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PS2's Programmable DMA

For those who don't know, DMA stands for Direct Memory Access, and it refers to logic circuits in a computer that allow for the automated transfer of system memory to and from peripherals. DMAs are beneficial because they are simple circuits that do work in parallel to the CPU -- while a DMA transfers data, the CPU is free to do other work.that requires more complex computations and logic. The end result is better utilization of the computer's maximum memory transfer bandwidth and computational/logical ability.

Traditionally DMAs are pretty simple. The Playstation 2's EmotionEngine, however, has an 'intelligent' programmable DMA controller (DMAC). Neatly translated, it means that the DMAC can do a lot more than just move raw data from place to place. It supports several modes of operation and has a number of special features to take advantage of the unique multi-core design of the EE. Furthermore, the EE's DMAC is much more tightly integrated with its memory bus than traditional DMAs, allowing it to transfer data with exceptional efficiency. These two features combined make the EE's DMAC a key component to PS2 games developers -- in quite a few games, the DMAC actually does more raw work than the EE Core CPU (R5900).

How The Real Thing Works

While emulating the actual hardware of the DMAC isn't usually needed, it can still be helpful to understand exactly how the PS2's real DMAC works at a hardware level. The EE DMAC operates at 147mhz (1/2th the EE's core clock speed), and transfers 128 bits (16 bytes) of memory per cycle; meaning that the theoretical maximum transfer rate of the DMAC is 2.4 GB/s (147mhz * 16 bytes). It's a nice number, but is technically unattainable even in ideal conditions. Further explanation will make it clear why.

The DMAC connects the PS2's 32 MB of Main Memory (RAM) to various peripheral interfaces, such as VIF (VPU), SIF (IOP), GIF (GS), and IPU (mpeg decoder). VIF, GIF, and IPU are all part of the Emotion Engine and operate at 147mhz, same as the DMAC itself. Thus each of those interfaces can send/receive data at roughly 2.4GB/s. SIF is limited by the IOP's own DMA controller and memry bus, which operates at 1/8th the speed of the EE's DMAC, or about 154MB/s.

Peripheral FIFOs

Each peripheral (VIF, GIF, SIF, IPU, etc) has a 128 or 256 byte FIFO. The FIFO helps mitigate occasional latency differences between Main Memory/SPRAM and the peripheral (some peripherals, in particular the GIF, can incur cycle stalls depending on data sent to them). Thanks to the FIFOs, data can be burst to/from memory in 128-byte blocks, which helps maximize data transfer rates since the EE's memory bus was built to operate most efficiently in those conditions. However, the maximum bandwidth of Main Memory (32MB) in ideal conditions is only ~1.2GB/s (half of the DMAC), and has additional memory bank related latencies, reducing its effective transfer rates even further. If DMA transfers are only done to/from Main Memory, the DMAC will only be able to come within about 40% of its theoretical maximum throughput.

Enter the Scratchpad!

The Scratchpad (SPRAM) is 16KB of memory integrated directly into the EmotionEngine. Because it is directly integrated on-die, it has no read/write latencies and can always be accessed at the maximum transfer rate of 2.4gb/s. The integrated nature of the SPRAM means it has to be small in order to fit -- and its lack of size is what limits its usefulness.

So in order to utilize the bandwidth potential of the EE DMAC, a PS2 programmer must find ways to use a combination of Main Memory and Scratchpad transfers in parallel: When main memory stalls due to inherent latencies, the DMAC will automatically busy itself with a pending SPRAM transfer. Likewise, while the DMAC is transferring to/from SPRAM, the EE's Main Memory becomes available to the CPU, which further improves the system's CPU throughput.

The Scratchpad's MemoryFIFO (MFIFO)

The MemoryFIFO function of the EE DMAC performs and managed two simultaneous DMA transfers, as follows:

  • Scratchpad -> Main Memory (RAM)
  • Main Memory (RAM) -> Peripheral (VIF1 or GIF)

As the buffer in memory is filled by Scratchpad, it is simultaneously drained by the attached peripheral, either VIF1 or GIF. On the surface, the MFIFO can appear to be somewhat silly, since the DMAC already has the ability to transfer direcly from SPRAM -> Peripheral. Adding a stop in Main Memory might seem like a waste of the DMAC's bandwidth capacity, but in some situations the 'extra work' can result in a general improvement in overall transfer speeds.

The PS2 engineers introduced the MFIFO for two reasons:

1. The scratchpad is too small. MFIFO can be used by the EE core as a place to "empty" the scratchpad after its completed a set of data processing. While the data in the MFIFO awaits the DMAC to transfer it, the EE is free to load new raw data into Scratchpad for processing.

2. The GIF has additional bandwidth constraints since it has direct connections to three PATHs: the the VU1 co-processor (GIF PATH1), VIF1 FIFO (GIF PATH2), and the DMAC's GIF channel (GIF PATH3). When transfers are active on any one of the paths, the other two paths must idle/stall until the current path's transfer completes; meaning that DMAC transfers to both GIF and VIF1 channels can have unexpectedly long stalls.

So by using MFIFO, the EE core can mitigate the unpredictable GIF/VIF1 stalls while it works on entirely new sets of data in parallel. If a GIF transfer via DMA is stalled because of other PATH1 or PATH2 transfers, the DMAC can busy itself with other transfers in meantime, such as SPRAM->memory or memory->SPRAM. These transfers are nearly 'free' in a sense, since the DMAC would have been idle regardless -- but thanks to the MFIFO concept, the SPRAM itself will be free for use by the EE Core to continue processing data. Thus while the DMAC's overall productivity isn't affected, the EE's overall computational ability improves.

I'll talk a bit more on actual emulation details of the PS2's programmable DMA controller in future blogs, so this is To Be Continued...

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