Evolution of PDP instruction sets.pic.twitter.com/hsYTfwWoRM
"jms" was such a weird instruction, showing that this arch predates most modern compiler languages and memory models: "jms Y" copies PC+1 to address Y and continues execution at address Y+1.
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Also, "tad" is twos-complements add because, you know, some CPUs implemented integers differently, like sign-magnitude, or ones complement, or excess-K, or BCD with external sign, or ....
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From before the time before recurive functions were thought to be needed. The HP 2100 series also has the same thing with its JSB instruction where the return address is stored before the first word of the subroutine.
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When I wrote a Forth compiler for the PDP-8, I cheated. I used JMS for calls, thinking it was a temporary solution I would fix later. Somewhat to my surprised everything worked fine. Forth doesn't need reentrancy much.
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