BACKLOG

# BACKLOG: frames of 4 emitted reversed: (a,b,c,d) -> (d,c,b,a). 3 frames, 12 out.
# core0 READER: holds a in BAK, pushes b DOWN to core4 (stored there), reads c into
#   ACC, then emits d, c, and a. Order sent down = b, d, c, a.
# core4 STORE+RELAY: stores b; relays d and c straight through (MOV DOWN,UP leaves
#   ACC/BAK alone); emits b; relays a. Net output order at core8 = d, c, b, a.
# core8 writes the stream to output 0.
@1
L: MOV ACC, UP
SAV
MOV DOWN, UP
MOV ACC, UP
MOV DOWN, UP
MOV DOWN, ACC
XCH
MOV DOWN, ACC
JMP L
@4
N: MOV ACC, UP
MOV DOWN, UP
MOV DOWN, UP
MOV DOWN, ACC
MOV DOWN, UP
JMP N
@7
MOV DOWN, UP