Address Maps

Address maps define how the address space of a CPU is layed out. This article aims to explain how address maps are declared and modified. Before reading this article, you might also want to check out CPUs and Address Spaces.

This article is WIP.

Address Map Structure

A typical address map looks like this (this example is taken from the qix.c driver):

static ADDRESS_MAP_START( main_map, ADDRESS_SPACE_PROGRAM, 8 )
    AM_RANGE(0x8000, 0x83ff) AM_RAM AM_SHARE(1)
    AM_RANGE(0x8400, 0x87ff) AM_RAM
    AM_RANGE(0x8800, 0x8bff) AM_READNOP   /* 6850 ACIA */
    AM_RANGE(0x8c00, 0x8c00) AM_MIRROR(0x3fe) AM_READWRITE(qix_video_firq_r, qix_video_firq_w)
    AM_RANGE(0x8c01, 0x8c01) AM_MIRROR(0x3fe) AM_READWRITE(qix_data_firq_ack_r, qix_data_firq_ack_w)
    AM_RANGE(0x9000, 0x93ff) AM_READWRITE(pia_3_r, pia_3_w)
    AM_RANGE(0x9400, 0x97ff) AM_READWRITE(pia_0_r, qix_pia_0_w)
    AM_RANGE(0x9800, 0x9bff) AM_READWRITE(pia_1_r, pia_1_w)
    AM_RANGE(0x9c00, 0x9fff) AM_READWRITE(pia_2_r, pia_2_w)
    AM_RANGE(0xa000, 0xffff) AM_ROM
ADDRESS_MAP_END

As you can see, it relies heavily on macros to do the heavy lifting. In the current implementation (as of March, 2008), the macros expand into a small "constructor" function. In the future, they may just boil down to a simple data-driven tokenization. Regardless, don't worry about the actual behavior of the macros, just what they mean.

Each address map starts with an ADDRESS_MAP_START declaration. This declaration takes 3 parameters. The first parameter (main_map) is the name of the variable you are defining. Each memory map is associated with a variable name so that you can reference it in your machine configuration. The second parameter (ADDRESS_SPACE_PROGRAM) simply specifies which address space the memory map is intended for. This helps MAME ensure that you don't mix memory maps inappropriately. The final parameter (8) is the data bus width, which again is used as a cross-check against the CPU's defined data bus width for the address space you are working with.

Following the ADDRESS_MAP_START declaration is a list of address ranges. Each range starts with a begin/end address pair wrapped in an AM_RANGE macro, followed by a series of macros that describe how to handle memory accesses within that range. The details of each macro will be described in detail below.

Finally, there is an ADDRESS_MAP_END macro which ties everything up.

A few general comments about the address map above:

• First, note that this address map has everything listed in nice ascending order. This is not required, though it is usually recommended for readability.

• Second, note that there are no overlapping ranges. This is also not a requirement. Entries in the address map are always processed in reverse order, starting from the bottom and working up to the top. So any overlapping ranges which appear earlier in the list will take precedence over ranges which appear later.

Address Map Macros

Below is a comprehensive list of the supported macros and what they mean.

AM_RANGE

AM_RANGE(start, end) 

The primary purpose of this macro is to declare a memory range. Any AM_* macros which follow implicitly apply to the most recently declared range. The AM_RANGE macro takes two parameters which specify an inclusive range of consecutive addresses beginning with start and ending with end (that is, an address hits in this bucket if the address >= start and address <= end).

AM_READ, AM_WRITE, AM_READWRITE

AM_READ(readhandler)
AM_WRITE(writehandler)
AM_READWRITE(readhandler, writehandler)

These macros provide pointers to functions that will be called whenever a read or write within the current range is detected. The actual prototypes and behaviors of the readhandler and writehandler functions will be described later. However, it is important to note that there is strict typechecking on the function pointers, especially in terms of data bus width, to prevent you from specifying a 16-bit readhandler in an 8-bit address map (recall that the data bus width of the address map was specified in the ADDRESS_MAP_START macro).

Instead of passing the raw address to the read/write handlers, the memory system actually passes an offset relative to the start address provided in the AM_RANGE macro. This allows for common handlers regardless of where the component is actually mapped in the address space.

In addition to regular function pointers, a small number of static identifiers are also permitted. For example, in an 8-bit address map, you can specify a readhandler of MRA8_RAM to specify a dynamically allocated region of RAM, or a writehandler of MWA8_UNMAP to specify that the current address range is unmapped for writes. More information on the supported static handler types is provided later.

The AM_READWRITE macro is really just a shortcut for AM_READ followed by AM_WRITE.

AM_READ_PORT

AM_READ_PORT(tag)

This macro is an alternate way of specifying a read handler for an input port. Since it is preferred that input ports are referenced by tag, you can use this macro to have MAME automatically look up the tagged port and substitute the correct input port handler.

AM_DEVREAD, AM_DEVWRITE, AM_DEVREADWRITE

AM_DEVREAD(type, tag, readhandler)
AM_DEVWRITE(type, tag, writehandler)
AM_DEVREADWRITE(type, tag, readhandler, writehandler)

These three macros follow the same pattern as the previous set of macros, except that they are used for device-specific read/write handlers. The only difference between a regular read/write handler and a device read/write handler is that the former is passed a pointer to the currently live running_machine, while the latter is passed a pointer to a specific device. The intention here is that if you have allocated a device in your machine's configuration (via MDRV_DEVICE_ADD), then the read/write handlers appropriate for that device should be invoked with a reference to that devices rather than a global pointer to the machine.

