Time scales of a PDP-11/34 - 50 µs
50 microseconds - CPU startup phases
Because the Power-On sequence is quite slow we have to zoom in from 200 milliseconds to 50 microsconds to see a new level of detail.
Relevant here signals are:
PROCINIT - High means: CPU is held in reset.
PROCCLK - internal CPU clock, drives the micro machine
MPC - the micro program counter
MSYN, SSYN, Addr, BUScycle, DATA - UNIBUS signals
What you see
You can recognize three different phases:
Phase 1: Reset
While PROC INIT is High, the CPU is held in reset. There's no signal activity.
Phase 2: micro coded selftest
As PROC INIT goes inactive, the CPU clock PROC CLK begins to oscillate (marker "A").
For the first 10 microseconds, there's a activity on MPC without UNIBUS signals.
Question: What is the CPU doing if it's not fetching program code from its BOOT ROM?
Answer: You see an internal selfest running, which is totally implemented in microcode. Here the basic function of the CPU is checked by clearing and incrementing the Program counter. A good part of the data path module is checked this way. See part of the self test micro program flow chart:
In case of an error, the micro code would clear the instruction register IR. Since opcode 000000 is a HALT, the CPU would stop then.
Phase 3: Start code execution
After selftest, the CPU begins fetching code and executing it, UNIBUS activity is visible (marker "E").
You see the characteristic patterns of UNIBUS signals: Overlapping MSYN and SSYN signals, as well as changing addresses and data lines.
Signal MSYN here means: "CPU has put a valid address onto the UNIBUS", while SSYN is the repsonse from the BOOT ROM: "Data read and valid".
At first an address is fetched from fixed vector 24, then execution starts from this address. If a Boot ROM card M9312 is inserted, the start address is manipulated point to one of the ROMs on M9312.