dosfs.c

这是一个同样来自贝尔实验室的和UNIX有着渊源的操作系统, 其简洁的设计和实现易于我们学习和理解

#include "all.h"
#include "io.h"
#include "mem.h"

#include "dosfs.h"

#define	GSHORT(p) (((p)[1]<<8)|(p)[0])
#define	GLONG(p) ((GSHORT(p+2)<<16)|GSHORT(p))
#define	GLSHORT(p) (((p)[0]<<8)|(p)[1])
#define	GLLONG(p) ((GLSHORT(p)<<16)|GLSHORT(p+2))

/*
 *  debugging
 */
#define chatty	0
#define	chat	if(chatty)print

/*
 *  block io buffers
 */
typedef struct Clustbuf	Clustbuf;

struct Clustbuf
{
	int	flags;
	int	age;
	Devsize	sector;
	uchar *	iobuf;
	Dos *	dos;
	int	size;
	int	bufsize;
};

enum
{
	Nbio=	16,
	LOCKED=	1,
	MOD=	2,
	IMMED=	4,
};

static void	puttime(Dosdir*);

static Clustbuf	bio[Nbio];

/*
 *  write an io buffer and update its flags
 */
static void
writeclust(Clustbuf *p)
{
	Dos *dos;
	Off addr;

	dos = p->dos;
	addr = (p->sector+dos->start)*dos->sectbytes;
	chat("writeclust @ %lld addr %lld...", (Wideoff)p->sector,
		(Wideoff)addr);
	if((*dos->seek)(dos->dev, addr) < 0)
		panic("writeclust: seek");
	if((*dos->write)(dos->dev, p->iobuf, p->size) != p->size)
		panic("writeclust: write");
	p->flags &= ~(MOD|IMMED);
	chat("OK\n");
}

/*
 *  write any dirty buffers
 */
static void
syncclust(void)
{
	Clustbuf *p;

	for(p = bio; p < &bio[Nbio]; p++){
		if(p->flags & LOCKED)
			panic("syncclust");
		if(p->flags & MOD)
			writeclust(p);
	}
}

/*
 *  get an io buffer, possibly with valid data
 */
static Clustbuf*
getclust0(Dos *dos, Off sector)
{
	Clustbuf *p, *oldest;

	chat("getclust0 @ %lld\n", (Wideoff)sector);

	/*
	 *  if we have it, just return it
	 *  otherwise, reuse the oldest unlocked entry
	 */
	oldest = 0;
	for(p = bio; p < &bio[Nbio]; p++){
		if(sector == p->sector && dos == p->dos){
			if(p->flags & LOCKED)
				panic("getclust0 locked");
			chat("getclust0 %lld in cache\n", (Wideoff)sector);
			p->flags |= LOCKED;
			return p;
		}
		if(p->flags & LOCKED)
			continue;
		if(oldest == 0 || p->age <= oldest->age)
			oldest = p;
	}
	p = oldest;
	if(p == 0)
		panic("getclust0 all locked");
	p->flags |= LOCKED;
	if(p->flags & MOD)
		writeclust(p);

	/*
	 *  make sure the buffer is big enough
	 */
	if(p->iobuf==0 || p->bufsize < dos->clustbytes){
		p->bufsize = dos->clustbytes;
		p->iobuf = ialloc(p->bufsize, 0);
	}
	if(sector >= dos->dataaddr)
		p->size = dos->clustbytes;
	else
		p->size = dos->sectbytes;
	p->dos = 0;	/* make it invalid */
	return p;
}

/*
 *  get an io block from an io buffer
 */
static Clustbuf*
getclust(Dos *dos, Off sector)
{
	Clustbuf *p;
	Off addr;

	p = getclust0(dos, sector);
	if(p->dos){
		p->age = MACHP(0)->ticks;
		return p;
	}
	addr = (sector+dos->start)*dos->sectbytes;
	chat("getclust seek addr %lld\n", (Wideoff)addr);
	if((*dos->seek)(dos->dev, addr) < 0){
		chat("can't seek block\n");
		return 0;
	}
	chat("getclust read addr %lld\n", (Wideoff)addr);
	if((*dos->read)(dos->dev, p->iobuf, p->size) != p->size){
		chat("can't read block\n");
		return 0;
	}

	p->age = MACHP(0)->ticks;
	p->dos = dos;
	p->sector = sector;
	chat("getclust %lld read\n", (Wideoff)sector);
	return p;
}

/*
 *  get an io block from an io buffer;
 *  any current data is discarded.
 */
static Clustbuf*
getclustz(Dos *dos, Off sector)
{
	Clustbuf *p;

	p = getclust0(dos, sector);
	p->age = MACHP(0)->ticks;
	p->dos = dos;
	p->sector = sector;
	memset(p->iobuf, 0, p->size);
	p->flags |= MOD;
	chat("getclustz %lld\n", (Wideoff)sector);
	return p;
}

/*
 *  release an io buffer
 */
static void
putclust(Clustbuf *p)
{
	if(!(p->flags & LOCKED))
		panic("putclust lock");
	if((p->flags & (MOD|IMMED)) == (MOD|IMMED))
		writeclust(p);
	p->flags &= ~LOCKED;
	chat("putclust @ sector %lld...", (Wideoff)p->sector);
}

