dos: int 25/26 may be register-dirty; wrap them in assembly

int 25h and int 26h are known to be register-dirty for some versions
of DOS -- unlike int 21h, which is usually clean.  As such, wrap those
in assembly functions.

Signed-off-by: H. Peter Anvin <hpa@zytor.com>

4 files changed, 102 insertions, 31 deletions

diff --git a/dos/Makefile b/dos/Makefile
index 6d87d64a..9d8ce33c 100644
--- a/dos/Makefile
+++ b/dos/Makefile
@@ -30,7 +30,8 @@ SRCS     = syslinux.c \
 	   ../libinstaller/mbr_bin.c \
            $(wildcard ../libfat/*.c)
 OBJS	 = crt0.o $(patsubst %.c,%.o,$(notdir $(SRCS)))
-LIBOBJS	 = conio.o memcpy.o memset.o  skipatou.o atou.o malloc.o free.o \
+LIBOBJS	 = int2526.o conio.o memcpy.o memset.o  skipatou.o atou.o \
+	   malloc.o free.o \
 	   argv.o printf.o __divdi3.o __udivmoddi4.o
 
 VPATH = .:../libfat:../libinstaller
diff --git a/dos/int2526.S b/dos/int2526.S
new file mode 100644
index 00000000..bcb7707e
--- /dev/null
+++ b/dos/int2526.S
@@ -0,0 +1,76 @@
+/* ----------------------------------------------------------------------- *
+ *
+ *   Copyright 1998-2008 H. Peter Anvin - All Rights Reserved
+ *   Copyright 2009 Intel Corporation; author: H. Peter Anvin
+ *
+ *   This program is free software; you can redistribute it and/or modify
+ *   it under the terms of the GNU General Public License as published by
+ *   the Free Software Foundation, Inc., 53 Temple Place Ste 330,
+ *   Boston MA 02111-1307, USA; either version 2 of the License, or
+ *   (at your option) any later version; incorporated herein by reference.
+ *
+ * ----------------------------------------------------------------------- */
+
+/*
+ * int 0x25 and 0x26 direct sector access
+ *
+ * Use assembly wrapper functions for these system calls, since unlike
+ * int 0x21 calls they are "dirty" and can destroy unrelated registers.
+ *
+ * NOTE: these all assume the data buffer is in the data segment, i.e.
+ * %ds == %es == dio.bufseg.
+ *
+ * Usage: int int25_read_sector(drive, dio)
+ * Usage: int int26_write_sector(drive, dio)
+ */
+
+	.code16gcc
+	.text
+
+	.globl	int25_read_sector
+	.type	int25_read_sector, @function
+int25_read_sector:
+	pushl	%ebp
+	pushl	%edi
+	pushl	%esi
+	pushl	%ebx
+
+	decw	%ax		/* AL = drive number (0 = A:) */
+	movw	%dx, %bx	/* BX = dio structure */
+	movw	6(%bx), %dx	/* DX = data buffer */
+	movw	$-1, %cx
+	int	$0x25
+	jc	1f
+	xorw	%ax, %ax	/* Error code: 0 = no error */
+1:
+	popfw
+	movzwl	%ax, %eax
+	popl	%ebx
+	popl	%esi
+	popl	%edi
+	popl	%ebp
+	retl
+
+	.globl	int26_write_sector
+	.type	int26_write_sector, @function
+int26_write_sector:
+	pushl	%ebp
+	pushl	%edi
+	pushl	%esi
+	pushl	%ebx
+
+	decw	%ax		/* AL = drive number (0 = A:) */
+	movw	%dx, %bx	/* BX = dio structure */
+	movw	6(%bx), %dx	/* DX = data buffer */
+	movw	$-1, %cx
+	int	$0x26
+	jc	1f
+	xorw	%ax, %ax	/* Error code: 0 = no error */
+1:
+	popfw
+	movzwl	%ax, %eax
+	popl	%ebx
+	popl	%esi
+	popl	%edi
+	popl	%ebp
+	retl
diff --git a/dos/mystuff.h b/dos/mystuff.h
index fbf4e75b..25344413 100644
--- a/dos/mystuff.h
+++ b/dos/mystuff.h
@@ -1,6 +1,8 @@
 #ifndef MYSTUFF_H
 #define MYSTUFF_H
 
