MBR: shave another ~4 bytes through an ugly error message hack

Use the "return address points to data" trick to shave a few bytes off
the generation of error messages.

diff --git a/mbr/mbr.S b/mbr/mbr.S
index 06e1980..424c46d 100644 (file)
--- a/mbr/mbr.S
+++ b/mbr/mbr.S
@@ -105,7 +105,10 @@ next:
        call    scan_partition_table
 
        /* If we get here, we have no OS */
-       jmp     missing_os
+missing_os:
+       call    error
+       .ascii  "Missing operating system.\r\n"
+       .byte   0
 
 /*
  * read_sector: read a single sector pointed to by %eax to 0x7c00.
@@ -241,6 +244,11 @@ scan_partition_table:
        popal
        ret
 
+too_many_active:
+       call    error
+       .ascii  "Multiple active partitions.\r\n"
+       .byte   0
+
 /*
  * boot: invoke the actual bootstrap. (%si) points to the partition
  *      table entry, and 28(%bp) has the partition table base.
@@ -260,20 +268,17 @@ boot:
        cli
        jmp     bootsec
 
-/*
- * error messages
- */
-missing_os:
-       movw    $missing_os_msg, %si
-       jmp     error
 disk_error:
-       movw    $disk_error_msg, %si
-       jmp     error
-too_many_active:
-       movw    $too_many_active_msg, %si
-       /* jmp error */
+       call    error
+       .ascii  "Operating system load error.\r\n"
+       .byte   0
 
+/*
+ * Print error messages.  This is invoked with "call", with the
+ * error message at the return address.
+ */
 error:
+       popw    %si
 2:
        lodsb
        andb    %al, %al
@@ -288,13 +293,3 @@ error:
 die:
        hlt
        jmp     die
-
-missing_os_msg:
-       .ascii  "Missing operating system.\r\n"
-       .byte   0
-disk_error_msg:
-       .ascii  "Operating system load error.\r\n"
-       .byte   0
-too_many_active_msg:
-       .ascii  "Multiple active partitions.\r\n"
-       .byte   0