The principle of write circuit is to assert voltage of one of the columns to a low level. This can be achieved by connecting either or to ground through transistor M3 and either of M2 or M1. The transistor M3 is driven by the column decoder selecting the specified column. The transistor M1 is on only in the presence of the write enable signal and when the data bit to be written is '0'. The transistor M2 is on only in the presence of the write signal and when the data bit to be written is '1'. The circuit for write operation is shown in Figure 28.61
or 
to ground through transistor M3 and either of M2 or M1. The transistor M3 is driven by the column decoder selecting the specified column. The transistor M1 is on only in the presence of the write enable signal 
and when the data bit to be written is '0'. The transistor M2 is on only in the presence of the write signal 
