fold MBR/ext table reading loops together.  Tested against
s1,s2,s3,s4,s5 partitions created by disk/fdisk.

Reference: /n/sources/patch/applied/9load-mbrpart
Date: Tue Apr 26 04:11:46 CES 2005

--- /sys/src/boot/pc/part.c	Tue Apr 26 04:10:42 2005
+++ /sys/src/boot/pc/part.c	Tue Apr 26 04:10:37 2005
@@ -152,88 +152,53 @@
 static int
 mbrpart(SDunit *unit)
 {
-	ulong mbroffset;
 	Dospart *dp;
-	ulong start, end;
-	ulong epart, outer, inner;
+	ulong taboffset, start, end;
+	ulong firstxpart, nxtxpart;
 	int havedos, i, nplan9;
 	char name[10];
 
-	if(tsdbio(unit, &unit->part[0], mbrbuf, 0, 1) < 0)
-		return -1;
-
-	mbroffset = 0;
+	taboffset = 0;
 	dp = (Dospart*)&mbrbuf[0x1BE];
-	for(i=0; i<4; i++, dp++)
-		if(dp->type == DMDDO) {
-			mbroffset = 63*512;
-			if(trace)
-				print("DMDDO partition found\n");
-			if(tsdbio(unit, &unit->part[0], mbrbuf, mbroffset, 1) < 0)
-				return -1;
-			i = -1;	/* start over */
-		}
+	if(1) {
+		/* get the MBR (allowing for DMDDO) */
+		if(tsdbio(unit, &unit->part[0], mbrbuf, (vlong)taboffset*unit->secsize, 1) < 0)
+			return -1;
+		for(i=0; i<4; i++)
+			if(dp[i].type == DMDDO) {
+				if(trace)
+					print("DMDDO partition found\n");
+				taboffset = 63;
+				if(tsdbio(unit, &unit->part[0], mbrbuf, (vlong)taboffset*unit->secsize, 1) < 0)
+					return -1;
+				i = -1;	/* start over */
+			}
+	}
 
+	/*
+	 * Read the partitions, first from the MBR and then
+	 * from successive extended partition tables.
+	 */
 	nplan9 = 0;
 	havedos = 0;
-	epart = 0;
-	dp = (Dospart*)&mbrbuf[0x1BE];
-	if(trace)
-		print("%s mbr ", unit->name);
-	for(i=0; i<4; i++, dp++) {
-		if(trace)
-			print("dp %d...", dp->type);
-		start = mbroffset/512+GLONG(dp->start);
-		end = start+GLONG(dp->len);
-
-		if(dp->type == PLAN9) {
-			if(nplan9 == 0)
-				strcpy(name, "plan9");
+	firstxpart = 0;
+	for(;;) {
+		if(tsdbio(unit, &unit->part[0], mbrbuf, (vlong)taboffset*unit->secsize, 1) < 0)
+			return -1;
+		if(trace) {
+			if(firstxpart)
+				print("%s ext %lud ", unit->name, taboffset);
 			else
-				sprint(name, "plan9.%d", nplan9);
-			sdaddpart(unit, name, start, end);
-			p9part(unit, name);
-			nplan9++;
-		}
-
-		/*
-		 * We used to take the active partition (and then the first
-		 * when none are active).  We have to take the first here,
-		 * so that the partition we call ``dos'' agrees with the
-		 * partition disk/fdisk calls ``dos''. 
-		 */
-		if(havedos==0 && isdos(dp->type)){
-			havedos = 1;
-			sdaddpart(unit, "dos", start, end);
+				print("%s mbr ", unit->name);
 		}
-		if(isextend(dp->type)){
-			epart = start;
+		nxtxpart = 0;
+		for(i=0; i<4; i++) {
 			if(trace)
-				print("link %lud...", epart);
-		}
-	}
-	if(trace)
-		print("\n");
+				print("dp %d...", dp[i].type);
+			start = taboffset+GLONG(dp[i].start);
+			end = start+GLONG(dp[i].len);
 
-	/*
-	 * Search through the chain of extended partition tables.
-	 */
-	outer = epart;
-	while(epart != 0) {
-		if(trace)
-			print("%s ext %lud ", unit->name, epart);
-		if(tsdbio(unit, &unit->part[0], mbrbuf, (vlong)epart*unit->secsize, 1) < 0)
-			break;
-		inner = epart;
-		epart = 0;
-		dp = (Dospart*)&mbrbuf[0x1BE];
-		for(i=0; i<4; i++, dp++) {
-			if(trace)
-				print("dp %d...", dp->type);
-			start = GLONG(dp->start);
-			if(dp->type == PLAN9){
-				start += inner;
-				end = start+GLONG(dp->len);
+			if(dp[i].type == PLAN9) {
 				if(nplan9 == 0)
 					strcpy(name, "plan9");
 				else
@@ -242,22 +207,34 @@
 				p9part(unit, name);
 				nplan9++;
 			}
-			if(havedos==0 && isdos(dp->type)){
-				start += inner;
-				end = start+GLONG(dp->len);
+
+			/*
+			 * We used to take the active partition (and then the first
+			 * when none are active).  We have to take the first here,
+			 * so that the partition we call ``dos'' agrees with the
+			 * partition disk/fdisk calls ``dos''. 
+			 */
+			if(havedos==0 && isdos(dp[i].type)){
 				havedos = 1;
 				sdaddpart(unit, "dos", start, end);
 			}
-			if(isextend(dp->type)){
-				epart = start + outer;
+
+			/* nxtxpart is relative to firstxpart (or 0), not taboffset */
+			if(isextend(dp[i].type)){
+				nxtxpart = start-taboffset+firstxpart;
 				if(trace)
-					print("link %lud...", epart);
+					print("link %lud...", nxtxpart);
 			}
 		}
 		if(trace)
 			print("\n");
-	}
-		
+
+		if(!nxtxpart)
+			break;
+		if(!firstxpart)
+			firstxpart = nxtxpart;
+		taboffset = nxtxpart;
+	}	
 	return nplan9 ? 0 : -1;
 }