Starting with how the Retail IPL determines that the system and disk are valid.
1. Check that the expansion PAK RAM is present.
2. Try to Read the Real Time Clock on the Disk Drive Controller. Test that the Controller is functioning, no diagnostic failures and no communication failures.
3. Read the Disk ID in Sector Mode (Disk LBA 14, Alternate 15)
4. Try to Read LBA 12 in Sector Mode with no retries. This will need to result in an unrecoverable read error for a valid retail disk.
4. Read the Disk System Data in Sector Mode with no retries (LBA 0, Alternates 1,8,9) For System Data to be valid the first four bytes must be 0xE848D316. The Disk Type (byte 5, bits 7-4) must be 1, 2 or 3.
5. Load the Disk Program starting at LBA 24 in Block/Track mode. The Block Count is given by System Data bytes 6/7. The Destination is given by System Data word 0x1C.
6. Check the Controller ID Register. It must be equal to 3.
7. Check the IPL ROM byte at offset 0x9FF00. It must equal 0xC3.
8. If the Disk ID sector byte 7, bit 7 is set the disk program is encrypted. The loader will decrypt this in two stages.
9. Reset the system state and jump to the Disk Program.
To read Development Disks the IPL ROM would need in step 4 to have no error reading LBA 12, in step 5 the first four bytes of system data must be 0x00000000 and be read from LBA 2 (Alternates 3,10,11) and in step 6 the controller ID register would return the value 4.