[GloriousCow] has started working on a series of investigations into the various historical floppy disk copy protection schemes used in the early days of the IBM PC and is here with the first of these results, specifically Formaster’s Copy-Lock.
This is the starting sector of track 6. It looks empty, but it’s not quite.
The game in question is King’s Quest by Sierra Entertainment, which used a ‘booter disk’ with the Copy-Lock protection scheme. Instead of having to boot DOS separately, you could just insert this disk and the game would launch automatically. Early copy protections often used simple methods, like adding sectors with non-standard sizes or tampering with sector CRC values to create disk errors. Copy-Lock employed several such tricks together, making it challenging for standard floppy disk hardware to replicate. In the case of Copy-Lock, Sector 1 on track 6 was intentionally written as only 256 bytes, with a 256-byte blank section to fill the gap. Additionally, the CRC was also altered to add another layer of protection.
When attempting to read the disk, the PC BIOS interrupt routine assumes it’s looking for a standard 512-byte sector, so when a “read sector” command is issued to locate the sector, it never finds it. To detect a dodgy copy, the game bypasses the BIOS and talks directly to the floppy disk controller using some custom code. The first part of the code uses the standard INT 13h routine to seek to track 6, sector 1, where it expects a fail since there is no valid sector there. Next, the floppy controller sends the “read track” command to perform a raw dump of all 512 bytes at thi ..
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