BBS:      TELESC.NET.BR
Assunto:  Looking for screenshots/a
De:       Ito
Data:     Fri, 27 Feb 2026 04:29:00 -0500
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  hbRenb: hcLooking for screenshots/a
  bBynb: hcNightfox bto cDaiTengu bon cWed Feb 25 2026 12:17 pmn

 > re-configure their IRQs).  But I recall seeing that the company that made
 > MajorBBS offered some hardware that allowed a lot more serial ports, and
 > their MajorBBS software naturally supported that.  I'm not sure how the
 > hardware accomplished that, since typically there were limited IRQs
 > available.

MajorBBS didn't use the IRQ's that are normally used for serial ports.  It used the IRQ that was hooked up to the programmable interval timer, which normally ran at 18.2 times per second.  It would change the interval so it ran often enough to get a byte from the serial port in time.  So for 1200 baud, it'd be at least 120 times per second.  And each time the timer IRQ triggered, it would loop through all the serial ports.  If there was a new character to read, it would add it to the input buffer.  If the port was ready for a character to be written, it would take it from the output buffer and send it to the serial port.

To keep the system clock from running too fast, at the end of the interrupt service routine, it would only call the previously attached ISR at every Xth invocation such that it would only get called at the expected 18.2 times per second.  So it set the programmable interval timer to a multiple of 18.2 so that it could easily divide it back out.
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  gSynchronetn  This is my tagline so I don't get any complaints when I post

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