By Jeff K.
The IBM360/30 gets stuck in a microcode loop. The documentation indicates that a branch should be taken if the Z-bus is zero, and the branch should be taken. The branch is not being taken.
A previous annoyance was that the microcode would stop at address 0xB46. As the documentation indicates for that location, it is checking that a register is zero. Hmm… checking for zero? That is the problem with the loop not stopping. So I dug deeper into this stop. There was a stuck bit! And here is what I found:
The circuit is an AND-OR-Invert gate and the output of the AND was high. The above circuit is the AND gate. If any of the inputs on the bottom go low the output should go low. The output was not going low. However, there is nothing on this circuit to force it low, but rather it allows the output to go low. So, the problem was the input to the OR gate:
Aha! That transistor with an X on it is not good. Fortunately, we have spares of this SLT module, and replacing it fixed the problem with the first microcode stop. However, the microcode loop with the non-taken branch is still not working as documented. Dig deeper…