BIO: The Bao I/O Coprocessor

Hello again HN, I'm bunnie! Unfortunately, time zones strike again...I'll check back when I can, and respond to your questions.

This is actually super cool, you can use those as both math accelerators and as io, and them being in lockstep you can kind of use them as int only shader units. I don't know how this is useful yet.

Btw I am curious what about edge cases. Maybe I have missed that from the article but what is the size of the FIFO?

Or the more dangerous part that is you have complex to determine timing now for complex cases like each reqd from FIFO is and ISR and you have until the next read from the FIFO amount of instructions otherwise you would stall the system and that looks to me too hard to debug.

I loved this article and had wanted to play with PIO for a long time (or at least, learn from it through playing!).

One thing jumped out here - I assumed CISC inside PIO had a mental model of "one instruction by cycle" and thus it was pretty easy to reason about the underlying machine (including any delay slots etc...).

For this RISC model using C, we are now reasoning about compiled code which has a somewhat variable instruction timing (1-3 cycles) and that introduces an uncertainty - the compiler and understanding its implementation.

I think this means that the PIO is timing-first, as timing == waveform where BIO is clarity-first with C as the expression and then explicit hardware synchronization.

I like both models! I am wondering about the quantum delays however that are being used to set the deadlines - here, human derived wait delays are utilized knowledge of the compiled instructions to set the timing.

Might there not be a model of 'preparing the next hardware transaction' and then 'waiting for an external synchronization' such as an external signal or internal clock, so we don't need to count the instruction cycles so precisely. On the external signal side, I guess the instruction is 'wait for GPIO change' or something, so the value is immediately ready (int i = GPIO_read_wait_high(23) or something) and the external one is doing the same, but synchronizing (GPIO_write_wait_clock( 24, CLOCK_DEF)) as an alternative to the explicit quantum delays.

This might be a shadow register / latch model in more generic terms - prep the work in shadow, latch/commit on trigger.

Anyway, great work Bunnie!

I appreciate the intro, motivation and comparison to the PIO of the RP2040/2350. How would this compare to the (considerably older, slower, but more flexible) Parallax P8X32A ("Propeller")?

> Above is the logic path isolated as one of the longest combination paths in the design, and below is a detailed report of what the cells are.

which is an argument that "fpga_pio" is badly implemented or that PIO is unsuitable for FPGA impls. Real silicon does not need to use a shitton of LUT4s to implement this logic and it can be done much more efficiently and closes timing at higher clocks (as we know since PIO will run near a GHz)

3 comments on this, from 2d ago, https://news.ycombinator.com/item?id=47469744

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