CPUs and Address Spaces

MAME supports many different types of CPUs. This article aims to explain how the memory and address spaces for these CPUs is managed in MAME.

This article is WIP.

Address Spaces

Currently, MAME supports CPUs with up to three distinct address spaces:

1. Program space (ADDRESS_SPACE_PROGRAM) is by definition the address space where all code lives. On Von Neumann architecture CPUs, it is also where data is stored. Most CPUs are Von Neumann architecture systems, and thus comingle code and data in a single address space.
2. Data space (ADDRESS_SPACE_DATA) is a separate address space where data is stored for Harvard architecture CPUs. An example of a Harvard architecture CPU in MAME is the ADSP2100.
3. I/O space (ADDRESS_SPACE_IO) is a third address space for CPUs that have separate I/O operations. For example, the Intel x86 architecture has IN and OUT instructions which are effectively reads and writes to a separate address space. In MAME, these reads and writes are directed to the I/O space.

Note that a number of CPU architectures also have internal memory that is in a separate space or domain from the other three. Memory referenced in this way is expected to be maintained internally in the CPU core and is not exposed through the memory system in MAME.

CPUs and Bus Width

Everyone has probably heard about the "8-bit" Z80 CPU or the "32-bit" 80386 CPU. But where does this notion of "8-bit" and "32-bit" come from? When referring to CPUs, there are three metrics worth considering, all of which might be used to describe a CPU, depending on which sounds better to the marketing department (seriously).

The first possible metric is the size of the internal arithmetic units in the CPU. For example, the Motorola 68000 CPU can do arithmetic operations on 32-bit numbers internally. Does this make it a 32-bit CPU? Depends on who you ask, though most people would probably say "no", because it is at odds with the other two metrics.

The second possible metric is the width of the address bus. When a CPU goes to fetch memory, it has to tell the outside world what address it wishes to access. To do this, it drives some of the pins on the chip to specify in binary the address it wants to access, and then signals a read or a write to actually cause the memory access to occur. The number of pins available on the chip for sending these addresses is referred to as the address bus width, and ultimately controls how much memory the CPU can access. For example, the original Intel 8086 has 20 address pins on it, and could access 2²⁰ bytes (or 1 MB) of memory; thus, we say it had a 20-bit address bus. When Intel created the 80286, it increased the address bus width to 24-bit (16 MB), and then to 32-bit (4 GB) with the introduction of the 80386. Which is one reason why the 80386 is called a "32-bit" CPU.

The third possible metric is the width of the data bus. This describes how many bits of data the CPU can fetch at one time. Again, this is related to pins on the chip, so a CPU that has an 8-bit data bus can fetch 8 bits (or one byte) at a time, and has 8 pins on the CPU which either send or receive the data that goes out to memory. Almost all CPUs access memory either in 8-bit, 16-bit, 32-bit, or 64-bit chunks (though there are a few oddballs that don't follow these rules). For example, the original Motorola 68000 accessed memory in 16-bit chunks, meaning it had 16 pins on the CPU which sent/received data, and thus we say it had a 16-bit data bus width. When Motorola introduced the 68020, it doubled that data bus width to 32-bits, meaning that it could fetch twice the amount of data in a single memory access. This is why the 68020 is called a "32-bit" CPU.

So why do you need to know all of this for working with address maps? Well, the first metric is irrelevant because it doesn't apply to memory accesses, but the second two metrics describe how the CPUs deal with memory, and some of those details leak into the address maps.

MAME today supports any address bus width from 1-32 bits, and it supports data bus widths of 8, 16, 32, and 64-bit. For CPUs with oddball data bus widths, we generally round up to the next highest and clean up the details in the CPU core.

Also note that each address space can have different properties, even within the same CPU. For example, the Intel 80386 has a program address space with a 32-bit address bus width, but it has an I/O address space with a 16-bit address bus width.

Memory Layout in MAME

Hoo boy, this is a controversial topic. MAME was originally written with only 8-bit CPUs in mind. The nice thing about 8-bit CPUs is that from the CPU's perspective, all memory accesses are a single byte wide, so it doesn't matter if the system running the emulator was little endian or big endian. Eventually, however, MAME expanded and an emulator for the Motorola 68000 was added. This brought up the tough question: how do you model memory accesses for a CPU with a 16-bit data bus?

There are a few things to keep in mind here. First, because the concept of an 8-bit byte is so firmly grounded in microprocessors, almost all CPUs that have larger data busses also support accessing individual bytes. So while a 16-bit CPU can access 16 bits at a time, it also has the ability to individually access either the upper 8 bits or the lower 8 bits, effectively permitting byte-level access to memory and peripherals. This is generally taken one step further with CPUs that have a 32-bit data bus; for the most part, they can access any of the four 8-bit bytes independently, or either of the two 16-bit words independently.

