VRAM

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VRAM is a memory segment on graphics systems in computers that is used to hold pattern data (for tiled VDPs) or bitmap data, in the case of a bitmap VDP. It is common for video adaptors to not map the video memory directly into I/O space, nor allow instant read/writes to it during active display. Most times, VRAM is DRAM or SDRAM, although in rare cases it may be SRAM or another variation of DRAM.

Mega Drive VDP VRAM

The two µPD41264 graphics memories on a Model 1 Mega Drive

The Mega Drive's VDP has 64 KB of dual-port graphics buffer memory. To get the required 8 bits of memory bandwidth, two µPD41264's are used in ZIP package, with addresses and control signals shared. The DRAM has two data ports, one for serial reading and a regular 4 bit random access port, a basic dual-port memory. The DRAM requires 256 (one per row) refresh cycles per 4 ms which is accomplished through /CAS before /RAS refresh. During active display, a limited number of access slots are available to the VRAM, one per 32 pixels.