It didn’t clear the return code. In mainframe jobs, successful executions are expected to return zero (in the machine R15 register).

So in this case fixing the bug required to add an instruction instead of removing one.

Just to boast my old timer credentials.

There is an utility program in IBM’s mainframe operating system, z/OS, that has been there since the 60s.

It has just one assembly code instruction: a BR 14, which means basically ‘return’.

The first version was bugged and IBM had to issue a PTF (patch) to fix it.

The underlining linear algebra routines are written in… FORTRAN.

Pl/1 did it right:

Dcl 1 mybools, 3 bool1 bit(1) unaligned, 3 bool2 bit(1) unaligned, … 3 bool8 bit(1) unaligned;

All eight bools are in the same byte.