memset.c

上一个上传的有问题,这个是好的。visopsys包括系统内核和GUI的全部SOURCE code ,还包括一些基本的docs文档。里面src子目录对应所有SOURCE code.对于想研究操作系统的朋

// 
//  Visopsys
//  Copyright (C) 1998-2007 J. Andrew McLaughlin
//  
//  This library is free software; you can redistribute it and/or modify it
//  under the terms of the GNU Lesser General Public License as published by
//  the Free Software Foundation; either version 2.1 of the License, or (at
//  your option) any later version.
//
//  This library is distributed in the hope that it will be useful, but
//  WITHOUT ANY WARRANTY; without even the implied warranty of
//  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU Lesser
//  General Public License for more details.
//
//  You should have received a copy of the GNU Lesser General Public License
//  along with this library; if not, write to the Free Software Foundation,
//  Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
//
//  memset.c
//

// This is the standard "memset" function, as found in standard C libraries

// The description from the GNU man page reads as follows:
// The  memset() function fills the first n bytes of the memory area pointed
// to by s with the constant byte c. Returns a pointer to the memory area s.


#include <string.h>
#include <errno.h>

#define writeBytes(value, dest, count)   \
  __asm__ __volatile__ ("pushal \n\t"    \
			"pushfl \n\t"    \
			"cld \n\t"       \
			"rep stosb \n\t" \
			"popfl \n\t"     \
			"popal"          \
			: : "a" (value), "D" (dest), "c" (count))

#define writeDwords(value, dest, count)  \
  __asm__ __volatile__ ("pushal \n\t"    \
			"pushfl \n\t"    \
			"cld \n\t"       \
			"rep stosl \n\t" \
			"popfl \n\t"     \
			"popal"          \
			: : "a" (value), "D" (dest), "c" (count))


void *memset(void *dest, int value, size_t bytes)
{
  unsigned dwords = (bytes >> 2);
  unsigned tmpDword = 0;

  // Check params
  if (dest == NULL)
    {
      errno = ERR_NULLPARAMETER;
      return (NULL);
    }

  if (dwords)
    tmpDword = ((value << 24) | (value << 16) |	(value << 8) | value);

  if (((unsigned) dest % 4) || (bytes % 4))
    writeBytes(value, dest, bytes);
  else
    writeDwords(tmpDword, dest, dwords);

  // Return success
  return (dest);
}