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cembeddedhardware-port

Reading and writing on I/O ports

I am trying to understand the following code:

#include<stdio.h>
#include<stdlib.h>
#include<sys/io.h>

#define baseport 0x378

int main()
{
    int b;
    if(ioperm(baseport,3,1))
    {
        perror("ioperm");
        exit(1);
    }
    outb(0,baseport);

    usleep(1000000);
    printf("\n the status: %x,\n",inb(baseport));

    if (ioperm(baseport,3,0)) {perror("ioperm"); exit(1);}

    exit(0);
}

The output is 0xff, 255 in decimal, whether I write on Port 1 or Port 0 (using outb()). I cannot understand why it is 255 when I am writing 0 to it.

Solution

  • The result of doing inb(0x378) is hardware-dependent. Some chips return the value you have written previously with outb, and some other chips just return garbage. In any case, it is not the port to read bytes from a potentially connected device.