TC08 DECtape bootloader question

Kyle Owen kylevowen at
Sat Mar 20 19:51:42 CDT 2021

On Sat, Mar 20, 2021 at 7:19 PM Kyle Owen <kylevowen at> wrote:

> However, it appears as though word count will be hit by the loading of the
> first block. In fact, my instrumented version of SimH says it's overwritten
> with a zero. If that's the case, it would seem as though the word count
> overflow flag will never get set. Not to mention, the current address will
> be updated next, causing data to be redirected to yet another position.

To continue this thought, it appears as though SimH does the following for
a read data break:
0. Increment WC
1. Increment CA
2. Compute memory address from the extended memory bits from the TC08 and CA
3. Store the data from the tape at the memory address specified from 2.
4. Check if WC equals zero, and handle that accordingly

>From what I can tell, both the PDP-8/E and the PDP-8/I set the WC overflow
based on the carry out from the if WC happens to be overwritten
with a zero, a carry out will never happen in the real hardware. In SimH,
the entire WC is tested and compared to zero. This behavior seems different.


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