[dtt-github]

What

In Why is it this value?, a value read from a memory (assign dout = mem[rptr]) used to end the chain with mem = ?. Now it names the word that was read and lets you follow it back:

dout = 17 from cycle 1: line 35 of fifo4.v wrote it: assign dout = mem[rptr]
It used mem[0] = 17, rptr = 0.
mem[0] = 17 from cycle 1: line 27 of fifo4.v wrote it: mem[wptr] <= din
It ran because !rst_n was false (rst_n = 1), and do_wr was true (do_wr = 1). It used din = 17.

"Why wasn't it N?" lists the words read in the same way.

How

• Simulator (wafer-sim):
  • Each memory word's changes are recorded like a signal's (SimResult::words, named tb.dut.mem[2], with what wrote each value).
  • Each time an assignment that reads memory words runs, it records which words it read (SimResult::word_reads, in Origin::words order). That log is capped at MAX_WORD_READS.
  • Only memories that are actually used get recorded.
• WASM: new getters wordNames, wordWidths, wordTimes, wordValues, wordBy, wordReadTimes, wordReadOrigins and wordReadWords; words is added to the origins JSON.
• Web: why.ts looks words up like signals and expands a memory read into the words read by the run that made the value.

Checks

• New simulator test, memory_words_are_recorded_like_signals: a memory with a non-zero base index, an index expression, two word reads in one assignment, and a bit-select of a word.
• Checked in the browser on the FIFO starter: dout → mem[0] → mem[wptr] <= din → din, as shown above.
• fmt, clippy, workspace tests, typecheck and web build all pass.
• Docs: user guide (design view), architecture and CHANGELOG.

🤖 Generated with Claude Code