Due to the pipelining latency, by the time that the v-to-c
messages of a block column are computed by the subtractor,
the v-to-c messages of the same block column for the previous
layer stored in RAM A1 have already been read out for the
a posteriori message computation. Hence the newly computed
v-to-c messages can be stored into RAM A1 without loosing
any data. Therefore, a single copy of RAM A is adequate to
hold the v-to-c messages