Shave an additional ~7 bytes off the MBR

diff --git a/mbr/mbr.S b/mbr/mbr.S
index ea74a86..b512c11 100644 (file)
--- a/mbr/mbr.S
+++ b/mbr/mbr.S
@@ -98,9 +98,8 @@ next:
        mulw    %cx             /* Heads*sectors -> sectors per cylinder */
        pushw   %ax             /* Save sectors/cylinder on the stack */
 
-       xorl    %eax, %eax
-       pushl   %eax            /* Base */
-       pushl   %eax            /* Root */
+       xorl    %eax, %eax      /* Base */
+       xorl    %edx, %edx      /* Root: %edx <- 0 */
        call    scan_partition_table
 
        /* If we get here, we have no OS */
@@ -158,15 +157,19 @@ read_partition_table:
 /*
  * scan_partition_table:
  *     Scan a partition table currently loaded in the partition table
- *     area.  Preserve 16-bit registers.
+ *     area.  Preserve all registers.
  *
- *     On stack:
- *       18(%bp) - root (offset from MBR, or 0 for MBR)
- *       22(%bp) - base (location of this partition table)
+ *      On entry:
+ *       %eax - base (location of this partition table)
+ *       %edx - root (offset from MBR, or 0 for MBR)
+ *
+ *      These get pushed into stack slots:
+ *        28(%bp) - %eax - base
+ *       20(%bp) - %edx - root
  */
 
 scan_partition_table:
-       pusha
+       pushal
        movw    %sp, %bp
 
        /* Search for active partitions */
@@ -175,7 +178,7 @@ scan_partition_table:
        xorw    %ax, %ax
 5:
        testb   $0x80, (%di)
-       jz 6f
+       jz      6f
        incw    %ax
        movw    %di, %si
 6:
@@ -203,21 +206,18 @@ scan_partition_table:
           partition table and resume scan. */
 8:
        movl    8(%di), %eax            /* Partition table offset */
-       movl    18(%bp), %edx           /* "Root" */
+       movl    20(%bp), %edx           /* "Root" */
        addl    %edx, %eax              /* Compute location of new ptab */
        andl    %edx, %edx              /* Is this the MBR? */
        jnz     10f
        movl    %eax, %edx              /* Offset -> root if this was MBR */
 10:
-       pushl   %eax                    /* Push new base */
-       pushl   %edx                    /* Push new root */
        call    read_partition_table
-       jc 11f
+       jc      11f
        call    scan_partition_table
 11:
-       addw    $8, %sp
        /* This returned, so we need to reload the current partition table */
-       movl    22(%bp), %eax
+       movl    28(%bp), %eax
        call    read_partition_table
 
        /* fall through */
@@ -227,7 +227,7 @@ scan_partition_table:
        loopw   7b
 
        /* Nothing found, return */
-       popa
+       popal
        ret
 
 /*