mirror of
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538221861b
* Made it an error to specify address map and explicit ROM region. * Made it an error if explicitly specified ROM region is not found. * Made the ROM region tag apply relative to the current device. -Cleaned up formatting for a couple more documentation pages.
114 lines
3.3 KiB
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114 lines
3.3 KiB
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The device_rom_interface
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========================
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.. contents:: :local:
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1. Capabilities
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---------------
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This interface is designed for devices that 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 impractical (graphics decode caching, for instance). The
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interface provides the capability to connect a ROM region, connect an
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address map, or dynamically set up a block of memory as ROM. In the
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region/memory block cases, banking is handled automatically.
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2. Setup
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--------
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.. code-block:: C++
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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 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 non-zero) 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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.. code-block:: C++
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void set_map(map);
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Use that method at machine configuration time to provide an address map
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for 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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.. code-block:: C++
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void set_device_rom_tag(tag);
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Used to specify a ROM region to use if a device address map is not
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given. Defaults to ``DEVICE_SELF``, i.e. the device’s tag.
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.. code-block:: C++
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ROM_REGION(length, tag, flags)
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If a ROM region with the tag specified using ``set_device_rom_tag`` if
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present, or identical to the device tag otherwise, is provided in the
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ROM definitions 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 configuration.
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.. code-block:: C++
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void override_address_width(u8 width);
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This method allows the address bus width to be overridden. It must be
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called from within the device before **config_complete** time.
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.. code-block:: C++
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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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set up 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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.. code-block:: C++
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u8 read_byte(offs_t addr);
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u16 read_word(offs_t addr);
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u32 read_dword(offs_t addr);
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u64 read_qword(offs_t addr);
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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 can access, banking is automatically set up.
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.. code-block:: C++
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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 space zero (0, or
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``AS_PROGRAM``) is used by the ROM interface, and don’t forget to call
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the base ``memory_space_config`` method.
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For devices which have outputs that can be used to address ROMs but only
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to forward the data to another device for processing, it may be helpful
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to disable the interface when it is not required. This can be done by
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overriding ``memory_space_config`` to return an empty vector.
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