MSX Video access method
The ColecoVision, SG-1000, CreatiVision, and first-generation MSX computers use the TMS9918A Video Display processor (VDP), which has its own 16 KiB of video memory that was not shared with main memory. Compared to the unified system and video memory used by other 8-bit computers of the time, such as the Apple II, ZX Spectrum, and Commodore 64, separate memory has the advantage of freeing up of the Z80 processor's 64 KiB address space for main RAM, and the VDP does not need to steal CPU cycles to access video memory. The disadvantage is that the program has to use the CPU's dedicated I/O instructions to command the VDP to manipulate the contents of the video RAM. This not only slows down video access but also makes the porting of games from unified-memory platforms more difficult. Attempts of porting ZX Spectrum games were often thwarted by this difference. Also, programmers had to learn to optimally use the more advanced capabilities of the VDP.