From 16cd04d1f97a4b4ecabd74cf6e2e16f12f2f35be Mon Sep 17 00:00:00 2001
From: Joel Sing <jsing@cvs.openbsd.org>
Date: Mon, 29 Oct 2012 13:54:57 +0000
Subject: Make cdboot(8) work correctly when it is larger than 64KB in size.
 When relocating use blocks that are a maximum of 32KB in size and increment
 the segment registers after relocating each block. This keeps us within the
 confines of the %cx register and the real mode segmented addressing.

---
 sys/arch/amd64/stand/cdboot/srt0.S | 49 ++++++++++++++++++++++++++++----------
 1 file changed, 36 insertions(+), 13 deletions(-)

(limited to 'sys/arch/amd64')

diff --git a/sys/arch/amd64/stand/cdboot/srt0.S b/sys/arch/amd64/stand/cdboot/srt0.S
index 9a3d241a263..82ff009ab16 100644
--- a/sys/arch/amd64/stand/cdboot/srt0.S
+++ b/sys/arch/amd64/stand/cdboot/srt0.S
@@ -1,4 +1,4 @@
-/*	$OpenBSD: srt0.S,v 1.2 2012/10/11 12:08:24 jsing Exp $	*/
+/*	$OpenBSD: srt0.S,v 1.3 2012/10/29 13:54:56 jsing Exp $	*/
 
 /*
  * Copyright (c) 1997 Michael Shalayeff
@@ -58,13 +58,10 @@ _start:
 	 * Like the standard /boot, we are linked to run at 0x40120
 	 * (load address 0x40000), so we relocate to there.
 	 *
-	 * From 0x7C00 to 0x40000 is 0x38400 (230400) bytes, so don't
+	 * From 0x7C00 to 0x40000 is 0x38400 (230400) bytes, so we don't
 	 * have to worry about an overlapping copy until cdboot is
 	 * over 225 KB.
 	 *
-	 * Note that there are other reasons to be worried if
-	 * sizeof(/boot) > 64 KB.  So currently we copy a maximum of 64 KB.
-	 *
 	 * Our cdbr CD-ROM boot sector passes us the drive number to use
 	 * in %dl.
 	 */
@@ -73,20 +70,46 @@ _start:
 	movw	%ax, %ss		/* CPU disables interrupts till... */
 	movl	$CDBOOTADDR-4, %esp	/* after this instruction */
 
-	movw	$(CDBOOTADDR >> 4), %ax
-	movw	%ax, %ds
-	xorw	%si, %si		/* Where we're coming from */
+	pushl	%edx			/* Preserve the drive number. */
 
-	movw	$(LINKADDR >> 4), %ax
-	movw	%ax, %es		/* Set %es = 0x4000 */
-	xorw	%di, %di		/* Where we're going to */
+	movw	$(CDBOOTADDR >> 4), %ax /* Reloc from %ds = 0x7c0. */
+	movw	$(LINKADDR >> 4), %bx	/* Reloc to %es = 0x4012. */
 
-	movl	$_C_LABEL(end), %ecx
-	subl	$_C_LABEL(_start), %ecx	/* How big are we? */
+	movl	$_C_LABEL(end), %edx
+	subl	$_C_LABEL(_start), %edx	/* How big are we? */
+
+	/*
+	 * Relocate in blocks that are a maximum of 32KB in size, incrementing
+	 * the segment registers after each block. The 'rep; movsb' instruction
+	 * uses %cx, which limits us to a maximum block size of 0xfff0, even
+	 * though we can address the full 64KB within a single segment.
+	 */
+#define RELOC_BLOCK_SIZE 0x8000
+reloc_loop:
+	movl	%edx, %ecx
+	jcxz	reloc_done
+	cmpl	$RELOC_BLOCK_SIZE, %ecx
+	jbe	reloc_notrunc
+	movl	$RELOC_BLOCK_SIZE, %ecx
+reloc_notrunc:
+	subl	%ecx, %edx
+
+	movw	%ax, %ds		/* Where we're coming from */
+	xorw	%si, %si
+
+	movw	%bx, %es		/* Where we're going to */
+	xorw	%di, %di
 
 	cld
 	rep; movsb			/* Copy into place */
 
+	addw	$(RELOC_BLOCK_SIZE >> 4), %ax
+	addw	$(RELOC_BLOCK_SIZE >> 4), %bx
+
+	jmp	reloc_loop
+
+reloc_done:
+	popl	%edx
 	jmpl	$(LINKADDR >> 4), $(relocated-_start)	/* Now relocate */
 
 relocated:
-- 
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