SignalChapter 12: Shared buses

97HELP DESK

The task

b asks s for the root of each X: it writes X on the bus and reads back the answer.

P must receive each whole square root: 15 sends 3, and 16 sends 4. Q must receive X.

s is an SC-4, with C and D as well. b is a standard core cut to 4 lines.

The board

The board as the puzzle screen lays it out. Cyan pins read, lemon pins write, violet pins go both ways. On a narrow screen, scroll the board sideways to see all of it.

Key idea

Request and reply: s answers b before it passes X on to c, and keeps X in C meanwhile.

Hints

Each hint goes a step further than the one before. Open them one at a time.

Hint 1Show

Take away 1, then 3, then 5, and so on: how many odd numbers X has room for is its root.

Hint 2Show

While b waits for its answer, what s writes goes to b, as b is farther left than c.

Hint 3Show

So s answers b before it sends X on to c, and keeps X in C meanwhile.

Solution

Reveal the Full SolutionThe complete program for HELP DESK. Open it only when you want the answer.

The reference program

The game's own solution. It passes every test and scores exactly par on every benchmark.

sSC-4 quad

IN P0
MOV C, A
MOV B, A
MOV A, 1
L: XCH A, B
SUB B
JL G
XCH A, B
ADD 2
JMP L
G: XCH A, B
DIV 2
OUT P0
MOV A, C
OUT P0

bSC-2 standard

IN P1
OUT P0
IN P0
OUT P2

cSC-2 standard

IN P0
OUT P1

CYCLES: 1066; at par
ENERGY: 9342; at par
NODES: 3; at par
INSTRUCTIONS: 21; at par