Emulated system memory and address spaces management

1. Overview

The memory subsystem (emumem and addrmap) combines multiple functions useful for system emulation:

  • address bus decoding and dispatching with caching

  • static descriptions of an address map

  • RAM allocation and registration for state saving

  • interaction with memory regions to access ROM

Devices create address spaces, e.g. decodable buses, through the device_memory_interface. The machine configuration sets up address maps to put in the address spaces, then the device can do read and writes through the bus.

2. Basic concepts

2.1 Address spaces

An address space, implemented in the class address_space, represents an addressable bus with potentially multiple sub-devices connected requiring a decode. It has a number of data lines (8, 16, 32 or 64) called data width, a number of address lines (1 to 32) called address width and an Endianness. In addition an address shift allows for buses that have an atomic granularity different than a byte.

Address space objects provide a series of methods for read and write access, and a second series of methods for dynamically changing the decode.

2.2 Address maps

An address map is a static description of the decode expected when using a bus. It connects to memory, other devices and methods, and is installed, usually at startup, in an address space. That description is stored in an address_map structure which is filled programmatically.

2.3 Shares, banks and regions

Memory shares are allocated memory zones that can be put in multiple places in the same or different address spaces, and can also be directly accessed from devices.

Memory banks are zones that indirect memory access, giving the possibility to dynamically and efficiently change where a zone actually points to.

Memory regions are read-only memory zones in which ROMs are loaded.

All of these have names allowing to access them.


Views are a way to multiplex different submaps over a memory range with fast switching. It is to be used when multiple devices map at the same addresses and are switched in externally. They must be created as an object of the device and then setup either statically in a memory map or dynamically through install_* calls.

Switchable submaps, aka variants, are named through an integer. An internal indirection through a map ensures that any integer value can be used.

3. Memory objects

3.1 Shares - memory_share

class memory_share {
    const std::string &name() const;
    void *ptr() const;
    size_t bytes() const;
    endianness_t endianness() const;
    u8 bitwidth() const;
    u8 bytewidth() const;

A memory share is a named allocated memory zone that is automatically saved in save states and can be mapped in address spaces. It is the standard container for memory that is shared between spaces, but also shared between an emulated CPU and a driver. As such one has easy access to its contents from the driver class.

required_shared_ptr<uNN> m_share_ptr;
optional_shared_ptr<uNN> m_share_ptr;
required_shared_ptr_array<uNN, count> m_share_ptr_array;
optional_shared_ptr_array<uNN, count> m_share_ptr_array;

[device constructor] m_share_ptr(*this, "name"),
[device constructor] m_share_ptr_array(*this, "name%u", 0U),

At the device level, a pointer to the memory zone can easily be retrieved by building one of these four finders. Note that like for every finder calling target() on the finder gives you the base pointer of the memory_share object.

memory_share_creator<uNN> m_share;

[device constructor] m_share(*this, "name", size, endianness),

A memory share can be created if it doesn’t exist in a memory map through that creator class. If it already exists it is just retrieved. That class behaves like a pointer but also has the target(), length(), bytes(), endianness(), bitwidth() and bytewidth() methods for share information.

memory_share *memshare(string tag) const;

The memshare device method retrieves a memory share by name. Beware that the lookup can be expensive, prefer finders instead.

3.2 Banks - memory_bank

class memory_bank {
    const std::string &tag() const;
    int entry() const;
    void set_entry(int entrynum);
    void configure_entry(int entrynum, void *base);
    void configure_entries(int startentry, int numentry, void *base, offs_t stride);
    void set_base(void *base);
    void *base() const;

A memory bank is a named memory zone indirection that can be mapped in address spaces. It points to nullptr when created. configure_entry associates an entry number and a base pointer. configure_entries does the same for multiple consecutive entries spanning a memory zone. Alternatively set_base sets the base for entry 0 and selects it.

set_entry allows to dynamically and efficiently select the current active entry, entry() gets that selection back, and base() gets the associated base pointer.

required_memory_bank m_bank;
optional_memory_bank m_bank;
required_memory_bank_array<count> m_bank_array;
optional_memory_bank_array<count> m_bank_array;

[device constructor] m_bank(*this, "name"),
[device constructor] m_bank_array(*this, "name%u", 0U),

At the device level, a pointer to the memory bank object can easily be retrieved by building one of these four finders.

memory_bank_creator m_bank;

[device constructor] m_bank(*this, "name"),

A memory share can be created if it doesn’t exist in a memory map through that creator class. If it already exists it is just retrieved.

memory_bank *membank(string tag) const;

The membank device method retrieves a memory bank by name. Beware that the lookup can be expensive, prefer finders instead.

