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disk/format: create Fatinfo sector for fat32

--- a/sys/src/cmd/disk/format.c

+++ b/sys/src/cmd/disk/format.c

@@ -72,6 +72,24 @@

 	uchar	type[8];

 };

+enum

+{

+	FATINFOSIG1	= 0x41615252UL,

+	FATINFOSIG	= 0x61417272UL,

+};

+

+typedef struct Fatinfo Fatinfo;

+struct Fatinfo

+{

+	uchar	sig1[4];

+	uchar	pad[480];

+	uchar	sig[4];

+	uchar	freeclust[4];	/* num frre clusters; -1 is unknown */

+	uchar	nextfree[4];	/* most recently allocated cluster */

+	uchar	resrv[4*3];

+};

+

+

 #define	PUTSHORT(p, v) { (p)[1] = (v)>>8; (p)[0] = (v); }

 #define	PUTLONG(p, v) { PUTSHORT((p), (v)); PUTSHORT((p)+2, (v)>>16); }

 #define	GETSHORT(p)	(((p)[1]<<8)|(p)[0])

@@ -537,6 +555,8 @@

 		 */

 		fatbits = 12;

 Tryagain:

+		if(fatbits == 32)

+			nresrv++;	/* for FatInfo */

 		volsecs = length/secsize;

 		/*

 		 * here's a crock inside a crock.  even having fixed fatbits,

@@ -609,6 +629,8 @@

 			bb = (Dosboot32*)buf;

 			PUTLONG(bb->fatsize, fatsecs);

 			PUTLONG(bb->rootclust, 2);

+			PUTSHORT(bb->fsinfo, nresrv-1);

+			PUTSHORT(bb->bootbak, 0);

 			bb->bootsig = 0x29;

 			bb->driveno = (t->media == 0xF8) ? getdriveno(disk) : 0;

 			memmove(bb->label, label, sizeof(bb->label));

@@ -762,6 +784,25 @@

 	 *  write the fats and root

 	 */

 	if(commit) {

+		if(fatbits == 32){

+			Fatinfo *fi;

+

+			fi = malloc(secsize);

+			if(fi == nil)

+				fatal("out of memory");

+

+			memset(fi, 0, secsize);

+			PUTLONG(fi->sig1, FATINFOSIG1);

+			PUTLONG(fi->sig, FATINFOSIG);

+			PUTLONG(fi->freeclust, clusters - fatlast);

+			PUTLONG(fi->nextfree, fatlast);

+	

+			if(seek(disk->wfd, (nresrv-1)*secsize, 0) < 0)

+				fatal("seek to fatinfo: %r\n");

+			if(write(disk->wfd, fi, secsize) < 0)

+				fatal("writing fat #1: %r");

+			free(fi);

+		}

 		if(seek(disk->wfd, nresrv*secsize, 0) < 0)

 			fatal("seek to fat #1: %r");

 		if(write(disk->wfd, fat, fatsecs*secsize) < 0)