Wasn't so much the 8-bit address bus, as the fact that all the "main" memory was only accessible by asking the video chip for it. The TMS9900 CPU could use a chunk of memory as just registers - in a sense it was a bit like the RISC processors that would come later, but at the time it was based around a Texas Instruments minicomputer architecture, shrunk down to a single chip. To actually get anything from RAM you had to write the address into two registers and then read a third to get the value. For *every single thing* that you wanted to read... Bit of a shame, if it hadn't been for this "papering the hall through the letterbox" memory arrangement it would have been a very powerful machine - almost as powerful as the Atari ST, but at least four years earlier!