3.3 Regions - memory_region

class memory_bank {
    u8 *base();
    u8 *end();
    u32 bytes() const;
    const std::string &name() const;
    endianness_t endianness() const;
    u8 bitwidth() const;
    u8 bytewidth() const;
    u8 &as_u8(offs_t offset = 0);
    u16 &as_u16(offs_t offset = 0);
    u32 &as_u32(offs_t offset = 0);
    u64 &as_u64(offs_t offset = 0);

A region is used to store read-only data like ROMs or the result of fixed decryptions. Their contents are not saved, which is why they should not being written to from the emulated system. They don’t really have an intrinsic width (base() returns an u8 * always), which is historical and pretty much unfixable at this point. The as_* methods allow for accessing them at a given width.

required_memory_region m_region;
optional_memory_region m_region;
required_memory_region_array<count> m_region_array;
optional_memory_region_array<count> m_region_array;

[device constructor] m_region(*this, "name"),
[device constructor] m_region_array(*this, "name%u", 0U),

At the device level, a pointer to the memory region object can easily be retrieved by building one of these four finders.

memory_region *memregion(string tag) const;

The memregion device method retrieves a memory region by name. Beware that the lookup can be expensive, prefer finders instead.

3.4 Views - memory_view

class memory_view {
    memory_view(device_t &device, std::string name);
    memory_view_entry &operator[](int slot);

    void select(int entry);
    void disable();

    const std::string &name() const;

A view allows to switch part of a memory map between multiple possibilities, or even disable it entirely to see what was there before. It is created as an object of the device.

memory_view m_view;

[device constructor] m_view(*this, "name"),

It is then setup through the address map API or dynamically. At runtime, a numbered variant can be selected using the select method, or the view can be disabled using the disable method. A disabled view can be re-enabled at any time.

4. Address maps API

4.1 General API structure

An address map is a method of a device which fills an address_map structure, usually called map, passed by reference. The method then can set some global configuration through specific methods and then provide address range-oriented entries which indicate what should happen when a specific range is accessed.

The general syntax for entries uses method chaining:

map(start, end).handler(...).handler_qualifier(...).range_qualifier();

The values start and end define the range, the handler() block determines how the access is handled, the handler_qualifier() block specifies some aspects of the handler (memory sharing for instance) and the range_qualifier() block refines the range (mirroring, masking, lane selection, etc.).

The map follows a “last one wins” principle, where the handler specified last is selected when multiple handlers match a given address.

4.2 Global configurations

4.2.1 Global masking

map.global_mask(offs_t mask);

Specifies a mask to be applied to all addresses when accessing the space that map is installed in.

4.2.2 Returned value on unmapped/nop-ed read

map.unmap_value(u8 value);

Sets the value to return on reads to an unmapped or nopped-out address. Low means 0, high ~0.

4.3 Handler setting

4.3.1 Method on the current device

(...).rw(FUNC(my_device::read_method), FUNC(my_device::write_method))

uNN my_device::read_method(address_space &space, offs_t offset, uNN mem_mask)
uNN my_device::read_method(address_space &space, offs_t offset)
uNN my_device::read_method(address_space &space)
uNN my_device::read_method(offs_t offset, uNN mem_mask)
uNN my_device::read_method(offs_t offset)
uNN my_device::read_method()

void my_device::write_method(address_space &space, offs_t offset, uNN data, uNN mem_mask)
void my_device::write_method(address_space &space, offs_t offset, uNN data)
void my_device::write_method(address_space &space, uNN data)
void my_device::write_method(offs_t offset, uNN data, uNN mem_mask)
void my_device::write_method(offs_t offset, uNN data)
void my_device::write_method(uNN data)

Sets a method of the current device or driver to read, write or both for the current entry. The prototype of the method can take multiple forms making some elements optional. uNN represents u8, u16, u32 or u64 depending on the data width of the handler. The handler can be narrower than the bus itself (for instance a 8-bit device on a 32-bit bus).

