[RFT] Port 0x80 I/O speed

[Rene Herman <rene.herman@gmail.com>]

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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.

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/* 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;
}