ASA Print Control Characters
Most mainframe printers are set to print a fixed width of 132 characters. The files passed to the printer have a record length of 133 characters. The reason for the one character difference is that mainframe printers expect all of the lines to be preceded by a control character.
There are two different types of printer control characters that you can use (machine and asa) but asa control characters are the most commonly used. The asa control characters control the movement of the paper through the printer in order to determine where on the page that each line will print.
The following is a list of the asa control characters that control the spacing of the next line relative to the previous line printed:
blank - next line 0 - skip one line - - skip two lines + - overtype previous line
Each mainframe printer also has twelve channels that can be assigned to a fixed position on the page allowing the printer to skip a variable distance down the page to a fixed location. These channels are referenced using control characters of 1 through 9 and A through C (for channels 10, 11, and 12). When the appropriate control character is specified in column one the page will be moved down to the position specified for that channel. If that position on the current page is already passed then the next page will be loaded and the paper moved down to the appropriate position. Channel 1 is usually assigned to the top of the page so that specifying a 1 in the first column of a print line will ensure that the line prints at the top of the next page. Channels 2 through 12 are not usually assigned.
Early mainframe printers had a continuous paper tape mounted that would rotate once for each page printed. Holes in the tape indicated the page position to be skipped to for each channel. Channel positions could be changed to match different stationery by replacing the channel tape.