Evolution of PDP instruction sets.pic.twitter.com/hsYTfwWoRM
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"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.
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 ....