altering the values returned, the SCSI BIOS can make all the disks on the SCSI bus visible to the system. In this particular case, the BIOS itself may call INT 13h to provide a list of possible boot devices to the user, and because the SCSI BIOS has hooked the interrupt the user will be able to choose not only which standard system devices to boot from, but also which SCSI disks as well. This is because, as suggested in Appendix D of the Boot BIOS Specification, the BIOS could populate the IPL table with device and vendor information from INT 13h calls to the different disks, paired with the Hard Disk Number (80h, 81h ...), to allow any hard disk device to be booted from, rather than just the first disk of the first controller to hook INT 13h (the highest priority item in the BCV table), referred to as a BIOS Aware IPL Device (BAID) in the specification. Multiple controllers can hook INT 13h at once. For instance, after the SCSI controller, an AHCI controller can also hook INT 13h by putting a call to the previous handler, which was stored in the IDT at entry 13h by the SCSI controller, at the end of its own handler, before it puts the address of its own handler into the IDT at entry 13h. The first controller to hook INT 13h will see that 0 disks have been installed by checking the byte at 0040:0075, which resides in the BIOS Data Area (BDA), and if it has 4 disks to enumerate, it will assign the range of disk numbers 80h–83h and store '4' in the BDA. If the second controller to hook INT 13h has 2 disks, it will read '4' from the BDA, assign the disk numbers 84h and 85h, and store '6' in place of the '4'. Now if INT 13h is called with DL = 83h, then the handler of the second controller, which did not assign disk number 83h, will relay the call to the previous handler; that handler, which did assign disk number 83h, will handle the call itself. With any number of controllers' ISRs hooked into INT 13h, the ISRs will each pass control to the next one until the one that assigned the specified drive number recognizes the number, handles the call, and returns from the interrupt. Network boot ROM Another common