20 Mar
2021
20 Mar
'21
5:51 p.m.
On Sat, Mar 20, 2021 at 7:19 PM Kyle Owen <kylevowen at gmail.com> 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 adders...so 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.
Kyle