I read through the wiki page for videocore but still did not get a complete picture of what this is. Is it a GPU? If its a GPU or something that specializes in processing video/audio why is it in charge of the boot process? Doesn't it usually happen the other way around, the boot ROM loads the host OS which in turn initializes the other firmware?
It's a vector processor and a GPU. The vector processor is intended for video decoding and therefore has a very big SIMD capability. It has some nice gimmicks like the 'square' register file: you can access rows or columns for SIMD operations. The VPU is I believe what boots first and normally runs an RTOS called "ThreadX".
In particular we are talking about "Scalar/Vector (VPU)" "Dual Core VideoCore IV® Multimedia Co-Processor" (Figure 3B). As far as I understand, that's the thing that boots the device and usually runs ThreadX OS in the closed-source bootloader. The VPU is a dual core processor with scalar and integer vector instructions.
The VPU is more-or-less independent of the GPU, although I'm not sure whether the VPU gets involved with some GPU scheduling tasks.
The GPU has four "QPU" QuadProcessor pipelines, which from memory can do floating-point vector processing.
