Author: Christophe Theron
Date: 19:01:18 03/10/00
Go up one level in this thread
On March 10, 2000 at 02:54:42, Eugene Nalimov wrote:
>On March 10, 2000 at 02:39:39, Eugene Nalimov wrote:
>
>>On March 10, 2000 at 02:00:32, Christophe Theron wrote:
>>
>>>On March 09, 2000 at 19:23:01, John Coffey wrote:
>>>
>>>>
>>>>>So by your definition, a PC with SDRAM is 64-bit, whereas a new Pentium III with
>>>>>Rambus memory is 16-bit. Or maybe 256-bit, due to the interface to the L2 cache?
>>>>>Gets confusing quickly...
>>>>>
>>>>
>>>>
>>>>I am not going to make any claims to modern processors that I don't program.
>>>>The 68000 fetched data and instructions from memory 16 bits at a time. I don't
>>>>see anything wrong with my definition.
>>>>
>>>>John
>>>
>>>
>>>I have heard that some 64Kb limitations (for code or data) apply to the 68000. I
>>>don't understand why, as it has 32 bits registers. The 16 bits address bus is
>>>very similar to the 386sx 16 address bus (the 386sx had 32 bits registers).
>>>
>>>What's the problem with the 68000? Are there really such limitations?
>>>
>>>
>>> Christophe
>>
>>First 32k of address space (and of course last 32k) can directly addressed used
>>2 bytes address included in the instruction, not 4 bytes address, so typical
>>instruction occupies 4 bytes instead of 6. So if you'll manage to allocate all
>>your globals there, your program would be noticeable shorter. Important
>>consideration for the systems with limited ROM/RAM.
>>
>>Eugene
>
>Addition: 68000 has address mode "reg32+offset16", but does not have
>"reg32+offset32". So, to index the array that is located in first/last 32k, you
>can use address mode directly, i.e.
> mov.l 0x1234(a5), d0
>(1 instruction, 4 bytes)
>
>If array is located out of this space, you have to use something like
> mov.l a5, a3
> add.l #0x12345678, a3
> mov.l (a3), d0
>(3 instructions, 10 bytes, one extra register used)
>
>Eugene
OK! I understand better now what the problem is. Adressing an array bigger than
64K is not elegant. Reminds me a lot about these horrible segmented addressing
system of the x86 family and the near/far pointers.
Does the same problem apply to code addressing (jumps or calls limited to 16
bits offsets from current IP or something similar)?
Thanks for the info.
Christophe
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