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Microprocessors normally use two methods to connect
external devices: memory mapped and port mapped I/O. To understand
how to emulate microprocessors (for gaming or other purposes) it
is important to understand this subtle difference. As far
as the peripheral is concerned, both methods are really identical.
A device connected to a microprocessor must decode its address from
various numbers of address lines and read/write its data from various
numbers of data lines. The difference between the two schemes
occurs within the microprocessor. Intel has, for the most
part, used the port mapped scheme for their microprocessors and
Motorola has used the memory mapped scheme. It is certainly
possible for a hardware engineer to design a system with memory
mapped I/O which uses a CPU supporting port mapped I/O; below you
can see why that would not necessarily be a good idea. Here
are the basic differences:
Memory Mapped I/O
I/O devices are mapped into the system memory map along with RAM
and ROM. To access a hardware device, simply read or write
to those 'special' addresses using the normal memory access instructions.
The advantage to this method is that every instruction which can
access memory can be used to manipulate an I/O device. The
disadvantage to this method is that the entire address bus must
be fully decoded for every device. For example, a machine
with a 32-bit address bus would require logic gates to resolve the
state of all 32 address lines to properly decode the specific address
of any device. This increases the cost of adding hardware
to the machine.
Port Mapped I/O
I/O devices are mapped into a separate address space. This
is usually accomplished by having a different set of signal lines
to indicate a memory access versus a port access. The address
lines are usually shared between the two address spaces, but less
of them are used for accessing ports. An example of this is
the standard PC which uses 16 bits of port address space, but 32
bits of memory address space. The advantage to this system
is that less logic is needed to decode a discrete address and therefore
less cost to add hardware devices to a machine. On the older
PC compatible machines, only 10 bits of address space were decoded
for I/O ports and so there were only 1024 unique port locations;
modern PC's decode all 16 address lines. To read or write
from a hardware device, special port I/O instructions are used.
From a software perspective, I feel that this is a slight disadvantage
because more instructions are required to accomplish the same task.
For instance, if you wanted to test one bit on a memory mapped port,
there is a single instruction to test a bit in memory, but for ports
you must read the data into a register, then test the bit.
The way this applies to arcade emulation is that
the port I/O handling routines of your CPU emulator do not need
to have special flags or logic for handling reads and writes to
the port address space since every access to a port will be for
the purpose of communicating with a hardware device. Therefore,
in the simplest case, the port I/O handler routine will simply be
a switch statement on the port address. Memory mapped I/O
is harder to emulate because you must maintain some sort of flag
list for each address to know if it is ROM, RAM or a 'special' address
referencing a hardware device. See my article on CPU
emulation for a reasonable way to code this into your emulator
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