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memory_types: add segue to address_space.md from memory type table

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Ilia Sharin 2026-04-25 14:15:57 -04:00
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01_hardware/common/memory_types.md
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@ -23,6 +23,8 @@ Understanding this distinction is not optional. It determines where screen buffe
| **Slow RAM** | Pseudo-Fast / Ranger / "Trapdoor" RAM | CPU only — but on the Chip RAM bus | ~3.5 MHz effective (still contended) | ✗ No (not used by DMA) | `$C00000`–`$C7FFFF` (512 KB) | | **Slow RAM** | Pseudo-Fast / Ranger / "Trapdoor" RAM | CPU only — but on the Chip RAM bus | ~3.5 MHz effective (still contended) | ✗ No (not used by DMA) | `$C00000`–`$C7FFFF` (512 KB) |
| **Fast RAM** | True Fast / Zorro / Accelerator RAM | CPU only — on a separate bus | Full CPU speed (7–50+ MHz, no contention) | ✗ No | `$200000`+ (Zorro II) or `$01000000`+ (Zorro III) | | **Fast RAM** | True Fast / Zorro / Accelerator RAM | CPU only — on a separate bus | Full CPU speed (7–50+ MHz, no contention) | ✗ No | `$200000`+ (Zorro II) or `$01000000`+ (Zorro III) |
> These address ranges are excerpts — for the complete 24-bit and 32-bit memory maps showing how Chip, Slow, Fast RAM, CIAs, custom chips, ROM, and Autoconfig space interleave, see [address_space.md](address_space.md).
### Why "Slow RAM" Exists ### Why "Slow RAM" Exists
The A500's internal 512 KB expansion (the "trapdoor" board) and the A500 external side-expansion both map to `$C00000`–`$C7FFFF`. This RAM is **on the same bus as Chip RAM**, so it suffers the same DMA contention — the CPU waits whenever the custom chips are using the bus. However, the custom chips **do not** fetch DMA data from this range. It's the worst of both worlds: too slow for CPU work (bus contention), but invisible to DMA (can't be used for screen buffers or audio). The A500's internal 512 KB expansion (the "trapdoor" board) and the A500 external side-expansion both map to `$C00000`–`$C7FFFF`. This RAM is **on the same bus as Chip RAM**, so it suffers the same DMA contention — the CPU waits whenever the custom chips are using the bus. However, the custom chips **do not** fetch DMA data from this range. It's the worst of both worlds: too slow for CPU work (bus contention), but invisible to DMA (can't be used for screen buffers or audio).