Raimon @ quifisraimon
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- [NEW] Afegida la rom de Combat. - [NEW] Afegit mòdul "rom" - [NEW] Ampliats stubs de "mem", "tia" i "pia"
38 lines
2.3 KiB
Plaintext
38 lines
2.3 KiB
Plaintext
From: https://forums.atariage.com/topic/192418-mirrored-memory/
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There are two bankswitching schemes commonly called 3E and 3F because those
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zero-page addresses are used as hotspots to trigger a bank switch. The original
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version was 3F, also called Tigervision because it was used by Tigervision, and
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there is an extended version of 3F, as well as 3E. (I'm no expert on these
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schemes, so this information may contain errors.) I'm not sure if extended 3F
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and 3E are the same, or separate schemes, but 3E adds extra RAM, whereas 3F and
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extended 3F don't necessarily add extra RAM.
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Anyway, these bankswitching schemes divide the 4K cartridge area into two 2K
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slots. The upper 2K area ($1800 - $1FFF) always points to the last 2K of the
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cartridge ROM, but the lower 2K area ($1000 - $17FF) can be selected from
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several 2K banks by writing the desired bank number into address $3F. I think
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the original version had four 2K banks, for a total cartridge size of 8K,
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whereas the extended version allows up to 256 2K banks, for a total cartridge
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size of 512K.
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The 3E version still uses address $3F to select between different 2K ROM banks,
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but it also allows you to select between different 1K banks of extra RAM by
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writing the desired RAM bank number into address $3E.
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The TIA write addresses are $00 - $2C, and the TIA read addresses are $0 - $D,
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so even though $3E and $3F fall within the memory area that's normally
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associated with the TIA chip, they don't actually correspond to any addresses
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or mirrors that are used by the TIA read or write registers. So in theory it
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should be safe to use $3E and $3F as bankswitching hotspots without interfering
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with the TIA addresses and mirrors.
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However, it's my understanding that any access below $40 (i.e., from $00 - $3F)
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can trigger a bank switch in these schemes-- although this might not be true
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for all cartridges that use 3E or 3F bankswitching (I don't know). For this
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reason, the TIA addresses are usually (or always?) relocated to $40 - $7F when
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using 3E or 3F bankswitching. That is, $40 - $6C are used for the TIA write
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registers, and either $40 - $4D or $70 - $7D are used for the TIA read
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registers. Since these are mirrors of the "real" TIA addresses, accessing them
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is the same (from the TIA's point of view) as accessing $00 - $2C and $00 - $0D.
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