HP-25 Question

[Greg]

It looks like the emulators are correct. The microcode for GTO does a "0 -> s 1" which clears s1 if c[m] is 0 (the Program Counter is step 00) and skips that "if c[m] # 0"; but then does a "clear s" in both cases.

If "clear s" resets s1, the "0 -> s 1" would be pointless and any GTO would always stop.
That isn't the case so "clear s" can't be clearing s1.

My guess is that real world HP-25's will run the test program and stop at step "00" just like the emulators.

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The emulators go to some trouble to clear everything but s1, s2, s5 and s15 when they "clear s". On a HP-25, s15 is connected to the keyboard (a key is pressed) and s5 to the power (battery low sensor). It makes sense that s5 and s15 aren't clearable given they are wired to things.

s3 is also wired up (to the PRGM-RUN switch) so despite "clear s" clearing s3; if it was set, it goes back to set straight after the "clear s".

There doesn't seem to be any good reason for "clear s" not clearing s1 and s2 though. Perhaps it was a feature to have some flags that would remain across a "clear s". Much like having test-cleared and normal flags in HP-67 user programs.

All the best,