🚀 go-pugleaf

On 2018-03-31, Andreas Kohlbach <a...@spamfence.net> wrote:

> On 31 Mar 2018 17:20:57 GMT, Charlie Gibbs wrote:
>
>> On 2018-03-30, Andreas Kohlbach <a...@spamfence.net> wrote:
>>
>>> On 29 Mar 2018 23:10:26 GMT, Charlie Gibbs wrote:
>>>
>>>> I tried getting my IMSAI going last summer; the processor seemed to
>>>> act OK, but I suspect the boot ROM has decayed.
>>>
>>> Boot rom? Pah! You key in your boot loader on the front panel switches. ;-)
>>
>> I wish I could.  Unfortunately, depositing bytes into memory from the
>> front panel switches required a hardware hack that was only present
>> on the original RAM-4A boards.  I replaced them with a single board
>> that held 64K and ran _much_ cooler.
>
> I watched some videos on Youtube concerning the IMSAI. The author showed
> how to key in some code to jump start the system so he could load
> programs from punched tape or modern devices like an 8" floppy. *g*
> Cannot remember if this needed a spacial board.

The IMSAI took some shortcuts when displaying and alterming memory
through the front panel.  IIRC it tricked the 8080 chip into fetching
a byte as if it were an instruction, then somehow froze things and
displayed the byte on the lights.  Writing to memory required an
even weirder hack which needed special circuitry on the memory board
itself.  This circuitry was present on the original IMSAI RAM-4A
boards, but nobody else duplicated it.  Thus if you upgraded to
other memory boards, you could still read from memory via the
front panel, but you couldn't deposit data into memory.

-- 
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