1 files changed, 76 insertions, 0 deletions
diff --git a/bootblocks/sysmbr.s b/bootblocks/sysmbr.s
new file mode 100644
index 0000000..eb609de
--- /dev/null
+++ b/bootblocks/sysmbr.s
@@ -0,0 +1,76 @@
+
+! The master boot sector will have setup a stack,
+! this is normally at 0:7c00 down.
+! DS, SS, CS and ES will all have value 0 so the execution address is 0:7c00
+! On entry the register SI will be pointer to the partition entry that
+! this sector was loaded from, DL is the drive.
+
+! Also if it's a standard Master boot DH will be the head, CX will be the
+! sector and cylinder, BX=7C00, AX=1, DI=7DFE, BP=SI. There's a reasonable
+! chance that this isn't true though.
+
+! The Master boot itself will have been loaded and run at $07c00
+! The BIOS must have setup a stack because interrupts are enabled
+! Little else can be assumed because DOS doesn`t assume anything either
+
+! Lowest available is $0500, MSDOS appears to use $0600 ... I wonder why?
+ORGADDR=$0500
+
+org ORGADDR
+
+sysboot_start:
+  cli                   ! Assume _nothing_!
+  cld
+  mov   bx,#$7C00       ! Pointer to start of BB.
+  xor   ax,ax           ! Segs all to zero
+  mov   ds,ax
+  mov   es,ax
+  mov   ss,ax
+  mov   sp,bx           ! SP Just below BB
+  mov   cx,#$100        ! Move 256 words
+  mov   si,bx           ! From default BB
+  mov   di,#ORGADDR     ! To the correct address.
+  rep
+   movsw
+  jmpi  cont,#0         ! Set CS:IP correct.
+cont:
+  sti                   ! Let the interrupts back in.
+
+! This is where the code will be overlaid, the default is a hang
+public codestart
+codestart:
+  j	codestart
+
+! Partition table
+public bootblock_magic
+
+.blkb sysboot_start+0x1B6-*
+table_start:
+.blkb 2			! Dirty bits
+.blkb 4			! Volume Serial Number
+.blkb 2                 ! Possible Magic number
+
+.blkb sysboot_start+0x1BE-*
+partition_1:
+.blkb 8			! IN,SH,SS,ST,OS,EH,ES,ET
+.blkw 2			! Linear position (0 based)
+.blkw 2			! Linear length
+.blkb sysboot_start+0x1CE-*
+partition_2:
+.blkb 8			! IN,SH,SS,ST,OS,EH,ES,ET
+.blkw 2			! Linear position (0 based)
+.blkw 2			! Linear length
+.blkb sysboot_start+0x1DE-*
+partition_3:
+.blkb 8			! IN,SH,SS,ST,OS,EH,ES,ET
+.blkw 2			! Linear position (0 based)
+.blkw 2			! Linear length
+.blkb sysboot_start+0x1EE-*
+partition_4:
+.blkb 8			! IN,SH,SS,ST,OS,EH,ES,ET
+.blkw 2			! Linear position (0 based)
+.blkw 2			! Linear length
+
+.blkb sysboot_start+0x1FE-*
+bootblock_magic:
+.blkb 2