The SMJ44C251B/MT42C4256 multiport video RAM is a high-speed, dual-ported memory device. It consists of a dynamic random-access memory (DRAM) organized as 262144words of 4 bits each interfaced to a serial-data register or serial access memory (SAM) organized as 512 words of 4 bits each.The SMJ44C251B/MT42C4256 supports three types of operation: random access to and from the DRAM, serial access to and from the serial register, and bidirectional transfer of data between any row in the DRAM and the serial register. Except during transfer operations, the SMJ44C251B/MT42C4256 can be accessed simultaneously and asynchronously from the DRAM and SAM ports.During a transfer operation, the 512 columns of the DRAM are connected to the 512 positions in the serial data register.The 512 4-bit serial-data register can be loaded from the memory row (transfer read), or the contents of the 512 4-bitserial-data register can be written to the memory row (transfer write).