D-class Operations in LINC
I'm hoping to do a series of articles / threads about how the PDP-12 works, by diving into its instruction set and other low-level behaviors. Here's the first one! The PDP-12 is a combination of the LINC and the PDP-8 -- both 12-bit machines. The practice was to describe memory using four octal (3-bit) values (rather than, say, 3 hexadecimal values). So, 111111111111 in binary is octal 7777 (not 0xFFF ) and 101010101010 in binary is 5252 (not 0xAAA ). The PDP-12 runs in either LINC mode or PDP-8 mode at any given time, but you can arbitrarily switch between modes (and instruction sets) at any time, using ops PDP (0002) in LINC and LINC (6141) in 8-mode. Some hardware only works in LINC mode, so this is quite common. The LINC and the PDP-8 are single-accumulator machines, and so is the PDP-12. I.e., it has one general purpose register -- the AC. Instead of multiple registers, many ops can refer to core (RAM) addresses 1-15 and use them somewhat like modern registers. Don'