& exch2_txYStackLo,exch2_tyYStackLo,
INTEGER exch2_tyYStackLo(W2_maxNbTiles )
exch2_tyYStackLo( tId ) = 1 + exch2_tBasey(tId) + fBaseY
tGy = exch2_tyYStackLo(tN)
jG = exch2_tyYStackLo(tN)+j-1