The forms control buffer (FCB) module is of variable length and contains vertical line spacing information (6, 8, or 12 lines per inch for the 3800 Model 1; 6 or 8 lines per inch for the 4248; and 6, 8, 10, or 12 lines per inch for the 3800 Model 3). The FCB module can also identify one of 12 carriage-control channel codes for each line. For the 4248 printer, the module also contains information on the horizontal copy feature and the printer speed.

The FCB module is created and stored in an image library, using the FCB and NAME utility control statements. For the 4248 FCB module, the INCLUDE and OPTION statements can also be coded to indicate that an existing FCB module (prefix FCB2 or FCB4) is to be used as a model.

For the 3800, IBM supplies one default FCB image in SYS1.IMAGELIB, called FCB3STD1. For the 4248, although the last FCB image loaded is reloaded by the printer when the power is turned on, IBM supplies two FCB images that may also be used by printers other than the 4248. For the 3262 Model 5, a default FCB image is also supplied.