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69
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and taking a "link" penalty, you find a nearby CLA instruction, change it from the [group 1](http://gunkies.org/wiki/PDP-8_architecture#Group_1_.22operate.22_instruction_operations) to the [group 2](http://gunkies.org/wiki/PDP-8_architecture#Group_2_.22operate.22_instruction_operations) form of the `OPR` instruction, give it a label in the assembly code, and jump indirectly through that label:
OS8ENT, 7600 / group 2 form of CLA, also the OS/8 entry point
... / some number of other instructions, but less than a page worth
JMP I OS8ENT / return control to OS/8
The `I` modifies the `JMP` instruction, telling the PDP-8 to load the value at the memory location referred to by the `OS8ENT` label, load up the value it finds there (7600₈) and jump to *that* address.
The `I` modifies the `JMP` instruction, telling the PDP-8 to load the value at the memory location referred to by the `OS8ENT` label (7600₈) into the processor's program counter, causing a jump to that address. Savings: one whole PDP-8 word. Woo!
Barf bags are in the seat pocket ahead of you.
Once you've relieved yourself, please read on, because we're not done yet.
# Level 3: The Instruction and Data Field Registers
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