[Coco] Kip's Single Board Computer

[Dave Philipsen]

Nope. It just keeps goin' and goin'.  When it hits FFFF it goes back to 
0000.


On , camillus wrote:
> Thnx dave ,
> 
> I get it now, the cpu always fetches from fffe and ffff, and in this
> case it will find 1212 as address at where it finds a NOP and
> increases the PC with one, only to find another NOP instruction. It
> does not need to be absolute 0x7E 0xXX 0xXX.
> 
> Does this also mean this is a loop, or will the cpu stop somehow when
> reaches the highest address?
> 
> cb
> 
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> On 9/8/2015 7:38:17 PM, Dave Philipsen <dave at davebiz.com> wrote:
> Just one correction: Tie D1 and D4 high with 4.7k pull ups and tie all
> the other data lines directly to ground to put 0x12 on the data bus (I
> had previously said D2 and D5 but that is incorrect).
> 
> Dave Philipsen
> 
> 
> On 9/8/2015 7:23 PM, Dave Philipsen wrote:
>> If you just pull up D2 and D5 this causes 0x12 to be present on the
>> data bus at all times. Initially, when the CPU tries to fetch the
>> reset vector on startup it will see 0x12 as the high byte and the low
>> byte of the reset vector. It will jump to address 0x1212 and will
>> again read 0x12 on the data bus which is a NOP. Thereafter, all reads
>> of subsequent addresses (0x1213, 0x1214, etc) will come up with 0x12
>> so the CPU will just step through addresses one at a time at whatever
>> rate the core is clocked at. If the CPU is clocked at 1 MHz you'll
>> see a 1 MHz waveform on A0, 500 KHz on A1, 250 KHx on A2, etc.
>> 
>> Dave Philipsen
>> 
>> 
>> On 9/8/2015 7:08 PM, camillus wrote:
>>> I have to think a bit over this, so if there is no defined level on
>>> any of the address pins of the cpu and on the data bus the pattern
>>> 0x1212 is hardcoded, then the cpu will eventually read the dataport?
>>> 
>>> Then how is it suppose to know from where to start fetching code.
>>> Without any address from where some code is to execute from the cpu
>>> is going wild, no?
>>> 
>>> Or am I missing something?
>>> ( maybe a brain...LOL )
>>> cb
>>> 
>>> Sent from Mailbird
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>>> On 9/8/2015 6:46:39 PM, tim lindner wrote:
>>> On Tue, Sep 8, 2015 at 4:29 PM, camillus wrote:
>>>> Sorry I was not clear in my question, I was referring to the reset
>>>> vector. From where will the cpu start if there is no defined level
>>>> at 0xFFFE and 0xFFFF.
>>>> 
>>>> Is there a hard coded address somewhere?
>>> 
>>> If _all_ reads return 0x12, then the reset vector will be 0x1212. At
>>> which point it will read 0x12 (nop).
>>> 
>>> 
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>>> tim lindner
>>> 
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>> 
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