Download CPU/68000 USER`S MANUAL 68000-BASED

ADDRESS BUS
The processor board supports an extended 24 bit address bus.
This allows the CPU to directly address up to 16 megabytes of
memory.
Such a vast address space eliminates the need for
cumbersome bank select schemes. Older boards responding to only
the 16 bit address bus may be used with this CPU but this would
restrict the total system address space to 64 kilobytes.
1/0 Space vs. Memory Space
The 68000 instruction set does not have an explicit
Input/Output instruction.
Motorola archi tects intended for all
68000 I/O to be memory mapped. Memory mapped I/O takes advantage
of the many powerful addressing modes for fast, efficient I/O
routines.
To support S-100 I/O mapped peripherals the processor board
dedicates the most significant 64 kilobytes of memory to I/O. As
a result, any memory access to hex address FFOOOO through FFFFFF
results in an I/O bus cycle. That is, such an access asserts
status outputs sINP or sOUT. This configuration allows efficient
memory mapped software while maintaining full compatibility with
existing I/O devices.
For example, hex address FF0002 corresponds to I/O port with
address 02. So the 68000 instruction:
MOVE.B
OFF0002H,DO
is similar to the 8080 instruction:
IN
02H
Note that 64 kilobytes of address space are dedicated to I/O
devices. This allows over 64 thousand input and output ports.
To support this many ports requires that I/O devices decode the
least significant 16 address bits. The IEEE specification allows
extended I/O addressing but does not require it.
The majority of current I/O boards decode only the least
significant 8 address bits. This gives 256 input and output
ports. The processor board can be used with such an I/O device.
Since the I/O board does not decode the full 16 bit I/O address
its ports address is replicated throughout the 64 kilobyte I/O
address space. The processor board functions quite well with
existing I/O boards and is capable of fully supporting future
extended I/O address boards as well.
If you are using an I/O board which only decodes the low
eight bits of the address then you can use the 16-bit word
addressing mode of the 68000. Since to the I/O board address
OFF0002 is indistinguishable from OFFFF02, and the 68000 sign
extends the word long address, you can also use the address
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