11 Feb
2025
11 Feb
'25
5:42 p.m.
The 8250, 6551, Amiga serial port, etc do not have
FIFOs and so generate one
interrupt per byte. The CPU has to respond to the interrupt and retrieve the
incoming byte before the next byte has arrived. At 9600 baud, that needs to
happen within 1ms. At 115,200 baud, it's 87µs.
Older operating systems -- especially from the era where non-FIFO serial
ports were still standard -- would leave interrupts disabled for extended
periods.
At least for the 6551 cartridges on the Commodore 64/128, they were wired to
send NMIs. 57600bps generally worked just fine with later ones like the
Turbo232 (the SwiftLink was limited to 38400).
The 6551 in the Plus/4 is rigged to send IRQs, but I've not used one for serial
communications, so I can't say if it was substantially different in its
performance.
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Cameron Kaiser * Floodgap Systems * www.floodgap.com * ckaiser(a)floodgap.com
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