[IBM Corporation]

Description
This 4Mb dual port Video RAM (VRAM) consists of a Dynamic Random Access Memory (DRAM) organized as 256K x 16 interfaced to a Serial Register / Serial Access Memory (SAM) organized as 256 x 16.
The VRAM supports three basic operations: Bidirectional Random Access to the DRAM, Bidirectional Serial Access to the SAM, and Bidirectional Data Transfer between any DRAM row and the SAM.
Full compatibility is provided between Half Depth SAM (256 x 16) and Full Depth SAM (512 x 16) by setting the VRAM in Serial Register Stop (SRS) mode with a stop address of 128 bit (or less).