Reality Display Processor
The Reality Display Processor, or RDP, is the portion of the RCP responsible for graphics tasks such as Z-buffering, texturing, blending, anti-aliasing, etc. It contains 4KB of texture memory (TMEM,) and is sent commands called "primitives".
The RDP interface is made of several memory-mapped registers that allows the VR4300 and RSP to control the RDP. Both VR4300 and RSP are in fact able to control RDP execution by accessing the interface: VR4300 through memory mapped registers, while RSP through its COP0.