A memory access request associated with a data unit is received from a first thread of a pixel shader. A processing status associated with the data unit is obtained from a window buffer. It is determined whether the data unit is being processed by a second thread. If so, a rejection procedure is performed to avoid the first thread gaining to access an attribute value associated with the data unit from/to a DRAM (Dynamic Random Access Memory). Otherwise, an acknowledgement procedure is performed to grant the first thread to access the attribute value associated with the data unit from/to the DRAM.