mirror of
https://github.com/holub/mame
synced 2025-04-27 10:43:07 +03:00
22513fb6fe
The last(?) two changes are: - Add a template parameter to everything (theoretically the address space width, in practice a level derived from it to keep as much compatibility between widths as possible) so that the shift size becomes a constant. - Change the syntax of declaring and initializing the caches and specifics so that they're embedded in the owner device. Solves lifetime issues and also removes one indirection (looking up the base dispatch pointer through the cache/specific pointer).
90 lines
3.1 KiB
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90 lines
3.1 KiB
ReStructuredText
The device_rom_interface
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========================
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1. Capabilities
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---------------
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This interface is designed for devices which expect to have a rom
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connected to them on a dedicated bus. It's mostly designed for sound
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chips. Other devices types may be interested but other considerations
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may make it impratical (graphics decode caching for instance). The
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interface provides the capability of either connecting a ROM_REGION,
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connecting an ADDRESS_MAP or dynamically setting up a block of memory
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as rom. In the region/block cases, banking is automatically handled.
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2. Setup
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--------
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| device_rom_interface<AddrWidth, DataWidth=0, AddrShift=0, Endian=ENDIANNESS_LITTLE>
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The interface is a template that takes the address bus width of the
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dedicated bus as a parameter. In addition the data bus width (if not
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byte), address shift (if not 0) and endianness (if not little endian
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or byte-sized bus) can be provided. Data bus width is 0 for byte, 1
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for word, etc.
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| **MCFG_DEVICE_ADDRESS_MAP**\ (AS_0, map)
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Use that method at machine config time to provide an address map for
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the bus to connect to. It has priority over a rom region if one is
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also present.
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| **MCFG_DEVICE_ROM**\ (tag)
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Used to select a rom region to use if a device address map is not
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given. Defaults to DEVICE_SELF, e.g. the device tag.
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| **ROM_REGION**\ (length, tag, flags)
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If a rom region with a tag as given with **MCFG_DEVICE_ROM** if
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present, or identical to the device tag otherwise, is provided in the
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rom description for the system, it will be automatically picked up as
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the connected rom. An address map has priority over the region if
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present in the machine config.
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| void **override_address_width**\ (u8 width)
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This method allows to override the address bus width. It must be
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called from within the device before **config_complete** time.
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| void **set_rom**\ (const void \*base, u32 size);
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At any time post- **interface_pre_start**, a memory block can be
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setup as the connected rom with that method. It overrides any
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previous setup that may have been provided. It can be done multiple
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times.
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3. Rom access
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-------------
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| u8 **read_byte**\ (offs_t byteaddress)
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| u16 **read_word**\ (offs_t byteaddress)
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| u32 **read_dword**\ (offs_t byteaddress)
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| u64 **read_qword**\ (offs_t byteaddress)
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These methods provide read access to the connected rom. Out-of-bounds
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access results in standard unmapped read logerror messages.
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4. Rom banking
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--------------
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If the rom region or the memory block in set_rom is larger than the
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address bus, banking is automatically setup.
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| void **set_rom_bank**\ (int bank)
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That method selects the current bank number.
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5. Caveats
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----------
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Using that interface makes the device derive from
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**device_memory_interface**. If the device wants to actually use the
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memory interface for itself, remember that AS_0/AS_PROGRAM is used by
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the rom interface, and don't forget to upcall **memory_space_config**.
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For devices which have outputs that can be used to address ROMs but
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only to forward the data to another device for processing, it may be
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helpful to disable the interface when it is not required. This can be
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done by overriding **memory_space_config** to return an empty vector.
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