Alphamosaic to Pi to Doom via Broadcom (2000-2021)
Dr. Jon Peddie
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Alphamosaic Ltd was a UK semiconductor company founded by Robert Swann and Steve Barlow in 2000 in Cambridge, UK. The company was a spin-out from Cambridge Consultants, and they developed low-power mobile multimedia processors based on their VideoCore architecture.
The VC01 chip centered around a novel 2D DSP architecture for low-power processing of video and images. Consumer devices, including phones from Samsung, used the chip, as did the first Apple video iPod to handle video record and playback, image capture and processing, audio capture and processing, graphics, games, and ringtones.
The second-generation chip, VC02, ran at 150 MHz (almost twice the speed of the 85-MHz operation of the VC01), displayed video on QVGA screens, and captured up to 8 Megapixel images from image sensors. The new chip also had more internal SRAM memory (10-Mbits vs. 8-Mbits in the previous part) and advanced image filters.
The VC02 had features heavily weighted toward video, and two of the device’s most noteworthy features were a direct TVout and an input for TV tuners. [i]
The VC02 also had dual 32-bit RISC processors, which had a dual-issue compiler. Like all mobile devices of the time, the CPU(s) were fixed-point only. However, like the predecessor single CPU part, the VideoCore II was a dual video DSP, with a 16 parallel data path VLIW vector processor tightly coupled (by shared registers) to 32-bit RISC scalar processors.
The photo shows the development board— the VC02 is the chip just above the display.
In September 2004, Alphamosaic was acquired by Broadcom for $123 million, forming its Mobile Multimedia group on the Cambridge Science Park site.
Broadcom launched its first new chip in the VideoCore line at 3GSM in February 2005. The chip was designed for mainstream phones but enabled advanced features such as video, 3D gaming, and multimedia—features previously associated with high-end phones. Broadcom branded it the BCM2705.
Broadcom reduced the amount of memory, reduced the JPEG encode and decode from 8 Megapixels to 4 Megapixels, and removed some peripheral support, including TV-out and USB. The chip would enable phones to display video at 30 fps on a 2-inch color LCD and to capture 4-Megapixel images. The chip was 100% programmable and had MPEG-4 encode and decode.
The VideoCore processor was a 150MHz dual-ALU, allowing the BCM2702 to function as a co-processor or as a stand-alone. The chip was manufactured in 130 nm CMOS and packaged in a 281-pin TFBGA package (10.9 mm x 10.1 mm).
Swann said, “We showed some pretty good 3D games before, and we’ve got better games now,”
Though pricing was a very relative thing, the chip was in the range of $10.
In 2012 hobbyists began exploiting a powerful little development kit known as Raspberry Pi created in the UK by the Raspberry Pi Foundation in association with Broadcom. As of May 2021, more than 40 million boards had been sold.
The Raspberry Pi had a Broadcom SoC with a VideoCore IV 3D graphics core and used a closed-source binary driver (a blob), which communicated with the hardware. The blob ran on the BCM2835 SOC’s VPU vector processor of the Raspberry Pi. Open-source graphics drivers were a thin shim running on the ARM11 via a driver in the Linux kernel. But the lack of an open-source graphics driver and documentation was a problem for Linux on ARM—it prevented users from fixing driver bugs, adding features, and generally understanding what their hardware was doing.
Then in February 2014, Broadcom announced they would give the VC4 to the community. They released all the documentation for graphics core and a complete source code of the graphics stack under a 3-clause BSD license—anyone could use it.
The source release targeted the BCM21553 cellphone chip, but it was straightforward to port it to the BCM2835 on the Pi. That allowed access to the graphics core without using the blob. As an incentive to do that work, the Raspberry PI organization offered a $10,000 prize for the first person to demonstrate to them that Quake III could run successfully at a playable frame rate on Raspberry Pi using those drivers.
In April 2014, only a month after the prize was offered, it was claimed by Simon Hall, a longtime Pi hacker.
Raspberry Pi 4 kits could be bought for as little as $25 in 2022.
[i]Alphamosaic’s dual CPU media processor, TechWatch, V.4.3, (February 9, 2004), p. 6.
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