The second thing to understand is that when a 16-bit CPU performs a 16-bit memory access, at the hardware level it sends out a single read or write request. The reason this becomes important is that when you write an emulator, each memory access generally translates into a function call which simulates the read/write behavior of the appropriate block of memory or I/O device. A naive approach to supporting 16-bit CPUs might be to keep all these function calls operating at a byte level only, and just perform two function calls to read two neighboring bytes if a 16-bit request is issued. The problem with this is that you make it difficult for the functions that simulate the hardware to know whether the orignal CPU request was for 8 or 16 bits, and in some cases it does make a difference.

The third thing to know is that at the hardware level, a CPU with a 16-bit databus always performs "aligned" memory accesses. That is, when communicating to outside memory or peripherals, it will only ask for even addresses. In fact, 16-bit CPUs don't actually even have a pin for the low bit of the address bus! But what about accessing individual bytes, you ask? Well, if you read a byte from an even address (say $3000), the CPU will request a read from address $3000, but will also request that only the 8 bits corresponding to the first byte be returned. If you read from an odd address (say $3001), it will also issue a read from address $3000, but will also request that only the 8 bits corresponding to the second byte be returned.

With that in mind, the way that MAME decided to support 16-bit CPUs was to define a new set of functions which were dedicated to reads and writes from 16-bit CPUs. These functions know that accesses may be either 8-bit or 16-bit, and know how to call out to special 16-bit memory handlers, which are functions that simulate the behavior of peripherals and other devices mapped into the CPU's address space. Seems straightforward enough, so where's the controversy?

The first controversial bit has to do with the 'offset' parameter to the read/write handlers. As mentioned earlier, 16-bit CPUs don't actually have a low bit on their address bus, they just specify which bytes within the word they actually want to access. To enforce this, the memory system could either have masked off the low bit, or else shifted the address one bit to the right to discard the bit. In MAME, the latter approach was taken, which can be a bit confusing. For example, an access to address $30 on a 16-bit CPU would mean that your read/write handler actually gets passed an address of $18 (and on a 32-bit CPU that would translate as an address of $0C: shifted right 2 bits). But this actually makes sense once the second controversy is explained.

The second controversial bit is how RAM and ROM memory is laid out. Because all accesses to memory are effectively 16 bits wide (potentially with some masking to extract either one byte or the other), the decision was made to store RAM and ROM natively in 16-bit chunks. What does this mean? Well, say you have a block of ROM on a Motorola M68000 (big-endian) with the following values:

$1234 $5678 $9ABC $DEF0

And let's say you have a pointer to it in your code:

UINT16 *memptr;

Then the intention is that, regardless of the endianness of the CPU that is running the emulator, if you read memptr[2], you will get the value $9ABC. That's because for a CPU with a 16-bit data bus, all memory is organized and accessed by the core in 16-bit chunks. As long as you access memory strictly through 16-bit pointers, everything will work fine. (Note also that because of the use of 16-bit pointers, having the 'offset' parameter shifted by 1 lets you use the offset directly as an array index to memory.)

To illustrate the mind-bending a bit more graphically, consider running the M68000 on an Intel x86-based CPU. The M68000 is big endian while the x86 is little endian. When accessing data as words, it goes like this:

M68000: $1234 $5678 $9ABC $DEF0
   x86: $1234 $5678 $9ABC $DEF0

But if you look at the byte level, you see it is different:

M68000: $12 $34 $56 $78 $9A $BC $DE $F0
   x86: $34 $12 $78 $56 $BC $9A $F0 $DE

For this reason, with 16-bit CPUs, you generally cannot take a UINT16 * pointer to memory, cast it to a UINT8 *, and access the individual bytes without getting incorrect results when the endianness of the native CPU is different from that of the emulated CPU.

Special Memory Types

RAM, ROM, and banks, and unmap

Read/Write Handlers

Before diving into the details of the address map macros, let's talk about read/write handlers. The purpose of an address map is to describe what MAME should do when memory within a certain range is accessed. In its most simplistic sense, it specifies a set of functions which should be called in response to memory accesses. These are the read/write handlers.

A read handler is a function which accepts an address and perhaps a mask, and returns the value obtained by "reading" memory at that address. Here is a prototype for an 8-bit read handler:

UINT8 my_read_handler(offs_t offset);

Notice a couple of things about this definition. First, I specifically said an "8-bit" read handler, and you can see that the function returns a UINT8. This means that yes, there are 4 different handler function types, one each for 8, 16, 32, and 64-bit memory accesses. Regardless of the size of data returned, however, all the functions take an offset of type "offs_t", which today is 32 bits (though in the future we may expand it to 64). Also note that it is called an "offset", not an "address". This is because the memory system in MAME always subtracts the beginning address of a memory range from the raw address before passing it into the read/write handlers. This means that the offset parameter is always the offset relative to the starting address of the range.

Similarly, a write handler is a function which accepts an address, a value, and perhaps a mask, and "writes" memory at that address.