The 62X1 instruction (CDF, Change Data Field) set the data field to X. Similarly 62X2 (CIF) set the instruction field, and 62X3 set both. Pre-existing programs would never execute CIF or CDF; the DF and IF registers would both point to the same field, a single field to which these programs were limited. The effect of the CIF instruction was deferred to coincide with the next JMP or JMS instruction, so that executing CIF would not cause a jump. It was more complicated for multiple-field programs to deal with field boundaries and the DF and IF registers than it would have been if they could simply generate 15-bit addresses, but the design provided backward compatibility and is consistent with the 12-bit architecture used throughout the PDP-8. Compare the later Intel 8086, whose 16-bit memory addresses are expanded to 20 bits by combining them with the contents of a specified or implied segment register. The extended memory scheme let existing programs handle increased memory with minimal changes. For example, 4K FOCAL normally had about 3K of code with only 1K left over for user program and data. With a few patches, FOCAL could use a second 4K field for user program and data. Moreover, additional 4K fields could be allocated to separate users, turning 4K FOCAL into a multi-user timesharing system. Virtualization On the PDP-8/E and later models, the Memory Extension Controller was enhanced to enable machine virtualization. A program written to use a PDP-8's entire resources can coexist with other such programs on the same PDP-8 under the control of a virtual machine manager. The manager can make all I/O instructions (including those that operated on the Memory Extension Controller) cause a trap (an interrupt handled by the manager). In this way, the manager can map memory references, map data or instruction fields, and redirect I/O to different devices. Each original program has complete access to a "virtual machine" provided by the manager. New I/O instructions to the Memory Extension Controller retrieve the current value of the data and instruction fields, letting software save and restore most of the machine state across a trap. However, a program can not sense whether the CPU is in the process of deferring the effect of a CIF instruction (whether it has executed a CIF and not yet executed the matching jump instruction). The manager has to include a complete PDP-8 emulator (not difficult