The use of half-cycles ensures that memory and I/O devices have predictable timing windows when the CPU will access the bus, while still allowing the CPU to perform internal operations in parallel.
Unlike most microprocessors, the 6502 does not make memory accesses on an "as needed" basis. It always does a fetch or store on every single clock cycle. When there isn't anything to be fetched or stored, a "garbage" fetch or store occurs.
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