Editing TRS-80 Color Computer (section)

===Disk interface===
The CoCo uses an external floppy disk controller that plugsd into the side expansion port. The controller uses a WD 1791 FDC and a ROM-based disk operating system which is immediately available at power-on and does not require an OS disk (the CoCo 2-3 controller also supports booting alternative RAM-resident DOSes in place of the ROM one by typing the DOS command in BASIC). The CoCo 1 uses Shugart SA-400 full-height 5.25" drives which are 35 track single-sided units and formatted disks to 160k. They are similar to the TRS-80 Model I disk drives except for the custom modification of a power switch in the front faceplate. The CoCo 2-3 replaced these with 40 track half-height drives, two per unit (the existing 35 track 160k format was retained for compatibility purposes with the CoCo 1). The stock drives can be replaced by any Shugart-standard 34 pin interface units for additional storage space although the ROM DOS only supports default 160k format. The CoCo 1 controller has an older version of the WD 1791 FDC chip manufactured in enhancement load [[NMOS logic|NMOS]] that needsa three voltages to operate: 12V, 5V, and -5V. The CoCo 2 switched to a newer FDC that used a single 5V power line, and the 12V line was removed from the expansion port. Consequently the CoCo 1 controller will not work on the CoCo 2-3 nor will the latter's controller work on the CoCo 1.

The ROM DOS functios as part of BASIC and all disk commands are implemented as BASIC statements. BASIC programs can be loaded from disk by the LOAD command and LOADM for machine language programs.