Index: head/sys/boot/i386/btx/lib/btxv86.h
===================================================================
--- head/sys/boot/i386/btx/lib/btxv86.h
+++ head/sys/boot/i386/btx/lib/btxv86.h
@@ -23,6 +23,14 @@
 #include <sys/types.h>
 #include <machine/psl.h>
 
+/*
+ * Memory buffer space for real mode IO.
+ * Just one page is not much, but the space is rather limited.
+ * See ../btx/btx.S for details.
+ */
+#define	V86_IO_BUFFER		0x8000
+#define	V86_IO_BUFFER_SIZE	0x1000
+
 #define V86_ADDR   0x10000	/* Segment:offset address */
 #define V86_CALLF  0x20000	/* Emulate far call */
 #define V86_FLAGS  0x40000	/* Return flags */
Index: head/sys/boot/i386/libi386/bioscd.c
===================================================================
--- head/sys/boot/i386/libi386/bioscd.c
+++ head/sys/boot/i386/libi386/bioscd.c
@@ -309,9 +309,6 @@
 	return (0);
 }
 
-/* Max number of sectors to bounce-buffer at a time. */
-#define	CD_BOUNCEBUF	8
-
 /* return negative value for an error, otherwise blocks read */
 static int
 bc_read(int unit, daddr_t dblk, int blks, caddr_t dest)
@@ -339,8 +336,9 @@
 		 * physical memory so we have to arrange a suitable
 		 * bounce buffer.
 		 */
-		x = min(CD_BOUNCEBUF, (unsigned)blks);
-		bbuf = alloca(x * BIOSCD_SECSIZE);
+		x = V86_IO_BUFFER_SIZE / BIOSCD_SECSIZE;
+		x = min(x, (unsigned)blks);
+		bbuf = PTOV(V86_IO_BUFFER);
 		maxfer = x;
 	} else {
 		bbuf = NULL;
Index: head/sys/boot/i386/libi386/biosdisk.c
===================================================================
--- head/sys/boot/i386/libi386/biosdisk.c
+++ head/sys/boot/i386/libi386/biosdisk.c
@@ -666,9 +666,6 @@
     return (0);
 }
 
-/* Max number of sectors to bounce-buffer if the request crosses a 64k boundary */
-#define FLOPPY_BOUNCEBUF	18
-
 static int
 bd_edd_io(struct disk_devdesc *dev, daddr_t dblk, int blks, caddr_t dest,
     int write)
@@ -732,7 +729,7 @@
 bd_io(struct disk_devdesc *dev, daddr_t dblk, int blks, caddr_t dest, int write)
 {
     u_int	x, sec, result, resid, retry, maxfer;
-    caddr_t	p, xp, bbuf, breg;
+    caddr_t	p, xp, bbuf;
     
     /* Just in case some idiot actually tries to read/write -1 blocks... */
     if (blks < 0)
@@ -754,17 +751,12 @@
 	 * as we need to.  Use the bottom half unless there is a break
 	 * there, in which case we use the top half.
 	 */
-	x = min(FLOPPY_BOUNCEBUF, (unsigned)blks);
-	bbuf = alloca(x * 2 * BD(dev).bd_sectorsize);
-	if (((u_int32_t)VTOP(bbuf) & 0xffff0000) ==
-	    ((u_int32_t)VTOP(bbuf + x * BD(dev).bd_sectorsize) & 0xffff0000)) {
-	    breg = bbuf;
-	} else {
-	    breg = bbuf + x * BD(dev).bd_sectorsize;
-	}
+	x = V86_IO_BUFFER_SIZE / BD(dev).bd_sectorsize;
+	x = min(x, (unsigned)blks);
+	bbuf = PTOV(V86_IO_BUFFER);
 	maxfer = x;		/* limit transfers to bounce region size */
     } else {
-	breg = bbuf = NULL;
+	bbuf = NULL;
 	maxfer = 0;
     }
     
@@ -779,14 +771,14 @@
 	    x = min(x, maxfer);		/* fit bounce buffer */
 
 	/* where do we transfer to? */
-	xp = bbuf == NULL ? p : breg;
+	xp = bbuf == NULL ? p : bbuf;
 
 	/*
 	 * Put your Data In, Put your Data out,
 	 * Put your Data In, and shake it all about 
 	 */
 	if (write && bbuf != NULL)
-	    bcopy(p, breg, x * BD(dev).bd_sectorsize);
+	    bcopy(p, bbuf, x * BD(dev).bd_sectorsize);
 
 	/*
 	 * Loop retrying the operation a couple of times.  The BIOS
@@ -820,7 +812,7 @@
 	    return(-1);
 	}
 	if (!write && bbuf != NULL)
-	    bcopy(breg, p, x * BD(dev).bd_sectorsize);
+	    bcopy(bbuf, p, x * BD(dev).bd_sectorsize);
 	p += (x * BD(dev).bd_sectorsize);
 	dblk += x;
 	resid -= x;