rand.c

一款MP3 Player Firmware 的原代码,非常有参考价值

/*
 * MP3 Player, Pseudo Random Numbers, http://www.pjrc.com/tech/mp3
 * Copyright (c) 2000, PJRC.COM, LLC
 *
 * This program is free software; you can redistribute it and/or
 * modify it under the terms of the GNU General Public License
 * as published by the Free Software Foundation; either version 2
 * of the License, or (at your option) any later version.
 *
 * This program 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 General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program; if not, write to the Free Software
 * Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA  02111-1307, USA.
 *
 * As a specific exception to the GPL, the executable object code built
 * from this source code may be combined with hardware configuration data
 * files.  A hardware configuration data file is a set of data that is
 * transmitted to an intergrated circuit that is not a general purpose
 * microprocessor or microcontroller, in order to establish its normal
 * operation.  The process of combining the executable ojbect code built
 * from the GPL licensed source code with the hardware configuration data
 * shall be considered an aggregation of another work not based on the
 * Program.  While the GPL does not restrict use of the program, any
 * use restriction associated with the hardware configuration data (for
 * example, that it only be used with particular hardware) shall apply
 * to the combined file which includes a copy of the hardware configuration
 * data.
 *
 * Contact: [e-mail]
 */

#include "as31glue.h"
#include "main.h"
#include "printf.h"
#include "rand.h"
#include "params.h"
#include "playlist.h"
#include "display.h"

volatile xdata unsigned int rand16_seed;


void restore_rand_settings(void)
{
        rand16_seed = (int)read_param_1byte(PARM_RAND16_SEED_HI, 0x42) << 8
        	| read_param_1byte(PARM_RAND16_SEED_LO, 0x13);
	print("restore rand16 seed = ");
	print_hex16(rand16_seed);
	print_crlf();
	rand16();	// always advance at least one in the sequence
	update_rand_seed_needed = 0;
}


void backup_rand_settings(void)
{
	print("save rand16 seed = ");
	print_hex16(rand16_seed);
	print_crlf();
	ibuf[0] = (unsigned char)(rand16_seed >> 8);
	write_flash_param(PARM_RAND16_SEED_HI, ibuf);
	ibuf[0] = (unsigned char)(rand16_seed & 255);
	write_flash_param(PARM_RAND16_SEED_LO, ibuf);
	update_rand_seed_needed = 0;
}



/* call this at "random" times... such as the moment the IDE disk
 * finishes spinning up.  It takes the raw rapidly incrementing
 * value of timer2 and XORs the rand16_seed by it... so if this
 * is called at "unpredictable" times, the seed will change in a
 * random way
 */

sfr at 0xCC TL2;
sfr at 0xCD TH2;

void tweak_rand_seed(void)
{
#if 0
	unsigned int x;
	x = (((unsigned int)TH2 << 8) | TL2);
	rand16_seed ^= x;
	print("Tweaking rand16_seed by ");
	print_hex16(x);
	print_crlf();
#else
	_asm
	mov	dptr, #_rand16_seed
	movx	a, @dptr
	xrl	a, _TL2
	movx	@dptr, a
	inc	dptr
	movx	a, @dptr
	xrl	a, _TH2
	movx	@dptr, a
	_endasm;
#endif
}



#pragma SAVE
#pragma LESS_PEDANTIC

unsigned int rand16(void) {
_asm
	mov	dptr, #_rand16_seed
	movx	a, @dptr
	jnz	rand16b
	inc	dptr
	movx	a, @dptr
	jnz	rand16b
	cpl	a
	movx	@dptr, a
rand16b:
	mov	dptr, #_rand16_seed
	movx	a, @dptr
	anl	a, #0x08
	mov	b, a
	inc	dptr
	movx	a, @dptr
	anl	a, #0xD0
	orl	a, b
	mov	c, p	
	mov	dptr, #_rand16_seed
	movx	a, @dptr	
	rlc	a
	movx	@dptr, a
	mov	b, a
	inc	dptr				
	movx	a, @dptr
	rlc	a
	movx	@dptr, a	
	mov	dpl, b
	mov	dph, a
	_endasm;
}

#pragma RESTORE