TC08 DECtape bootloader question

Kyle Owen kylevowen at
Sat Mar 20 19:19:05 CDT 2021

My understanding of the OS/8 TC08 bootloader (MI8-EC) is as follows:
0. Rewind tape
1. Set current address (07755) to 07600
2. Set word count (07754) to -0200
3. Read block 0 and wait for flag
4. Continue executing at 07600

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.

But according to SimH, a write to the current address, 07755, never
happens. How can this be?

Any help would be appreciated!


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