input latency of BBC Micro? (keypress to mode 7 update)

for bbc micro/electron hardware, peripherals & programming issues (NOT emulators!)
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BigEd
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input latency of BBC Micro? (keypress to mode 7 update)

Post by BigEd » Mon Jan 01, 2018 5:46 pm

Anyone an idea of how long it takes a Beeb to process a keystroke?

(This article is all about input latency and how the Apple II is much better than a modern PC, and I wondered how the Beeb compares.)

crj
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Re: input latency of BBC Micro? (keypress to mode 7 update)

Post by crj » Tue Jan 02, 2018 4:10 am

Well, the Beeb has a slight disadvantage by having a field rate of 50Hz rather than 60Hz, so the unavoidable display latency is an average of 10ms, compared with the Apple II's 8.3ms. Despite that, I imagine the Beeb would be incredibly fast doing OSWORD 0 in MODE 7. It'll get a key pressed interrupt within (on average 8?) microseconds, which will update the keyboard driver state. After on average 5ms, the 100Hz timer will trigger and perform keyboard handling. That will drop a character into the input buffer, which will be immediately noticed by OSWORD 0 in the foreground.

I'll stick my neck out and say a BBC B will average less than 20ms latency, maybe as low as 16ms.

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BigEd
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Re: input latency of BBC Micro? (keypress to mode 7 update)

Post by BigEd » Tue Jan 02, 2018 7:46 am

My camera has a 100fps movie mode, but it sounds like that's not fast enough in this case.

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scarybeasts
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Re: input latency of BBC Micro? (keypress to mode 7 update)

Post by scarybeasts » Thu Jun 21, 2018 11:28 pm

I just added some notes about this in another thread:

viewtopic.php?f=4&t=14559&p=206586#p206586

Either I've gotten it wrong, or the MOS 1.2 code actually takes TWO 10ms timer interrupt ticks after the keyboard interrupt, to get the key press written to the buffer. Quirk? MOS 1.2 bug? Or I'm totally wrong?


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BigEd
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Re: input latency of BBC Micro? (keypress to mode 7 update)

Post by BigEd » Fri Jun 22, 2018 4:29 am

Interesting finding! I wonder if the intent is to filter out bouncing contacts? Or if it's an unintended misfeature which was never noticed. We'll see what others have to say over there.

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