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PostPosted: Thu Jun 18, 2020 10:56 am 
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Joined: Thu Jun 18, 2020 10:34 am
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I heard about that many years ago while working on device drivers for a multi tasking 68K box. It wasn't until I created a 6800 in logisim that the idea made perfect sense. Since most of the instructions, or opcodes, that cause a register, or memory, to loaded go through the ALU. The CLR instruction is implemented as an exclusive OR so that CLRA performs A XOR A and the result is 0 which is what the CLR instruction result is. This was important to know when dealing with hardware. Most of the Motorola I/O chips had a single address to let you read one side and write to the other side. Since CLR was a Read/Write Op it could write unintended data to the I/O chip.

Another way to say all of this is in the MC6800 from $40 to $FF almost everything went through the ALU on the way to a register and CLR was one of them.


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