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This Sector is where the "magic" happens. It defines the very first Sector on the CD as the default Boot Sector, and for the BIOS to treat the CD as a Floppy Disk.

It is only because ISO 9660 treats the first 16 (2 kiB) Sectors as "reserved" that this trick works: since the BIOS looks for the Primary and Boot Volumes from the 17th Sector, we can put whatever we like before it:

  • This Demonstrator;
  • A complete OS;
  • A loader for the rest of the CD, perhaps with an ISO driver;
  • Anything! Over to you...


; Pad/ISO/
; This file defines the Boot Catalog Sector for the ISO image. This basically
; points to the first sector, which is where the standard boot sector is anyway.
; Note that ISO.Boot.Default.Media has been set to Floppy.1440. This option
; works for VMware, VirtualPC and VirtualBox. Other options work for VMware only
%define         ISO.Catalog.ID          Version.Author
ISO.Sector.Catalog   EQU                ISO.Sectors
%assign              ISO.Sectors        ISO.Sectors + 1
; Add Validation entry, and calculate Checksum for us
ISO.Boot.Validation ISO.Validation      ISO.Catalog.ID, ISO.Boot.Platform.x86
.Bootable       DB              88h
.Media          DB              ISO.Boot.Media.Floppy.1440 ; HardDisk ; NoEmulation
.Load           DW              0000h   ; This could be 7C00h. 0000h defaults it
.System         DB              00h
                DB              0
.Count          DW              1       ; You can use this to load MORE!
.LBA            DD              0       ; Start sector to load
.Reserved       TIMES           (1Fh-0Ch+1) DB 00h
ISO.Boot.Catalog.Padding EQU    ISO.Sector.Size - ($ - ISO.Boot.Catalog)
                TIMES           ISO.Boot.Catalog.Padding DB 00h
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