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W65C02S Address selection

ElectroRetroNerd

Hello World! My name is Kaleb Papesh and I have decided to build my very own 6502 based computer from the ground up after watching the Apple 1 episodes of TBHS. I know that the 6502 is alive and kicking with Western Design Center currently making the chips however, after looking at the data sheets, I don't see any CS or CE lines for me to use a 3-8 decoder to select ram. Has anyone else used a W65C02S chip in there own 6502 based computers? I don't fully understand the entire addressing architecture for the 6502 so if anyone has in depth information on how it works and how to use a 3-8 decoder to select and use ram, the better! Thank you for any help you can provide!

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  • 0 in reply to D_Hersey

    First of all: small SRAM chips (by todays standards) are still available. Most engineers who build their own 6502 system use a 62256 (32KB) but smaller chips can easily be found, too. Interestingly, 64K SRAM chips (such as the 62512) are surprisingly difficult to find, but 128K chips are easy again.

     

    Second: Regardless of how big of a RAM chip you use in a 6502 system, you will still need address decoding, because every 6502 system needs ROM, RAM and I/O. The original Apple-1 used 8K RAM and 256 bytes (yes bytes) of ROM, but the PIA takes up 4 addresses starting at $D010. Even if you use a 64KB RAM chip, you still need an address decoder to turn the RAM chip off for address areas where the ROM or the PIA (or whatever other chips you want to put into your system) need to be, because only one chip can be active on the data bus.

     

    ===Jac