The device_rom_interface

1. Capabilities

This interface is designed for devices that expect to have a ROM connected to them on a dedicated bus. It’s mostly designed for sound chips. Other devices types may be interested but other considerations may make it impractical (graphics decode caching, for instance). The interface provides the capability to connect a ROM region, connect an address map, or dynamically set up a block of memory as ROM. In the region/memory block cases, banking is handled automatically.

2. Setup

device_rom_interface<AddrWidth, DataWidth=0, AddrShift=0, Endian=ENDIANNESS_LITTLE>

The interface is a template that takes the address width of the dedicated bus as a parameter. In addition the data bus width (if not byte), address shift (if non-zero) and Endianness (if not little Endian or byte-sized bus) can be provided. Data bus width is 0 for byte, 1 for word, etc.

void set_map(map);

Use that method at machine configuration time to provide an address map for the bus to connect to. It has priority over a ROM region if one is also present.

void set_device_rom_tag(tag);

Used to specify a ROM region to use if a device address map is not given. Defaults to DEVICE_SELF, i.e. the device’s tag.

ROM_REGION(length, tag, flags)

If a ROM region with the tag specified using set_device_rom_tag if present, or identical to the device tag otherwise, is provided in the ROM definitions for the system, it will be automatically picked up as the connected ROM. An address map has priority over the region if present in the machine configuration.

void override_address_width(u8 width);

This method allows the address bus width to be overridden. It must be called from within the device before config_complete time.

void set_rom(const void *base, u32 size);

At any time post-interface_pre_start, a memory block can be set up as the connected ROM with that method. It overrides any previous setup that may have been provided. It can be done multiple times.

3. ROM access

u8 read_byte(offs_t addr);
u16 read_word(offs_t addr);
u32 read_dword(offs_t addr);
u64 read_qword(offs_t addr);

These methods provide read access to the connected ROM. Out-of-bounds access results in standard unmapped read logerror messages.

4. ROM banking

If the ROM region or the memory block in set_rom is larger than the address bus can access, banking is automatically set up.

void set_rom_bank(int bank);

That method selects the current bank number.

5. Caveats

Using that interface makes the device derive from device_memory_interface. If the device wants to actually use the memory interface for itself, remember that space zero (0, or AS_PROGRAM) is used by the ROM interface, and don’t forget to call the base memory_space_config method.

For devices which have outputs that can be used to address ROMs but only to forward the data to another device for processing, it may be helpful to disable the interface when it is not required. This can be done by overriding memory_space_config to return an empty vector.