alt Hacker NewsAll those layers add some latency, but 8 mhz is really slow. USB 4 can do extremely low latency, otherwise PCIe over USB wouldn't work.
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lysace • yesterday at 11:24 PM
This looks quite interesting:
https://www.reddit.com/r/vintagecomputing/comments/jbaw38/is... (2020)
ISA-over-USB, using real ISA cards in an emulator
The problem is that a lot of those old ISA drivers were peeking and poking card registers as fast as the bus would allow, which was over 1 MHz (less than 1 microsecond per transaction), not DMAing data to RAM (though some did that too).
With a modern 4 GHz CPU, that means your budget is only 4000 clocks per interrupt. The context save alone can make it hard to hit 1MHz, and you really want to have your interrupt code pinned to the L1 cache because a few memory reads will blow your entire budget.
As nice as USB4 is, I’ve never heard anyone claim it has single-digit microsecond latency, to say nothing of sub-microsecond latency. The typically-quoted ~20 usec latency of USB4 limits it to around 50 kHz for an emulated ISA card.