The offset passed in is built from the access address. It starts at zero at the start of the range, and increments for each uNN unit. An u8 handler will get an offset in bytes, an u32 one in double words. The mem_mask has its bits set where the accessors actually drive the bit. It’s usually built in byte units, but in some cases of I/O chips ports with per-bit direction registers the resolution can be at the bit level.

4.3.2 Method on a different device

(...).r(m_other_device, FUNC(other_device::read_method))
(...).r("other-device-tag", FUNC(other_device::read_method))
(...).w(m_other_device, FUNC(other_device::write_method))
(...).w("other-device-tag", FUNC(other_device::write_method))
(...).rw(m_other_device, FUNC(other_device::read_method), FUNC(other_device::write_method))
(...).rw("other-device-tag", FUNC(other_device::read_method), FUNC(other_device::write_method))

Sets a method of another device, designated by an object finder (usually required_device or optional_device) or its tag, to read, write or both for the current entry.

4.3.3 Lambda function

(...).lr{8,16,32,64}(NAME([...](address_space &space, offs_t offset, uNN mem_mask) -> uNN { ... }))
(...).lr{8,16,32,64}([...](address_space &space, offs_t offset, uNN mem_mask) -> uNN { ... }, "name")
(...).lw{8,16,32,64}(NAME([...](address_space &space, offs_t offset, uNN data, uNN mem_mask) -> void { ... }))
(...).lw{8,16,32,64}([...](address_space &space, offs_t offset, uNN data, uNN mem_mask) -> void { ... }, "name")
(...).lrw{8,16,32,64}(NAME(read), NAME(write))
(...).lrw{8,16,32,64}(read, "name_r", write, "name_w")

Sets a lambda called on read, write or both. The lambda prototype can be any of the six available for methods. One can either use NAME() over the whole lambda, or provide a name after the lambda definition. The number is the data width of the access, e.g. the NN.

4.3.4 Direct memory access


Selects the range to access a memory zone as read-only, write-only or read/write respectively. Specific handler qualifiers specify the location of this memory zone. There are two cases when no qualifier is acceptable:

  • ram() gives an anonymous RAM zone not accessible outside of the address space.

  • rom() when the memory map is used in an AS_PROGRAM space of a (CPU) device which names is also the name of a region. Then the memory zone points to that region at the offset corresponding to the start of the zone.

(...).rom().region("name", offset)

The region qualifier causes a read-only zone point to the contents of a given region at a given offset.


The share qualifier causes the zone point to a shared memory region identified by its name. If the share is present in multiple spaces, the size, bus width, and, if the bus is more than byte-wide, the Endianness must match.

4.3.5 Bank access


Sets the range to point at the contents of a memory bank in read, write or read/write mode.

4.3.6 Port access


Sets the range to point at an I/O port.

4.3.7 Dropped access


Sets the range to drop the access without logging. When reading, the unmap value is returned.

4.3.8 Unmapped access


Sets the range to drop the access with logging. When reading, the unmap value is returned.

4.3.9 Subdevice mapping

(...).m(m_other_device, FUNC(other_device::map_method))
(...).m("other-device-tag", FUNC(other_device::map_method))

Includes a device-defined submap. The start of the range indicates where the address zero of the submap ends up, and the end of the range clips the submap if needed. Note that range qualifiers (defined later) apply.

Currently, only handlers are allowed in submaps and not memory zones or banks.

4.4 Range qualifiers

4.4.1 Mirroring


Duplicate the range on the addresses reachable by setting any of the 1 bits present in mask. For instance, a range 0-0x1f with mask 0x300 will be present on 0-0x1f, 0x100-0x11f, 0x200-0x21f and 0x300-0x31f. The addresses passed in to the handler stay in the 0-0x1f range, the mirror bits are not seen by the handler.

