SoylentNews

An Anonymous Coward writes:

https://www.pagetable.com/?p=406 [pagetable.com]

The MOS 6502 CPU was introduced in September of 1975, and while the documentation described the three shift/rotate instructions ASL, LSR and ROL, the ROR instruction was missing – the documentation said that ROR would be available in chips starting in June 1976. In fact, the reason for this omission was that the instruction, while being present, didn’t behave correctly. Only few 6502s with the defect are in existence, and nobody seemed to have checked what was actually going on in these chips.

Simon C got my KIM-1 working again, which has a 6502 from week 51 of 1975. There are 512 possible inputs to ROR (8 bit A plus 1 bit C; assuming it doesn’t have dependencies on other registers), and roughly two bytes of output: the 8 bit result and the processor status (flags) register. We ran the following programs on the KIM-1 – note that we had to split the task into several programs, because the KIM-1 doesn’t have enough RAM to hold all results.https://www.pagetable.com/?p=406

Original Submission