Reading and writing on I/O ports

Question

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.

What's supposed to be behind that port, and is it supposed to return anything else than 0xFF? — Mat, Mar 17 '12 at 14:38

score 5 · Accepted Answer · answered Mar 17 '12 at 15:40

5

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.

answered Mar 17 '12 at 15:40

jørgensen

10,149
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if inb does not read byte on the port then for what it is used....please explain as i am a newbie....... – karan421 Mar 17 '12 at 17:13
4

@karan - Some ports are just for output, like one connected to a printer. What would you be reading from the printer? – Bo Persson Mar 17 '12 at 18:14
@BoPersson: its status, but then again, that is at a different, read-only, port, IIRC. – ninjalj Mar 31 '12 at 11:14

score -2 · Answer 2 · edited Jan 08 '23 at 03:59

-2

First you need to see the port's capabilities, input, output or both. If it can be configured as both, you have to set it to the respective mode and only then you can expect the right behavior.

edited Jan 08 '23 at 03:59

trxgnyp1

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answered Mar 19 '12 at 11:45

RKT

187
1
9

Reading and writing on I/O ports

2 Answers2