4 ms·

Thanks! I bought the book earlier today and am looking forward to it. One of the things I'm interested in is how to construct gate-level emulation in software

Thanks! I bought the book earlier today and am looking forward to it. One of the things I'm interested in is how to construct gate-level emulation in software. I saw a video about 6502 hardware emulation where the guy said his was half-cycle complete. Looking forward to understanding all of that.

clevernickname 10y ago
> I saw a video about 6502 hardware emulation where the guy said his was half-cycle complete. Looking forward to understanding all of that. To give you a little head start on that, where many digital circuits are clocked (or make one step) on just the rising edge of a clock cycle (where it transitions from low-to-high), the 6502 clocks on both the rising and falling edges; certain parts (like the hardware that initiates a memory access) will fire on the rising edge, then the part that needed that information will fire on the rising edge half a cycle later (like making the ALU start adding the value that was just fetched from memory to the accumulator)[1]. If you've ever heard someone say that a 6502 of one speed is roughly equivalent to a Z80 clocked at twice the speed, now you know why: the internal logic is essentially clocked twice as fast as the input clock. [1] IIRC this isn't completely correct, and external memory is allowed to take up to 3/4ths of a cycle to perform a memory access to accommodate slower ROMs, but you get the idea.
- Narishma 10y ago
  Is the 6502 unique in working that way? If so, why didn't other processors use the same technique?
- vardump 10y ago
  > If you've ever heard someone say that a 6502 of one speed is roughly equivalent to a Z80 clocked at twice the speed, now you know why: the internal logic is essentially clocked twice as fast as the input clock. 1 MHz 6502 is about as fast as 3 MHz Z80. Although your mileage my vary, some say up to 4 MHz Z80.

clevernickname 10y ago

> I saw a video about 6502 hardware emulation where the guy said his was half-cycle complete. Looking forward to understanding all of that. To give you a little head start on that, where many digital circuits are clocked (or make one step) on just the rising edge of a clock cycle (where it transitions from low-to-high), the 6502 clocks on both the rising and falling edges; certain parts (like the hardware that initiates a memory access) will fire on the rising edge, then the part that needed that information will fire on the rising edge half a cycle later (like making the ALU start adding the value that was just fetched from memory to the accumulator)[1]. If you've ever heard someone say that a 6502 of one speed is roughly equivalent to a Z80 clocked at twice the speed, now you know why: the internal logic is essentially clocked twice as fast as the input clock. [1] IIRC this isn't completely correct, and external memory is allowed to take up to 3/4ths of a cycle to perform a memory access to accommodate slower ROMs, but you get the idea.

Narishma 10y ago
Is the 6502 unique in working that way? If so, why didn't other processors use the same technique?
vardump 10y ago
> If you've ever heard someone say that a 6502 of one speed is roughly equivalent to a Z80 clocked at twice the speed, now you know why: the internal logic is essentially clocked twice as fast as the input clock. 1 MHz 6502 is about as fast as 3 MHz Z80. Although your mileage my vary, some say up to 4 MHz Z80.