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Featured Developer
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Dr. Fraser Dickin Ph.D. of Hewlett Packard’s Research Business Unit wins WDC’s Developer of the
‘Moth.′ He found a classic case where using Zero Page Indirect addressing modes, the 65C02 Simulator
addressed memory as if it was continuous in this mode.Since the 65C02 has an 8-bit data bus and a 16-bit address bus, the address is fetched from memory in two cycles, one for the low byte of the address and the other for the high byte. Since the Zero Page addressing mode of the 65C02 microprocessor can only use a single byte, it can only address memory locations 0-255d ($00-$FFh), hence, the name Zero Page.
When fetching the 16-bit address from location $00FF, the low byte of the program counter is
incremented without carry into the high byte. With Zero Page addressing, the program counter will
now contain the value $0000. This is the location where the high byte of the 16-bit address will come
from. However, the 65C02 simulator picked up the high byte from location $0100, therefore, loading an
address that wasn′t expected compared with what the actual hardware would do.When Fraser noticed this he contacted The Western Design Center and asked “Does the simulator exactly emulate the 65C02?” As it turned out, Fraser was correct in that the simulator did not emulate the 65C02 microprocessor correctly. The problem has since been fixed, thanks to the efforts Fraser winning him the title of WDC Developer of the Month of May. We thank him for his efforts and proudly recognize him for his enthusiasm in the 65xx product line. We have enjoyed working with him.
