What does PrimeTime's `write_changes` produce, and how is it applied in ICC2?

Short Answer

write_changes (PT) writes every netlist change made during the PrimeTime session as a change list, in a format chosen with -format. For ICC2 the format is icctcl, a Tcl script of netlist edits; ICC2 sources it, then places the changed cells with place_eco_cells -eco_changed_cells (ICC2) and reconnects them with route_eco (ICC2). The file is only a list of edits, so ICC2 still has to make them physically legal.

Technical Explanation

• The change list covers all ECO changes made in the PT session, not only the last command. Writing after setup fixing and again after hold fixing gives two files where the second repeats the first.
• Formats are ptsh (the default, a PT Tcl script), text, dctcl, icctcl for IC Compiler or IC Compiler II, eco (a binary read back by read_eco_changes (PT)) and aprtcl for third-party tools. Only icctcl is meant for ICC2.
• In physically aware mode the file carries locations; the PT user guide example shows an insert_buffer line with a -location coordinate pair and an add_buffer_on_route line with its location. Physically aware ECO needs a PrimeTime-ADV license (PT user guide, ECO chapter).
• In ICC2 you source (ICC2) the file. Each edited cell gets an ECO change status such as create_cell, add_buffer, change_link or size_cell, which is what place_eco_cells -eco_changed_cells (ICC2) picks up.
• place_eco_cells -eco_changed_cells -legalize_only (ICC2) only legalizes changed cells that already have a location, which suits a physically aware change list; any changed cell that is still unplaced is skipped with warning EPL-021, so search the log for it. For a logic-only change list, run it without -legalize_only so ICC2 places the new cells itself.
• After placement, insert fillers into the gaps, then run route_eco (ICC2) to reconnect the modified nets. Check legality and routing before extraction.
• With multiply instantiated module ECO enabled through eco_enable_mim (PT), write_changes (PT) writes a separate change list per module, prefixed with the module name, so check the file list, not only the file you named.

What To Check

• The format and the number of edits in the file, by type.
• Errors or warnings while sourcing in ICC2, such as unknown library cells or missing pins.
• Every ECO cell placed and legal after place_eco_cells (ICC2), with displacement in the expected range.
• No opens or DRCs after route_eco (ICC2).

Healthy, Suspicious & Hard-stop Results

• Healthy (illustrative): The script sources with no errors, every one of the changed cells is placed and legal, maximum displacement stays within a few microns and check_routes (ICC2) reports no opens.
• Suspicious (illustrative): Sourcing warns about a library cell name that matches in two libraries; set the PT variables that prepend library names and rewrite.
• Hard stop: Unplaced ECO cells after place_eco_cells (ICC2), or source errors that stop the script halfway. Do not route or extract a half-applied ECO.

What The Interviewer Is Testing

• Knowing the format ICC2 needs, and what the default format is.
• Can you explain how ICC2 knows which cells to place after sourcing?
• Understanding that the change list is cumulative for the PT session.

Follow-up Question & Model Response

"When would you use the eco binary format instead of icctcl?"

Candidate Model Response: The eco format is read back into PrimeTime with read_eco_changes (PT), not into ICC2. Replaying a change list in PrimeTime needs a PrimeTime-ADV license (PT UG licensing table). It is useful to replay an ECO in a fresh or restored PT session, for example to check the changes in other scenarios before implementing them. The PT user guide describes this flow: write eco, restore the session, replay, assess, then write icctcl for implementation. It is also how block-level ECOs are combined at the top level.