To specify which device you wish to pass to the read/write handler, you provide the device's type and tag, which is used to look up the device.

AM_MASK

AM_MASK(mask)

Specifies a bitmask which applies to the offset that is passed to the read/write handlers. By default, there is no mask, and the read/write handlers are passed in the raw address minus the start address of the current address range. If a mask is provided, this bitmask is applied in an AND operation after subtracting the start address. Thus, the value passed to the read/write handlers is really ((address - start) & mask).

AM_MIRROR

AM_MIRROR(mirror)

This macro specifies the "mirror mask" for the current address range. There are two ways to understand a mirror mask; hopefully at least one of them makes sense!

• A hardware-centric interpretation would describe a mirror mask as essentially a bitmask consisting of all bits that are ignored when the address is decoded by the hardware. Most arcade hardware does not fully decode each address; rather, in order to save on chip counts, the hardware is set up to do the minimum necessary work to separate accesses to different components in the system, and many bits are ignored. For example, in Pac-Man, bits 13 and 15 are not used at all when deciding whether an access should be directed to spriteram. Thus, the mirror mask is set as $A000.

• A software-centric interpretation would be that each bit in the mirror mask describes a "mirror" of the address range at a different address in the system. Looking again at the Pac-Man example, spriteram is traditionally thought of as existing at address $4FF0. But it turns out that you can also access it at $6FF0, $CFF0, and $EFF0, due to the fact that the hardware does not care whether bits 13 and 15 are 0 or 1. So the mirror mask of $A000 means that the memory system will replicate this address range to automatically create these mirrors by going through each bit of the mirror mask and mapping the range with that bit set to 0 and then to 1.

Note that the mirroring is by default completely hidden to the read/write handlers. This is done by making the default AM_MASK value for a mirrored range equal to the logic NOT of the mirror. In the case of Pac-Man above, for example, the mask would be ~$A000 = $5FFF. Looking at an example access to $CFF7, we would subtract the base address of $4FF0, giving an offset of $8007. Then we apply the mask of $5FFF to get the final offset of $0007.

If you want your read/write handler to see the full address with no masking, you can provide an explicit AM_MASK which will override the default value and enable you to specify which bits you wish to see.

AM_REGION

AM_REGION(region, offset)

This macro is only useful if you used AM_READ or AM_WRITE and specified a reference to RAM, ROM, or a BANK. By default, in these cases memory is either allocated (RAM and BANK) or assumed to point to the memory region corresponding to the relevant CPU (ROM). When you use the AM_REGION macro, you are overriding this default behavior, specifying instead a particular memory region and an offset within that region which corresponds to the start address of the memory range.

AM_SHARE

AM_SHARE(index)

This macro has limitations similar to AM_REGION, in that it only makes sense when used with RAM, ROM, or a BANK. However, instead of specifying an explicit memory region and offset, you instead specify a non-zero index. The first memory range that is encountered with an AM_SHARE allocates its memory in the default fashion. However, subsequent ranges which also use AM_SHARE and which reference the same index override this default behavior and point to the exact same memory that the first instance referenced.

This is primarily used to map shared memory between multiple CPUs. For example, if CPU #1 has RAM in the region $4000-$4fff, and CPU #2 has that same RAM mapped in the region $8000-$8fff, you can specify AM_SHARE(1) next to each one. When building up the memory system, AM_SHARE(1) is seen first for CPU #1, and it is allocated as normal RAM. Shortly afterwards, AM_SHARE(1) is see a second time for CPU #2, but instead of allocating memory, we simply point back to the same RAM that was allocated for CPU #1.

Note that multiple independent shared regions can be managed this way, by using a different value for index. Also note that this sharing technique only works between CPUs with the same data bus width (e.g., 8-bit to 8-bit, or 16-bit to 16-bit). If there is shared RAM between, say, an 8-bit CPU and a 16-bit CPU, then you need to write your own handlers to manage that RAM.

AM_BASE, AM_SIZE

AM_BASE(base)
AM_SIZE(size)

These macros are a convenience for driver writers. Since the memory system will allocate memory automatically for certain types of address ranges, you need a way to get ahold of a pointer to that memory so you can examine it. The AM_BASE macro takes a pointer to an appropriately-sized pointer (e.g., a pointer to a UINT8 * for 8-bit data bus), and fills it in after the memory system is initialized with the address of the memory that was allocated. In a similar fashion, the AM_SIZE macro takes a pointer to a size_t and returns in it the size (end + 1 - start) of the range referenced.

AM_BASE_MEMBER, AM_SIZE_MEMBER

AM_BASE_MEMBER(struct, member)
AM_SIZE_MEMBER(struct, member)

These two macros are variants of the standard AM_BASE and AM_SIZE macros which are designed to work in a newer more object-oriented style. Drivers now have a pointer in the running_machine object which contains driver-specific data. But since the memory for this data is allocated dynamically, you cannot use the regular AM_BASE and AM_SIZE macros to point to an address where the pointer or size should be stored. To make this work, you instead use the AM_BASE_MEMBER macro to specify the type of struct that will be allocated and the name of the struct member where you want the data to be stored.