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From: "Rod Pemberton" <dont_use_email AT xnothavet DOT cqm>
Newsgroups: comp.os.msdos.djgpp
Subject: Re: Using outportb() with djgpp
Date: Thu, 10 Jul 2014 22:08:13 -0400
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On Thu, 10 Jul 2014 16:20:13 -0400, Mateusz Viste  
<mateusz DOT viste AT localhost> wrote:

> I am trying to write to a hardware port from within djgpp, using  
> outportb(). It does work from time to time, but often outportb() will  
> make the whole PC reboot. Specifically I am writing to port 0x330 (to a  
> MPU401 adapter). If using the same code with turbo C, it works  
> perfectly. Hence I am wondering: are there any special "rules" I should  
> be aware of when trying to outportb() from within protected mode?

I'm not familiar with your port.

I use outportb() extensively in my OS project for keyboard, mouse,
RTC, PIT, PICs, VGA registers, etc.  Typically, I have both hardware
interrupts and software interrupts disabled when using outportb().
In at least one instance, only software interrupts are disabled.

Software interrupts are disabled with the x86 CLI instruction, but
with C compilers you should use the provided function.  Sometimes
they do more than just clear the interrupt.  For DJGPP, you should
use disable().  Other C compilers have similar functions.

Hardware interrupts (NMI - Non-Maskable Interrupts) are disabled or
enabled by changing a bit in CMOS memory, i.e., using out_portb()
and in_portb().

"Non Maskable Interrupt"
http://wiki.osdev.org/Non_Maskable_Interrupt

alt.os.development FAQ on NMI
http://aodfaq.wikispaces.com/boot#How%20do%20I%20disable%20NMI%20on%20a%20PC?


Rod Pemberton