mame/docs/source/techspecs/memory.rst

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
programatically.


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.

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 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 share()
method to get th memory_share object and the 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
allows to set a relationship between 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
assotiated 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 memshare device method retrieves a memory share 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 readonly 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 memshare device method retrieves a memory share by name.  Beware
that the lookup can be expensive, prefer finders instead.


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 defines
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, byte
selection...).

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


4.2 Global configurations
~~~~~~~~~~~~~~~~~~~~~~~~~

4.2.1 Global masking
''''''''''''''''''''

| map.global_mask(offs_t mask);

Allows to indicates 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_low();
| map.unmap_value_high();
| 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
''''''''''''''''''''''''''''''''''

| (...).r(FUNC(my_device::read_method))
| (...).w(FUNC(my_device::write_method))
| (...).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
less wide than the bus itself (for instance a 8-bits device on a
32-bits 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 drives 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 a finder
(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}(FUNC([...](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}(FUNC([...](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}(FUNC(read), FUNC(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 6 available for methods.  One can either use FUNC() 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
''''''''''''''''''''''''''

| (...).rom()
| (...).writeonly()
| (...).ram()

Selects the range to access a memory zone as readonly, writeonly or
readwrite respectively.  Specific handle qualifiers allow to tell
where this memory zone should be.  There are two cases when no
qualifier is acceptable:

* ram() gives an anonymous ram zone not accessibles 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 allows to make a readonly zone point to the
contents of a given region at a given offset.

| (...).rom().share("name")
| (...).writeonly.share("name")
| (...).ram().share("name")

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


4.3.5 Bank access
'''''''''''''''''

| (...).bankr("name")
| (...).bankw("name")
| (...).bankrw("name")

Sets the range to point at the contents of a bank is read, write or
readwrite mode.


4.3.6 Port access
'''''''''''''''''

| (...).portr("name")
| (...).portw("name")
| (...).portrw("name")

Sets the range to point at an i/o port.


4.3.7 Dropped access
''''''''''''''''''''

| (...).nopr()
| (...).nopw()
| (...).noprw()

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


4.3.8 Unmapped access
'''''''''''''''''''''

| (...).unmapr()
| (...).unmapw()
| (...).unmaprw()

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
'''''''''''''''

| (...).mirror(mask)

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.


4.4.2 Masking
'''''''''''''

| (...).mask(mask)

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


4.4.3 Selection
'''''''''''''''

| (...).select(mask)

Only valid with handlers, the range will be mirrored as with mirror,
but the mirror address bits are kept 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 actual
device with 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
''''''''''''''''''''''''''''''''''''''

| (...).cselect(16/32/64)

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 allows to tell 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).


5. Address space dynamic mapping API
------------------------------------

5.1 General API structure
~~~~~~~~~~~~~~~~~~~~~~~~~

A series of methods allow to change the bus decoding of an address
space on the fly.  They're powerful but have some issues:
* changing the mappings repeateadly 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 puts them all together as method
parameters.  To make things a little more readable lots of them are
optional though, the optional ones being written in italics.


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 12
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 of with mask, mirror and select.  In the
readwrite case both delegates must be of the same flavor (smo stuff)
to avoid a combinatorial explosion of method types.

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 readonly, 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 for reading, writing or both an existing memory bank 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 read, write or both ports by name.

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.