Exercise 2: Fixing a Missing Echo — Possible Solution
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WHAT ACTUALLY HAPPENS
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      TRAP x20   ; GETC
      TRAP x25   ; HALT

Per this chapter's own warn-box, GETC (TRAP x20) reads a character
from the keyboard into R0 silently -- it never echoes what was typed
to the screen on its own. The very next instruction is TRAP x25
(HALT), which immediately stops the fetch-decode-execute cycle. The
learner will observe: the program reads their keystroke, then halts
immediately, with NOTHING ever printed to the screen at all -- not
the character they typed, not any other output. It will look like
the program did nothing visible, even though it did correctly read
their input into R0.

THE FIX
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      TRAP x20   ; GETC — read the character into R0
      TRAP x21   ; OUT — explicitly echo R0's character to the screen
      TRAP x25   ; HALT

Inserting TRAP x21 (OUT) between the GETC and the HALT explicitly
prints whatever character is currently sitting in R0 -- which is
exactly the character GETC just read, since GETC's whole job is to
place the typed character into R0. This matches the chapter's own
stated fix: "if you want the typed character to actually show up, you
have to explicitly follow it with TRAP x21 (OUT)."

AN ALTERNATIVE FIX
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Replacing TRAP x20 with TRAP x23 (IN) instead would also work, since
IN reads AND echoes in a single call -- though it also prints its own
prompt first, which GETC+OUT does not.

WHY THIS WORKS AS AN ANSWER
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It describes precisely what the learner will observe (silence, then
an immediate halt) using GETC's own no-echo behavior from the
chapter's warn-box, and supplies the exact one-instruction fix the
chapter itself names, with a correctly ordered final sequence.