by using a line feed character. For ease of external editing of the source file, later versions of BASIC-PLUS also allowed the & character as a line-continuation character. Multiple statements could be placed on a single line using as the statement separator. For PDP-11 systems with virtual memory (RSTS/E), address space was limited to about 64 KB. With BASIC-PLUS, about half of this was used by the combined command interpreter and run-time library. This limited user programs to about 32 KB of memory. Older RSTS-11 systems lacked virtual memory, so the user program had to fit into whatever was left of physical memory after RSTS and BASIC-PLUS took up their share. For example, on a PDP-11/35 with 32K of physical memory, running RSTS-11 V04B-17, user programs were limited to 7 KB. Large programs could be broken up into various pieces by use of the "CHAIN" instruction. Programs could chain to specific line numbers in a secondary program. The use of a shared memory section called core common also allowed programs to pass data among each other as needed; disk files could also be used but were slower. The interpreter included a garbage collecting memory manager, used for both string data and byte-code. A running program could be interrupted, have variables examined and modified, and then be resumed. Many of the control structures used in other high-level languages existed in BASIC-PLUS, including WHILE and UNTIL. The language also supported the use of conditional modifiers on a single line. For example, the line "" would print the value of "I" unless I was less than 10. BASIC Plus 2 Basic Plus 2 (BP2 or BASIC-Plus-2) was later developed by DEC to add additional features and increase performance. It used true compilation into threaded code, and wrote its output to machine language object files. These were compatible with other object files on the system, and could be assembled into libraries. A linker (the TKB taskbuilder) then created executable files from them. TKB also supported overlays; this allowed individual routines to be swapped into the main memory space as needed. BP2 programs ran under RSX-11 or RSTS/E's RSX Run Time System. This RTS only occupied 8KB (later, 2KB) of the user's address space, leaving 56KB for the user's program. These two factors allowed individual BP2 programs to be much larger than BASIC-PLUS programs, often eliminating the need for CHAINing. Unlike BASIC-PLUS (which was only