Editing Sega CD (Model 1) (section)

== Technical specification (Model 1, BIOS v1.10) ==
{| class="wikitable styled-table" style="width:70%; text-align:center;"
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! style="background:#ddd;" |Processing
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| Main 68k | 12.500 MHz (25 MHz ÷ 2) wait-stated 3–4 cycles to word-RAM
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| Sub MCU | 3.58 MHz (NTSC crystal) – handles CD block decode, seek, audio FIFO
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| DMA | 5.0 MB·s⁻¹ peak host⇄word-RAM
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! style="background:#ddd;" |Memory map (simplified)
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| $0000 00–$1F FFFF | 2 MB Program RAM
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| $2000 00–$23 FFFF | 256 KB Word-RAM bank 0
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| $2400 00–$27 FFFF | 256 KB Word-RAM bank 1 (dual-port)
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| $2FFF E0–$2FFF FF | PCM regs & sample RAM (128 KB windowed)
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| $3E00 00–$3E3F FF | Backup SRAM 64 KB (battery)
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| $3F00 00–$3F3F FF | ASIC / host control regs
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| $3F00 40–$3F00 7F | Sub-CPU I/O FIFO
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| $FF00 00–$FFFF FF | BIOS ROM 128 KB (mirrored)
|}

=== Video timing ===
* Pixel clock 13.5 MHz (NTSC) / 13.32 MHz (PAL) via VDP master.  
* Scaled plane generated at 256×A & line-buffered into Genesis VDP CRAM.  
* V-blank interrupt every 262 lines NTSC (≈ 59.92 Hz).