In the US over 300 000 people were using DC's Internet and in Europe over 200 000. Then came the announcement of a free Dreamcast given to anyone who signed up for Dreamcast Internet for a minimum of 2 years. This was a deal Sony could not match and so this kept the Dreamcast going for a while.

But there was still the issue of price that made the Playstation 2 a more attractive offer. Even though the Dreamcast was still cheaper, people would prefer to pay a little more for the more promising-looking Playstation 2. Software sales for the Dreamcast were never good, which didn't help either. Sales of the Dreamcast dropped in late 2000 and things kept going downhill from there. Sega stopped production of the Dreamcast in February 2001 and then lowered the price, selling their consoles at a loss. After this, Sega decided to become a third-party producer of games for other consoles and not make any more consoles.
Sega Dreamcast Technical Specifications

    * CPU: 128-Bit Hitachi SuperH4 RISC (360 Mips, 800MB/sec Data Throughput)
    * CPU speed: 200MHz
    * RAM: 26 Megabyte (16MB main/8MB video/2 MB sound)
    * Sound: 64 Voice Yamaha Super Intelligent Sound Processor (45MHz, 40Mips, 64 voices, 16-bit 48KHz, 3D audio support)
    * Graphics processor: NEC PowerVR Series II (100MHz, renders up to 3.5 million polygons/sec)
    * MIPS: 360 Mips
    * FLOPS: 1.4 Billion
    * Polygons per second: 3 Million
    * CD-ROM Drive: 12 speed Proprietary Yamaha GD-ROM (Gigabyte Disc)
    * Stores up to 1.2 Gigabytes.
    * Controller ports: 4
    * Dimensions: 189mm x 195mm x 76mm (7 7/16" x 7 11/16" x 3")
    * Weight: 1.9kg (4.4lbs)
    * Internal Modem
    * Broadband/Ethernet Capability


The system's processor is a 200 MHz SH-4 with an on-die 128-bit vector graphics engine, 360 MIPS and 1.4 GFLOPS (single precision), using the vector graphics engine. The graphics hardware is a PowerVR2 CLX2 chipset, capable of 7.0 million polygons/second peak performance and trilinear filtering. Graphics hardware effects include gouraud shading, z-buffering, anti-aliasing, per-pixel translucency sorting (also known as order independent translucency) and bump mapping. The system supports approximately 16.78 million colors (24-bit) color output and displays interlaced or progressive scan video at 640x480 video resolution.

For sound, the system features a Yamaha AICA Sound Processor with a 32-Bit ARM7 RISC CPU operating at 45 MHz, 64 channel PCM/ADPCM sampler (4:1 compression), XG MIDI support and 128 step DSP.

The Dreamcast has 16 MB 64 Bit 100 MHz main RAM, 8 MB 4x16-bit 100 MHz video RAM and 2 MB 16-bit 66 MHz sound RAM. The hardware supports VQ Texture Compression at either asymptotically 2bpp or even 1bpp

The system reads media using a 12x maximum speed (Constant Angular Velocity) Yamaha GD-ROM Drive.

Dreamcast Block Diagram   

As you can see the PowerVR2DC graphics chip is the heart of the system, in that it contains specific circuitry for connecting all the devices together. This extra circuitry is called the System ASIC (Application Specific Integrated Circuit).

Internal Bandwidth

The diagram above shows the bandwidth between the processors, and their individual dedicated memories. This follows the segmented memory architectural design. The SMA design allows each major component (CPU, Graphics, Sound) to do it's 'job' without any data contention interference from each other.
Data PathBandwidth
CPU SH-4 to 16 MB Main Memory64-bits x 100 MHz = 800 MB/s
PVR2DC to 8 MB Graphics Memory64-bits x 100 MHz = 800 MB/s
ARM7 Sound to Sound Memory16-bits x 66 MHz = 132 MB/s
Total:1732 MB/s

Note also that a game can have textures stored in main memory and those textures can be moved into graphics memory at 64-bits x 100 MHz = 800 MB/s, as indicated on the diagram. The Dreamcast has exceptionally high internal bandwidth, and is even more impressive when paired with the incredibly bandwidth efficient 'tile' based rendering of the PowerVR graphics chip!

External Bandwidth

Controller Ports: These ports have a rated bandwidth of 2 Mbits/s, or 250 KBytes/s. This is 1/6th the speed of a PC USB (Universal Serial Bus) port, which can transfer data at 12 Mbits/s. Even though it is quite a bit slower then USB, it still more then fast enough for any data transfer that has to take place between the controllers and the main unit. A 128 KB VMS memory unit can transfer it's entire memory contents to the Dreamcasts main memory in half a second.

The port coming off of the SH-4 is the rear serial port, that has the same performance characteristics as the controller ports, which is about 2 Mbits/s, or 250 KBytes/s. This port can be used for networking two DC's together, or attaching external devices like a keyboard, etc.

Modem Port: This port is attached to a bus that is rated at 16-bits x 25 MHz = 50 MB/s, and this same bus is also used to transfer sound data from the GD-ROM to the sound system. The speed of this port clearly shows that it cannot support any processor or memory expansion at all, which is contrary to what others have suggested. It seems quite clear by this diagram that the Dreamcast has no external ports that can support a processor or memory expanision, as the ports are not fast enough.

ic.expert 发表于 2010-9-19 21:56:08

当年3Dfx和MEGA公司官司的事情我也听说了,当时很奇怪为啥那么不用voodoo2呢,先看看来还是带宽的问题,同时代的N64/PS2都是把桢缓冲区(Framebuffer)直接放在片上存储器中(on-chip Memory),我们现在无从知晓DC版本的Voodoo2是否也是使用了类似的方法,但是DC版本的PowerVR性能落后于PS2倒是不争的事实,而且性能差距巨大PowerVR DC之后 3.5 million Triangle/second,而 PS2的性能是这个十倍左右,大概是33million Triangle/second。显存容量也不占优势,PS2可是4MB的eDRAM,DC在片外的显存也就是 区区8MB而已。但是从可编程性的角度来说,DC的编程要比PS2方便太多了,PS2还是那种流式纹理(Streaming Texture),好在当年没有那么多厂商去做跨平台的游戏。
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