+#include <inttypes.h>
+
 #define NULL ((void *)0)
 
 unsigned int skip_atou(const char **s);
@@ -11,4 +13,12 @@ static inline int isdigit(int ch)
     return (ch >= '0') && (ch <= '9');
 }
 
+struct diskio {
+    uint32_t startsector;
+    uint16_t sectors;
+    uint16_t bufoffs, bufseg;
+} __attribute__ ((packed));
+int int25_read_sector(unsigned char drive, struct diskio *dio);
+int int26_write_sector(unsigned char drive, struct diskio *dio);
+
 #endif /* MYSTUFF_H */
diff --git a/dos/syslinux.c b/dos/syslinux.c
index ca2fd8e1..650c48f6 100644
--- a/dos/syslinux.c
+++ b/dos/syslinux.c
@@ -152,15 +152,8 @@ uint16_t data_segment(void)
     return ds;
 }
 
-struct diskio {
-    uint32_t startsector;
-    uint16_t sectors;
-    uint16_t bufoffs, bufseg;
-} __attribute__ ((packed));
-
 void write_device(int drive, const void *buf, size_t nsecs, unsigned int sector)
 {
-    uint8_t err;
     uint16_t errnum;
     struct diskio dio;
 
@@ -172,22 +165,18 @@ void write_device(int drive, const void *buf, size_t nsecs, unsigned int sector)
     dio.bufseg = data_segment();
 
     /* Try FAT32-aware system call first */
-    asm volatile("int $0x21 ; setc %0"
-		 : "=bcdm" (err), "=a" (errnum)
+    asm volatile("int $0x21 ; jc 1f ; xorw %0,%0\n"
+		 "1:"
+		 : "=a" (errnum)
 		 : "a" (0x7305), "b" (&dio), "c" (-1), "d" (drive),
 		   "S" (1), "m" (dio)
 		 : "memory");
 
-    if (err && errnum == 0x0001) {
-	/* Try legacy system call */
-	asm volatile("int $0x26 ; setc %0 ; popfw"
-		     : "=bcdm" (err), "=a" (errnum)
-		     : "a" (drive-1), "b" (&dio), "c" (-1), "d" (buf),
-		       "m" (dio)
-		     : "esi", "memory");
-    }
+    /* If not supported, try the legacy system call (int2526.S) */
+    if (errnum == 0x0001)
+	errnum = int26_write_sector(drive, &dio);
 
-    if (err) {
+    if (errnum) {
 	dprintf("rv = %04x\n", errnum);
 	die("sector write error");
     }
@@ -195,7 +184,6 @@ void write_device(int drive, const void *buf, size_t nsecs, unsigned int sector)
 
 void read_device(int drive, const void *buf, size_t nsecs, unsigned int sector)
 {
-    uint8_t err;
     uint16_t errnum;
     struct diskio dio;
 
@@ -207,21 +195,17 @@ void read_device(int drive, const void *buf, size_t nsecs, unsigned int sector)
     dio.bufseg = data_segment();
 
     /* Try FAT32-aware system call first */
-    asm volatile("int $0x21 ; setc %0"
-		 : "=bcdm" (err), "=a" (errnum)
+    asm volatile("int $0x21 ; jc 1f ; xorw %0,%0\n"
+		 "1:"
+		 : "=a" (errnum)
 		 : "a" (0x7305), "b" (&dio), "c" (-1), "d" (drive),
 		   "S" (0), "m" (dio));
 
-    if (err && errnum == 0x0001) {
-	/* Try legacy system call */
-	asm volatile("int $0x25 ; setc %0 ; popfw"
-		     : "=bcdm" (err), "=a" (errnum)
-		     : "a" (drive-1), "b" (&dio), "c" (-1), "d" (buf),
-		       "m" (dio)
-		     : "esi");
-    }
+    /* If not supported, try the legacy system call (int2526.S) */
+    if (errnum == 0x0001)
+	errnum = int25_read_sector(drive, &dio);
 
-    if (err) {
+    if (errnum) {
 	dprintf("rv = %04x\n", errnum);
 	die("sector read error");
     }