Editing IBM 3270 (section)

====Buffer addressing====
3270 displays and printers have a buffer containing one byte for every screen position. For example, a 3277 model 2 featured a screen size of 24 rows of 80 columns for a buffer size of 1920 bytes. Bytes are addressed from zero to the screen size minus one, in this example 1919. "There is a fixed relationship between each ... buffer storage location and its position on the display screen."<ref name=CompDesc />{{rp|p.13}} Most orders start operation at the "current" buffer address, and executing an order or writing data will update this address. The buffer address can be set directly using the ''Set Buffer Address (SBA)'' order, often followed by ''Start Field'' or ''Start Field Extended''. For a device with a 1920 character display a twelve bit address is sufficient. Later 3270s with larger screen sizes use fourteen or sixteen bits.

Addresses are encoded within orders in two bytes.{{sfn|3270 Data Stream|loc=Appendix D. 12-, 14-, and 16-Bit Addressing|pp=D-1—D-2}} For twelve bit addresses the high order two bits of each byte are set to form valid EBCDIC (or ASCII) characters. For example, address 0 is coded as X'4040', or space-space, address 1919 is coded as X'5D7F', or '{{not a typo|"}}'.<ref name=CompDesc />{{rp|pp.75–90}} Programmers hand-coding panels usually keep the table of addresses from the 3270 Component Description or the 3270 Reference Card handy. For fourteen and sixteen-bit address, the address uses contiguous bits in two bytes.