[Coco] Kip's Single Board Computer

[Dave Philipsen]

I'd have to look it up but I do know you'll get a nice square wave out 
of all of the address lines.

Dave Philipsen


On , RETRO Innovations wrote:
> 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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