[Simh] i1401 Bug: ZA and ZS Source Operand Space Character Handling

[Bob Abeles]

On 6/25/08, Paul Pierce posted 5 newly converted 1401 tape images  
from the "Cincinnati 1401 donation to the Computer History Museum".  
One of the tapes is a 1401 Autocoder V3 distribution tape.

The tape can be found here: http://www.piercefuller.com/library/ 
cincinnati-h.html

While attempting to build a system tape from the image, I ran into  
difficulties where a spurious "sequence error" message occurs. Here  
is the text of the message as it appears on the printer:

^^^^^^^^^;^^^^^^^^^;^^^^^^^^^;^^^^^^^^^L030527,508518,040040,04004010400 
36037233    SEQUENCE^ERROR

The root cause is an error in the implementation of the ZA and ZS  
instructions in "IBM 1401 simulator V3.8-0" and likely earlier  
versions. Specifically, the implementation translates each source  
character from binary to BCD, replacing blank characters with zeros.   
"A24-1403-5 1401 Reference Manual" and "R25-1495 1401 Instruction  
Logic" indicate that the qui-binary adder and its associated  
translation circuits are not used by these instructions and that zone  
bits are simply stripped from source operand characters. Correcting  
the implementation caused system tape generation to function properly.

Here is the source code patch:

bobspb:~/Historical/IBM_1401/simh-build/i1401 rabeles$ diff -c  
i1401_cpu_orig.c i1401_cpu.c
*** i1401_cpu_orig.c    Sat Jul  7 14:39:38 2007
--- i1401_cpu.c Fri Jun 27 15:58:47 2008
***************
*** 23,28 ****
--- 23,29 ----
      used in advertising or otherwise to promote the sale, use or  
other dealings
      in this Software without prior written authorization from  
Robert M Supnik.

+    27-Jun-08    REA     Fixed ZA/ZS bug, should not convert source  
to BCD
      07-Jul-07    RMS     Removed restriction on load-mode binary tape
      28-Jun-07    RMS     Added support for SS overlap modifiers
      22-May-06    RMS     Fixed format error in CPU history (found  
by Peter Schorn)
***************
*** 868,874 ****
               if (a & WM) wm = M[BS] = (M[BS] & WM) | BCD_ZERO;
               else {
                   a = M[AS];                              /* get A  
char */
!                 t = (a & CHAR)? bin_to_bcd[a & DIGIT]: 0;
                   wm = M[BS] = (M[BS] & WM) | t;          /* move  
digit */
                   MM (AS);
                   }
--- 869,875 ----
               if (a & WM) wm = M[BS] = (M[BS] & WM) | BCD_ZERO;
               else {
                   a = M[AS];                              /* get A  
char */
!                 t = a & DIGIT;                          /* isolate  
digit bits */
                   wm = M[BS] = (M[BS] & WM) | t;          /* move  
digit */
                   MM (AS);
                   }

Bob Abeles