From: moskewicz AT MEM DOT po DOT com
Date: Fri, 14 Jun 1996 01:18:16 -0400
Message-Id: <9606140518.AA12305@mercury>
To: djgpp AT delorie DOT com
Subject: Scale by 4 in asm faults?

The following code dies with this error:
[Exactly what I did]
D:\DJGPP2\PROJECTS\LIGHTS>make -ftestmake -W test.cc

gcc -Wall -O3 test.cc -o test
coff2exe test

D:\DJGPP2\PROJECTS\LIGHTS>test
Hello.
Exiting due to signal SIGSEGV
General Protection Fault at eip=000015a2
eax=76a1a69a ebx=00022080 ecx=00040000 edx=0000018a esi=002b9004
edi=001b8004
ebp=001b7fd8 esp=001b7fc4 cs=00a7 ds=00af es=00af fs=00af gs=00c7
ss=00af
Call frame traceback EIPs:
  0x000015a2   _main+74
  0x00001b03   ___crt1_startup+115
[End of What Got Me Into Trouble]
[The Code]
#include <stdlib.h>
#include <stdio.h>
#include <go32.h>

int main( void )
{

	printf("Hello.\n");
	char *buf1 = (char *)malloc(1024*1024);
	char *buf2 = (char *)malloc(1024*1024);
	int times = 1024*1024/4;
	int stamp1, stamp2;
	short _our_ds = _go32_my_ds();
	asm("
		movw %5,%%fs
		cli
		.byte 0x0f
		.byte 0x31 #//RDTSC
		movl %%eax, %0

		movl %%fs:(,%2,4),%%eax 
		#//           ^^This scale is the culprit. 
		#//Anything other than 1 for scale crashes.
		#//The .S file looks okay, too.

		.byte 0x0f
		.byte 0x31 #//RDTSC
		movl %%eax, %1
		sti

		"
		:"=m" (stamp1),"=m" (stamp2)
		:"D"(buf1),"S"(buf2)
		,"c"(times)
		,"m"(_our_ds),"m"(_our_ds)
		:"%eax","%ecx"
		);
	int cycles;
	if (stamp2 > stamp1)
		cycles = stamp2 - stamp1;
	else											 	
		cycles = stamp2  + (0xffffffff - stamp1) + 1;
	printf("Cycles for #2 %-8i\n",cycles - 12);
}
[End Code]
[Relevant Lines from test.S]
      .byte 0x0f
      .byte 0x31 #//RDTSC
      movl %eax, -16(%ebp)

      movl %fs:(,%edi,4),%eax

      .byte 0x0f
      .byte 0x31 #//RDTSC
[End test.S]

This puzzles me. Why can't I scale by four? I've tried other index 
registers, etc ... nothing works, except a scale of 1.
BTW, whats a better way to impliment RDTSC? My way seems ugly.
----
moskewicz AT mem DOT po DOT com