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86Box serial port passthrough make VM run at a crawl

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First post, by HwAoRrDk

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I'm trying to use 86Box (v5.2 on Windows 11) to test something using the serial port passthrough feature, but everything seems to go a bit weird when I do.

I have virtual COM1 set for "Host Serial Passthrough", with the relevant host COM port (COM5) selected, and appropriate baud rate (1200) and data/stop bits set (8, 1). I assume the pipe name is irrelevant for host passthrough.

The main problems is that it makes the entire VM run at a crawl. The speed indicator (or whatever it's called) in the top right of the window shows it's running at about 14% speed. It goes even lower (7-8%) during boot-up, which takes forever - literally a couple of minutes just to boot a default install of MS-DOS 6.22! Most definitely not what the PII-450 that I'm emulating should be performing like. It makes what I'm trying to test pointless, because the system is running to slow to respond usefully.

I assume it's not supposed to work like this. Perhaps I've mis-configured something? Or is serial port passthrough just buggy?

Reply 1 of 1, by Jo22

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Hi, is it connected to a real COM port or an USB-serial adapter?
Because it might make a difference here, not exactly sure.
USB-serial adapters don't support all V.24 data formats and may not be fully compatible to an 16550 FiFo (or up). Depends on the drivers, too.
Also, USB 1.x and 2.0 uses polling, while a real COM port uses interrupts.
A real 16550 in its native mode (not 8250 or 16450 mode) has the ability to use lower interrupt rate, that also causes lower CPU usage.

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In what to one race is no time at all, another race can rise and fall..." - The Minstrel

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