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    Subject[RFT] Port 0x80 I/O speed
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    Good day.

    Would some people on x86 (both 32 and 64) be kind enough to compile and run
    the attached program? This is about testing how long I/O port access to port
    0x80 takes. It measures in CPU cycles so CPU speed is crucial in reporting.

    Posted a previous incarnation of this before, buried in the outb 0x80 thread
    which had a serialising problem. This one should as far as I can see measure
    the right thing though. Please yell if you disagree...

    For me, on a Duron 1300 (AMD756 chipset) I have a constant:

    rene@7ixe4:~/src/port80$ su -c ./port80
    cycles: out 2400, in 2400

    and on a PII 400 (Intel 440BX chipset) a constant:

    rene@6bap:~/src/port80$ su -c ./port80
    cycles: out 553, in 251

    Results are (mostly) independent of compiler optimisation, but testing with
    an -O2 compile should be most useful. Thanks!

    Rene.
    /* gcc -W -Wall -O2 -o port80 port80.c */

    #include <stdlib.h>
    #include <stdio.h>

    #include <sys/io.h>

    #define LOOPS 10000

    inline unsigned long long rdtsc(void)
    {
    unsigned long long tsc;

    asm volatile ("rdtsc": "=A" (tsc));

    return tsc;
    }

    inline void serialize(void)
    {
    asm volatile ("cpuid": : : "eax", "ebx", "ecx", "edx");
    }

    int main(void)
    {
    unsigned long long start;
    unsigned long long overhead;
    unsigned long long output;
    unsigned long long input;
    int i;

    if (iopl(3) < 0) {
    perror("iopl");
    return EXIT_FAILURE;
    }

    asm volatile ("cli");
    start = rdtsc();
    for (i = 0; i < LOOPS; i++) {
    serialize();
    serialize();
    }
    overhead = rdtsc() - start;

    start = rdtsc() + overhead;
    for (i = 0; i < LOOPS; i++) {
    serialize();
    asm volatile ("outb %al, $0x80");
    serialize();
    }
    output = rdtsc() - start;

    start = rdtsc() + overhead;
    for (i = 0; i < LOOPS; i++) {
    serialize();
    asm volatile ("inb $0x80, %%al": : : "al");
    serialize();
    }
    input = rdtsc() - start;
    asm volatile ("sti");

    output /= LOOPS;
    input /= LOOPS;
    printf("cycles: out %llu, in %llu\n", output, input);

    return EXIT_SUCCESS;
    }
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      Last update: 2007-12-12 00:35    [W:0.033 / U:2.604 seconds]
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