/*
 *  walk the fat one level ( n is a current cluster number ).
 *  return the new cluster number or -1 if no more.
 */
static long
fatwalk(Dos *dos, int n)
{
	ulong k, sect;
	Clustbuf *p;
	int o;

	chat("fatwalk %d\n", n);

	if(n < 2 || n >= dos->fatclusters)
		return -1;

	switch(dos->fatbits){
	case 12:
		k = (3*n)/2; break;
	case 16:
		k = 2*n; break;
	default:
		return -1;
	}
	if(k >= dos->fatbytes)
		panic("getfat");

	sect = k/dos->sectbytes + dos->fataddr;
	o = k%dos->sectbytes;
	p = getclust(dos, sect);
	k = p->iobuf[o++];
	if(o >= dos->sectbytes){
		putclust(p);
		p = getclust(dos, sect+1);
		o = 0;
	}
	k |= p->iobuf[o]<<8;
	putclust(p);
	if(dos->fatbits == 12){
		if(n&1)
			k >>= 4;
		else
			k &= 0xfff;
		if(k >= 0xff8)
			k |= 0xf000;
	}
	k = k < 0xfff8 ? k : -1;
	chat("fatwalk %d -> %lud\n", n, k);
	return k;
}

/*
 *  write a value into each copy of the fat.
 */
static void
fatwrite(Dos *dos, int n, int val)
{
	Off k, sect;
	Clustbuf *p;
	int i, o;

	chat("fatwrite %d %d...", n, val);

	if(n < 2 || n >= dos->fatclusters)
		panic("fatwrite n");

	switch(dos->fatbits){
	case 12:
		k = (3*n)/2; break;
	case 16:
		k = 2*n; break;
	default:
		panic("fatwrite fatbits");
		return;
	}
	if(k >= dos->fatbytes)
		panic("fatwrite k");

	for(i=0; i<dos->nfats; i++, k+=dos->fatbytes){
		sect = k/dos->sectbytes + dos->fataddr;
		o = k%dos->sectbytes;
		p = getclust(dos, sect);
		if(p == 0)
			panic("fatwrite getclust");
		switch(dos->fatbits){
		case 12:
			if(n&1){
				p->iobuf[o] &= 0x0f;
				p->iobuf[o++] |= val<<4;
			}else
				p->iobuf[o++] = val;
			if(o >= dos->sectbytes){
				p->flags |= MOD;
				putclust(p);
				p = getclust(dos, sect+1);
				if(p == 0)
					panic("fatwrite getclust");
				o = 0;
			}
			if(n&1)
				p->iobuf[o] = val>>4;
			else{
				p->iobuf[o] &= 0xf0;
				p->iobuf[o] |= (val>>8)&0x0f;
			}
			break;
		case 16:
			p->iobuf[o++] = val;
			p->iobuf[o] = val>>8;
			break;
		}
		p->flags |= MOD;
		putclust(p);
	}
	chat("OK\n");
}

/*
 *  allocate a free cluster from the fat.
 */
static int
fatalloc(Dos *dos)
{
	Clustbuf *p;
	int n;

	n = dos->freeptr;
	for(;;){
		if(fatwalk(dos, n) == 0)
			break;
		if(++n >= dos->fatclusters)
			n = 2;
		if(n == dos->freeptr)
			return -1;
	}
	dos->freeptr = n+1;
	if(dos->freeptr >= dos->fatclusters)
		dos->freeptr = 2;
	fatwrite(dos, n, 0xffff);
	p = getclustz(dos, dos->dataaddr + (n-2)*dos->clustsize);
	putclust(p);
	return n;
}

/*
 *  map a file's logical sector address to a physical sector address
 */
static long
fileaddr(Dosfile *fp, Off ltarget, Clustbuf *pdir)
{
	Dos *dos = fp->dos;
	Dosdir *dp;
	Off p;

	chat("fileaddr %8.8s %lld\n", fp->name, (Wideoff)ltarget);
	/*
	 *  root directory is contiguous and easy
	 */
	if(fp->pdir == 0){
		if(ltarget*dos->sectbytes >= dos->rootsize*sizeof(Dosdir))
			return -1;
		p = dos->rootaddr + ltarget;
		chat("fileaddr %lld -> %lld\n", (Wideoff)ltarget, (Wideoff)p);
		return p;
	}
	if(fp->pstart == 0){	/* empty file */
		if(!pdir)
			return -1;
		p = fatalloc(dos);
		if(p <= 0)
			return -1;
		chat("fileaddr initial alloc %lld\n", (Wideoff)p);
		dp = (Dosdir *)(pdir->iobuf + fp->odir);
		puttime(dp);
		dp->start[0] = p;
		dp->start[1] = p>>8;
		pdir->flags |= MOD;
		fp->pstart = p;
		fp->pcurrent = p;
		fp->lcurrent = 0;
	}
	/*
	 *  anything else requires a walk through the fat
	 *  [lp]current will point to the last cluster if we run off the end
	 */
	ltarget /= dos->clustsize;
	if(fp->pcurrent == 0 || fp->lcurrent > ltarget){
		/* go back to the beginning */
		fp->lcurrent = 0;
		fp->pcurrent = fp->pstart;
	}
	while(fp->lcurrent < ltarget){
		/* walk the fat */
		p = fatwalk(dos, fp->pcurrent);
		if(p < 0){
			if(!pdir)
				return -1;
			p = fatalloc(dos);
			if(p < 0){
				print("file system full\n");
				return -1;
			}
			fatwrite(dos, fp->pcurrent, p);
		}
		fp->pcurrent = p;
		++fp->lcurrent;
	}

	/*
	 *  clusters start at 2 instead of 0 (why? - presotto)
	 */
	p = dos->dataaddr + (fp->pcurrent-2)*dos->clustsize;
	chat("fileaddr %lld -> %lld\n", (Wideoff)ltarget, (Wideoff)p);
	return p;
}

/*
 *  set up a dos file name
 */
static void
setname(char *name, char *ext, char *from)
{
	char *to;

	memset(name, ' ', 8);
	memset(ext, ' ', 3);

	to = name;
	for(; *from && to-name < 8; from++, to++){
		if(*from == '.'){
			from++;
			break;