Another 2 bytes off the MBR.

diff --git a/mbr/mbr.S b/mbr/mbr.S
index 854e39b..654393f 100644 (file)
--- a/mbr/mbr.S
+++ b/mbr/mbr.S
@@ -107,9 +107,10 @@ next:
 
 /*
  * read_sector: read a single sector pointed to by %eax to 0x7c00.
- * CF is set on error.  All registers clobbered.
+ * CF is set on error.  All registers saved.
  */
 read_sector:
+       pushal
 read_sector_cbios:
        xorl    %edx, %edx
        divl    (secpercyl)
@@ -132,18 +133,18 @@ read_sector_ebios:
 read_common:
        movb    (driveno), %dl
        int     $0x13
+       popal
        ret
 
 /*
  * read_partition_table:
  *     Read a partition table (pointed to by %eax), and copy
  *     the partition table into the ptab buffer.
- *     Preserve registers.
+ *     Clobbers %si, %di, and %cx, other registers preserved.
  */
 ptab   = _start+446
 
 read_partition_table:
-       pushal
        call    read_sector
        jc      20f
        movw    $bootsec+446, %si
@@ -151,7 +152,6 @@ read_partition_table:
        movw    $(16*4/2), %cx
        rep ; movsw
 20:
-       popal
        ret
 
 /*
@@ -173,18 +173,18 @@ scan_partition_table:
        movw    %sp, %bp
 
        /* Search for active partitions */
-       movw    $ptab, %di
+       movw    $ptab, %bx
        movw    $4, %cx
        xorw    %ax, %ax
-       push    %di
+       push    %bx
        push    %cx
 5:
-       testb   $0x80, (%di)
+       testb   $0x80, (%bx)
        jz      6f
        incw    %ax
-       movw    %di, %si
+       movw    %bx, %si
 6:
-       addw    $16, %di
+       addw    $16, %bx
        loopw   5b
 
        decw    %ax             /* Number of active partitions found */
@@ -192,10 +192,10 @@ scan_partition_table:
        jns     too_many_active
 
        /* No active partitions found, look for extended partitions */
-       popw    %di             /* %di <- ptab */
+       popw    %bx             /* %bx <- ptab */
        popw    %cx             /* %cx <- 4    */
 7:
-       movb    4(%di), %al
+       movb    4(%bx), %al
        cmpb    $0x0f, %al      /* 0x0f = Win9x extended */
        je      8f
        andb    $~0x80, %al     /* 0x85 = Linux extended */
@@ -206,7 +206,7 @@ scan_partition_table:
           try to scan it.  If the scan returns, re-load the current
           partition table and resume scan. */
 8:
-       movl    8(%di), %eax            /* Partition table offset */
+       movl    8(%bx), %eax            /* Partition table offset */
        movl    20(%bp), %edx           /* "Root" */
        addl    %edx, %eax              /* Compute location of new ptab */
        andl    %edx, %edx              /* Is this the MBR? */
@@ -224,7 +224,7 @@ scan_partition_table:
        /* fall through */
 9:
        /* Not an extended partition */
-       addw    $16, %di
+       addw    $16, %bx
        loopw   7b
 
        /* Nothing found, return */
@@ -239,9 +239,7 @@ boot:
        movl    8(%si), %eax
        addl    28(%bp), %eax
        movl    %eax, 8(%si)    /* Adjust in-memory partition table entry */
-       pushw   %si
        call    read_sector
-       popw    %si             /* ds:si -> partition table entry */
        jc      disk_error
        cmpw    $0xaa55, (bootsec+510)
        jne     missing_os              /* Not a valid boot sector */