Sega Genesis Color RAM

Discuss technical or other issues relating to programming the Nintendo Entertainment System, Famicom, or compatible systems.

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Sega Genesis Color RAM

Post by GUTY » Mon Aug 03, 2020 10:12 am

Sorry about the Sega Genesis topic, but i have a question that confusing me.

The real C-RAM of Sega Genesis is 72 Bytes (64x9 Bits = 576 Bits or 72 when converted into Bytes), but in other sites the number is 128 Bytes (?), why the number of C-RAM is other ?

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Re: Sega Genesis Color RAM

Post by lidnariq » Mon Aug 03, 2020 10:20 am

It's 64 palette entries, each holding 9 bits. 64·9=576 bits.

When you use DMA to upload things to it, those 9 bits are packed one palette entry per 16-bit word. 64·16÷8 = 128 bytes.

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Re: Sega Genesis Color RAM

Post by Quietust » Mon Aug 03, 2020 10:42 am

By the same token, NES palette RAM could be described as 32 bytes (the actual amount you're going to copy to PPU $3F00-$3F1F), 28 bytes (the number of uniquely addressible entries, excluding 3F10/3F14/3F18/3F1C), 25 bytes (the number of entries used during normal rendering, excluding 3F04/3F08/3F0c), or even 21 bytes (the number of SRAM bits in the PPU's palette memory - 168 - naively divided by 8).
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