HP-41 instruction print bug found

[hth]

At least with the HP82162A printer an XROM instruction with postfix argument is not printed properly when executed from the keyboard.

Say you have an XROM FOO instruction that has its header marked as taking a numeric postfix argument. In this case the instruction code is the 2-byte XROM code and the argument is passed on the side in register B.X. The PRT5 routine (in the HP-IL module) is supposed to print the executed instruction. However the actual printing of the argument picks the postfix operand from the second byte of the 2-byte XROM code, not the one that was passed in B.X.

If I invoke 'FOO 07' (where FOO is an XROM) and this instruction code is A3CC, it will print as "FOO IND 76", not "FOO 07". CC is 80 (IND), taking that away we have 4C which is 76 decimal.

Is it possible to alter the HP-IL module image on the 41CL somehow?

I imagine that the power on sequence could see if there is an HP-IL module inserted and then use the MMU to map an alternative (fixed) ROM image over it?

The offending code is:

Code:

   953 1454 PFK35   630 C=M                  PUT ARGUMENT
   954 1455         274 RCR    5              IN C[1:0]
   955 1456           1 GOLONG PRW930        PRINT NUMERICAL OPERAND
   955 1457           2                      *ILPRINTER:  PL2, @0156

Here it assumes the second byte has the postfix operand, but that is not true for XROMs (RCR 2 is needed for XROMs).