4.4.2 Masking


Only valid with handlers, the address will be masked with the mask before being passed to the handler.

4.4.3 Selection


Only valid with handlers, the range will be mirrored as with mirror, but the mirror address bits are preserved in the offset passed to the handler when it is called. This is useful for devices like sound chips where the low bits of the address select a function and the high bits a voice number.

4.4.4 Sub-unit selection

(...).umask16(16-bits mask)
(...).umask32(32-bits mask)
(...).umask64(64-bits mask)

Only valid with handlers and submaps, selects which data lines of the bus are actually connected to the handler or the device. The mask value should be a multiple of a byte, e.g. the mask is a series of 00 and ff. The offset will be adjusted accordingly, so that a difference of 1 means the next handled unit in the access.

If the mask is narrower than the bus width, the mask is replicated in the upper lines.

4.4.5 Chip select handling on sub-unit


When a device is connected to part of the bus, like a byte on a 16-bits bus, the target handler is only activated when that part is actually accessed. In some cases, very often byte access on a 68000 16-bits bus, the actual hardware only checks the word address and not if the correct byte is accessed. cswidth tells the memory system to trigger the handler if a wider part of the bus is accessed. The parameter is that trigger width (would be 16 in the 68000 case).

4.5 View setup

map(start, end).view(m_view);
m_view[0](start1, end1).[...];

A view is setup in a address map with the view method. The only qualifier accepted is mirror. The “disabled” version of the view will include what was in the range prior to the view setup.

The different variants are setup by indexing the view with the variant number and setting up an entry in the usual way. The entries within a variant must of course stay within the range. There are no other additional constraints. The contents of a variant, by default, are what was there before, i.e. the contents of the disabled view, and setting it up allows part or all of it to be overridden.

Variants can only be setup once the view itself has been setup with the view method.

A view can only be put in one address map and in only one position. If multiple views have identical or similar contents, remember that setting up a map is nothing more than a method call, and creating a second method to setup a view is perfectly reasonable. A view is of type memory_view and an indexed entry (e.g. a variant to setup) is of type memory_view::memory_view_entry &.

A view can be installed in another view, but don’t forget that a view can be installed only once. A view can also be part of “what was there before”.

5. Address space dynamic mapping API

5.1 General API structure

A series of methods allow the bus decoding of an address space to be changed on-the-fly. They’re powerful but have some issues:

  • changing the mappings repeatedly can be slow

  • the address space state is not saved in the saved states, so it has to be rebuilt after state load

  • they can be hidden anywhere rather that be grouped in an address map, which can be less readable

The methods, rather than decomposing the information in handler, handler qualifier and range qualifier, put them all together as method parameters. To make things a little more readable, lots of them are optional.

5.2 Handler mapping

uNN my_device::read_method(address_space &space, offs_t offset, uNN mem_mask)
uNN my_device::read_method_m(address_space &space, offs_t offset)
uNN my_device::read_method_mo(address_space &space)
uNN my_device::read_method_s(offs_t offset, uNN mem_mask)
uNN my_device::read_method_sm(offs_t offset)
uNN my_device::read_method_smo()

void my_device::write_method(address_space &space, offs_t offset, uNN data, uNN mem_mask)
void my_device::write_method_m(address_space &space, offs_t offset, uNN data)
void my_device::write_method_mo(address_space &space, uNN data)
void my_device::write_method_s(offs_t offset, uNN data, uNN mem_mask)
void my_device::write_method_sm(offs_t offset, uNN data)
void my_device::write_method_smo(uNN data)

readNN_delegate   (device, FUNC(read_method))
readNNm_delegate  (device, FUNC(read_method_m))
readNNmo_delegate (device, FUNC(read_method_mo))
readNNs_delegate  (device, FUNC(read_method_s))
readNNsm_delegate (device, FUNC(read_method_sm))
readNNsmo_delegate(device, FUNC(read_method_smo))

writeNN_delegate   (device, FUNC(write_method))
writeNNm_delegate  (device, FUNC(write_method_m))
writeNNmo_delegate (device, FUNC(write_method_mo))
writeNNs_delegate  (device, FUNC(write_method_s))
writeNNsm_delegate (device, FUNC(write_method_sm))
writeNNsmo_delegate(device, FUNC(write_method_smo))

