[Simh] Non-numeric character in IBM 1620 P & Q addresses

[Paul Koning]

> On Dec 25, 2015, at 2:21 AM, Bob Supnik <bob at supnik.org> wrote:
> 
> It's worse than that. The detailed logic is shown on 227-5631-0 pp 188ff. The decode chart for each decode switch (DSW) is:
> 
> DSW 0        0000
> DSW 1        0001
> DSW 2        0010
> DSW 3        X011
> DSW 4        X100
> DSW 5        X101
> DSW 6        X110
> DSW 7        X111
> DSW 8        1XX0
> DSW 9        1XX1
> 
> It's quite bizarre, actually. The decode switches are 4-input AND gates (sort of), so every decode could have been complete. Instead, the X inputs are not wired to anything.
> 
> The result is that for invalid digits:
> 
> 1010        nothing active, because DSW2 decode is complete

You're right that I didn't look closely enough.  As Michael Short points out, 1010 would decode as 8.  So for the original question, which is "what happens with a record mark (1010) in the address" the answer would appear to be that it is taken as an 8.

And yes, that does assume that some separate logic isn't triggering MAR Error on that case; I haven't looked.

	paul