[Coco] Kip's Single Board Computer

[RETRO Innovations]

On 9/8/2015 8:47 PM, Darren A wrote:
> On Tue, Sep 8, 2015 at 6:34 PM, Dave Philipsen wrote:
>
>> Since NOP is a single byte opcode and a single cycle instruction the CPU
>> will just move on to fetch the next instruction at an address one greater
>> than the previous.  Again, it will find 0x12 there and thus execute a NOP.
>> Since the CPU is stepping through addresses one at a time because of the
>> fact that the NOP is a single byte instruction you will see very regular
>> patterns when you view the address lines on a 'scope.
>>
>
> This is correct except that NOP is actually a 2-cycle instruction.  The 2nd
> cycle of each NOP will read the following location.  The sequence of
> addresses presented will be $1212 and $1213 for the first NOP, then $1213
> and $1214 for the second NOP, etc..  So, following the first NOP, the
> address bus will increment on every other CPU cycle.
Would it really increment the address bus counter like that on the 
external pins?  6502 is a 2 cycle NOP, but the address lines do not do 
that, they just (as you note) increment at clock/2.

Jim

>
> - Darren
>


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