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Subject[RFT] Port 0x80 I/O speed
From
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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