🚀 go-pugleaf

In article <1992Aug19.135359.24909@bas-a.bcc.ac.uk> nino@mph.sm.ucl.ac.uk (Nino Margetic) writes:
}I know that there is also a way to access the LPT port directly thru the
}in/out commands and its 3 I/O registers. Are there any advantages (in
}terms of speed) in using interrupt driven parralel I/O, as compared to
}INT17 BIOS service as I have used it - if there is, how much, and is
}it doable in C?? I can just about read assembler, but that's about it.

Writing directly to the ports can be an order of magnitude faster than doing
separate interrupt calls to check the status and actually write the character.
However, you probably will not be able to get reliable interrupt-driven
operation because (a) a majority of parallel ports don't reliable generate
the IRQ, and (b) the standard IRQ7 is also triggered whenever the interrupt
controller gets an interrupt request which is deasserted before the CPU
responds to the request.

}The speed of the application is crucial - I need to send as many bytes
}thru the LPT port as possible (order of 1e6). N.B. does anyone know the
}approximate number of bytes per second that can be sent thru the LPT port
}via INT17 and/or interrupt driven parallel I/O??

Maximum speed depends on the CPU speed and the speed of the device at the
other end of the parallel cable.  For example, if the other end is an
Epson FX80, the timing specifications for the strobe/ACK sequence set a
limit of about 75,000 characters per second.  Connecting a pair of 386/33s,
a data rate of 300,000 characters per second sounds feasible using tight
loops (you'd never be able to get near that speed in interrupt-driven
operation due to the overhead of handling an interrupt).

I've managed to pump nearly 2400 characters per second to said FX80 using
30% of the CPU cycles on a 4.77MHz 8088 (over one-third of that 30% was
the overhead of handling 2400 clock interrupts per second).  All parallel
port I/O was done using direct port accesses.

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