Editing TMS9918 (section)

==External interfaces==

===Video RAM===
The VDP has 16K × 8 bits of external video memory.  This memory is outside the address space of the [[Central processing unit|CPU]].  Having a separate address space means that the CPU has to do more work to write or read this memory, but it also means that the VDC doesn't slow the CPU down when it periodically reads this memory to generate the display.  Additionally, it leaves more address space available to the CPU for other memory and memory-mapped hardware.

Depending on the screen mode being used, not all of the video memory may be needed to generate the display.  In these cases, the CPU may use the extra video memory for other purposes.  For example, one use is as a scratch-pad for uncompressing graphics or sound data stored in cartridge ROM into.  Another popular use is to create a second copy of some or all of the display data to eliminate flickering and tearing, a technique known as [[Multiple buffering#Double buffering in computer graphics|double buffering]].

===CPU===
The CPU communicates with the VDP through an [[Bus (computing)|8-bit bus]].  A pin controlled by the CPU separates this bus into two "ports", a control port and a data port.  To write or read a byte of video memory, the CPU first has to write two bytes on the VDP's control port to the VDC's internal address [[Processor register|register]].  Next, the CPU performs the actual write or read on the VDP's data port.  As a data byte is written or read, the TMS9918 automatically increments the internal address register.  This auto-increment feature accelerates writes and reads of blocks of data.  The control port is also used to access various internal registers.