To be added to a map, a method call and the device it is called onto have to be wrapped in the appropriate delegate type. There are twelve types, for read and for write and for all six possible prototypes. Note that as all delegates, they can also wrap lambdas.

space.install_read_handler(addrstart, addrend, read_delegate, unitmask, cswidth)
space.install_read_handler(addrstart, addrend, addrmask, addrmirror, addrselect, read_delegate, unitmask, cswidth)
space.install_write_handler(addrstart, addrend, write_delegate, unitmask, cswidth)
space.install_write_handler(addrstart, addrend, addrmask, addrmirror, addrselect, write_delegate, unitmask, cswidth)
space.install_readwrite_handler(addrstart, addrend, read_delegate, write_delegate, unitmask, cswidth)
space.install_readwrite_handler(addrstart, addrend, addrmask, addrmirror, addrselect, read_delegate, write_delegate, unitmask, cswidth)

These six methods allow to install delegate-wrapped handlers in a live address space. Either plain or with mask, mirror and select. In the read/write case both delegates must be of the same flavor (smo stuff) to avoid a combinatorial explosion of method types. The unitmask and cswidth arguments are optional.

5.3 Direct memory range mapping

space.install_rom(addrstart, addrend, void *pointer)
space.install_rom(addrstart, addrend, addrmirror, void *pointer)
space.install_writeonly(addrstart, addrend, void *pointer)
space.install_writeonly(addrstart, addrend, addrmirror, void *pointer)
space.install_ram(addrstart, addrend, void *pointer)
space.install_ram(addrstart, addrend, addrmirror, void *pointer)

Installs a memory block in an address space, with or without mirror. _rom is read-only, _ram is read/write, _writeonly is write-only. The pointer must be non-null, this method will not allocate the memory.

5.4 Bank mapping

space.install_read_bank(addrstart, addrend, memory_bank *bank)
space.install_read_bank(addrstart, addrend, addrmirror, memory_bank *bank)
space.install_write_bank(addrstart, addrend, memory_bank *bank)
space.install_write_bank(addrstart, addrend, addrmirror, memory_bank *bank)
space.install_readwrite_bank(addrstart, addrend, memory_bank *bank)
space.install_readwrite_bank(addrstart, addrend, addrmirror, memory_bank *bank)

Install an existing memory bank for reading, writing or both in an address space.

5.5 Port mapping

space.install_read_port(addrstart, addrend, const char *rtag)
space.install_read_port(addrstart, addrend, addrmirror, const char *rtag)
space.install_write_port(addrstart, addrend, const char *wtag)
space.install_write_port(addrstart, addrend, addrmirror, const char *wtag)
space.install_readwrite_port(addrstart, addrend, const char *rtag, const char *wtag)
space.install_readwrite_port(addrstart, addrend, addrmirror, const char *rtag, const char *wtag)

Install ports by name for reading, writing or both.

5.6 Dropped accesses

space.nop_read(addrstart, addrend, addrmirror)
space.nop_write(addrstart, addrend, addrmirror)
space.nop_readwrite(addrstart, addrend, addrmirror)

Drops the accesses for a given range with an optional mirror.

5.7 Unmapped accesses

space.unmap_read(addrstart, addrend, addrmirror)
space.unmap_write(addrstart, addrend, addrmirror)
space.unmap_readwrite(addrstart, addrend, addrmirror)

Unmaps the accesses (e.g. logs the access as unmapped) for a given range with an optional mirror.

5.8 Device map installation

space.install_device(addrstart, addrend, device, map, unitmask, cswidth)

Install a device address with an address map in a space. The unitmask and cswidth arguments are optional.

5.9 View installation

space.install_view(addrstart, addrend, view)
space.install_view(addrstart, addrend, addrmirror, view)


Installs a view in a space. This can be only done once and in only one space, and the view must not have been setup through the address map API before. Once the view is installed, variants can be selected by indexing to call a dynamic mapping method on it.

A view can be installed into a variant of another view without issues, with only the usual constraint of single installation.