Re: 6502 trashing memory cycles...

["bieling@terra.es" <bieling@terra.es>]

On Mar 4, 9:11 pm, "Bryan Parkoff" <nos...@nospam.com> wrote:
>     Why do you think this way?  Let me give you the information below.  I
> don't think 3 out of the 6 cycles of an RTS are useless.
>
> Cycle  Address Bus  Data Bus      External Operation  Internal Operation
> 1       0300           Opcode          Fetch Opcode           Finish
> Previous Operation, 0301 + PC
>
> 2       0301           Discarded Data  Fetch Discarded Data   Decode RTS
>
> 3       01FD           Discarded Data  Fetch Discarded Data   Increment
> Stack Pointer to 01FE
>
> 4       01FE           02              Fetch PCL              Increment
> Stack Pointer to 01FF
>
> 5       01FF           01              Fetch PCH
>
> 6       0102           Discarded Data  Put out PC             Increment PC
> by 1 to 0103
>
> 7       0103           Next Opcode     Fetch Next Opcode
>

Yes, memory cycles 2,3 and 6 are useless, wasted.
An external circuit that knew that could put them to better use : to
generate sound, or to read from the video frame buffer, or for DMA,
or... ?

Jorge.