markers while the WD1791 only supports two, and some versions of TRSDOS for the Model I also use them. In addition, they are used by copy protection schemes. Software was available to allow Model I disks to be read on a Model III. The WD1791 supports the 500 bit/s bitrate needed for high-density floppy drives, but the controller is not capable of using them without extensive modifications. TRSDOS for the Model III was developed in-house by Radio Shack rather than being contracted out like the Model I's DOS. None of the code base from Model I DOS was reused and the Model III DOS was rewritten from scratch; this also created some compatibility issues since the Model III DOS's API was not entirely identical to the Model I DOS. This was primarily to avoid legal disputes with Randy Cook over ownership of the code as had occurred with Model I DOS and also because Radio Shack originally planned several features for the Model III such as 80 column text support that were not included. Two early versions, 1.1 and 1.2, were replaced by version 1.3 in 1981 which became the standard Model III OS. TRSDOS 1.3 is not format compatible with 1.1 and 1.2; a utility called XFERSYS is provided which converted older format disks to TRSDOS 1.3 format (this change is permanent and the resultant disks cannot be read with the older DOS versions). The Model III's boot screen was cleaned up from the Model I. Instead of displaying garbage on screen at power up, it displays a "Diskette?" prompt if a bootable floppy is not detected. The user can insert a disk and press any key to boot. On powerup or reset holding down the key will boot the computer into ROM-based Level II BASIC. This ability is useful if the disk drive is not functioning and cannot boot a TRSDOS disk (or if a boot disk is not available); it permits an operator familiar with the machine hardware to perform diagnostics using BASIC's PEEK and POKE commands. This works for the Model 4 as well, but not for the 4P. While Model I DOS is fairly flexible in its capabilities, Model III DOS is hard coded to only support 180K single sided floppies, a problem fixed by the many third party DOSes. To that end, when Radio Shack introduced hard disks for the TRS-80 line in 1982,