Conference hydra::amiga_v1

    True.

    Then what does it mean when they say the 68000 is 32 bits internal
    64 bits external??

Re: .0

Your friend is right.  It is quite common to define a family architecture
that defines a particular word length, and then provide implementations
that are built using hardware with smaller data paths.  All the machines
in the architecture appear to have the same word length, but some of them
are cheaper and perform poorer because they are based on narrower busses.

The 68000 is a 16 bit implementation of a 32 bit family architecture.
The 68020 is a 32 bit implementation of the same architecture.  I believe
that there is even a 68008 that is an 8 bit implementation.

The same sort of situation holds with the IBM pcs:  the PC is an 8 bit
processor, the AT is 16 bit, and the '386 high end PS/2 machine is 32
bit.

The early LSI PDP-11s were 8 bit processors that implemented a 16 bit
architecture.

    Very interesting!  So does that mean that there is a 32 bit data
    bus leading up to a 68000 which is actually 16 bits internally?

Re: .4

Sort of the other way around.  The 68000 has a 16 bit data bus and a 24
bit address bus coming off the chip.  I don't know how wide the data
paths are inside the chip; I've never seen a block diagram of the 68000.
It probably has a variety of sizes of data paths for different things
depending on functional and performance requirements.  I would expect
that it has at least 8 and 16 data paths on chip, with the possibility
of 24 and 32 bit data paths used sparingly.  (But then, what do I know?)

    
    
    	The Amiga has a 16 bit data buss. This buss connects directly
    to the 68000 ( 16 bit ) data buss. All the custom chips are also
    on a 16 bit buss. 
    
    	The Turbo Amiga and other 68020 upgrades to the Amiga have
    a local 32 bit buss that can be connected to special memory cards
    to give 32 bit performance. But when they access the Amiga buss
    they do 16 reads/writes.
    
    		Steve Peters
    

    1. The Amiga has a 16 bit bus, running at 14.28MHz.
    
    2. The Blitter and all other support chips run at 4 clocks per bus
    cycle, meaning 14.28/4 MHz.
    
    3. The 68000 likewise takes 4 clocks per bus cycle, meaning one
    memory access for every four ticks of the 14.28MHz system clock.
  
    4. The 68000 has a 16 bit ALU, so 32 bit operations are generally
    done 16 bits at a time. However, it has 32 bit registers, and some
    internal paths are 32 bits (i think).
    
    5. The 68000 has a 24 bit address space (16 Meg). Of this, 1/2 is
    reserved for fast RAM in the Amiga design, of the other half, 1M
    is used for CHIP (1/2 meg) and WILL-BE-CHIP (1/2 Meg in the 2000,500)
    
    6. The 68020 has full 32 bit internal paths, and can dynamically
    access memory 8, 16, 24 or 32 bits at a time.
    
    7. The 68020 also has 32 bit addressing, and supports virtual memory.
    
    8. The 68000, 68010 do not support the 68881 floating point processor.
    It is not much easier to use the 68881 with a 68000 than with a
    microprocessor of a different family.
    
    9. The 68020 supports the 68881 in hardware. ie you hook it up,
    and it looks like the processor does more instructions as far as
    the outside world is concerned.
    
    For more technical info on the 68020 etc, see the topic on 68020.
    You can also obtain the "68020 User's Manual" and "68881 User's
    Manual" by calling your local Motorola sales office and asking for
    it.

    68000's internal busses are all 32-bits wide.  As mentioned above,
    the only piece of 16-bit data path is the ALU.
    
    Steph