The Cromemco 4FDC Floppy Disk Controller is designed to interface both 5.25- and 8.0-inch floppy disk drives to the S-100 computer bus used in Cromemco and other IEEE 696 computers. It also contains an RS-232 serial I/O channel with software-selectable baud rates from 110 to 76,800. In addition, it has a 1 KB resident 2708 ROM containing Cromemco's RDOS, the resident disk operating system. The 4FDC was designed to drive Persci 277 8-inch single-density floppy drives. These drives were interesting in two respects: They used a fast voice coil actuator and not a stepper motor to position the drive read write head. The data separator electronics were on the drive itself. Due to the second fact, an unmodified 4FDC can not be used with 8-inch drives that don't have single-density data separators on the drive electronics. Later Cromemco disk controllers such as the 16FDC and 64FDC contained both single and double density data separators and the 64FDC also supplied write pre-compensation. An aftermarket add-on board, the FDCX4 Double Density Upgrade Board for the 4FDC, was designed and marketed by JVB Electronics. The FDCX4 was a daughter board assembly that replaced the WD1771 single density disk controller chip on the 4FDC with a FD1791 (early production) or Fujitsu MB8876A (later production) double-density controller chip. The FDCX4, in addition to using an analog phase-locked-loop data separator in all modes, also used write-precompensation. These features allowed the FDCX4 equipped 4FDC to reliably use the Persci 277 drives, as well as other drives, in double-density mode. Technical notes Four switches on the 4FDC interface card are used to set the operation of the card. Switch 1 is the RDOS DISABLE switch. When this switch is ON the 1 KB ROM containing RDOS cannot be accessed by the computer. When this switch is OFF the RDOS program resides in the computer memory space from address 0xC000 to 0xC3FF. Switch 2 is the RDOS disable after boot switch. If this switch is ON the 1 KB ROM containing RDOS will automatically be disabled after CDOS is bootstrapped in from a disk thus clearing memory space from 0xC000 to 0xC3FF for system use. (In this mode the ROM is actually disabled by an output to port 40H which is done automatically by CDOS). If switch 2 is OFF, RDOS remains in memory space even after CDOS is loaded. RDOS contains two programs; 1) the CDOS bootstrap program