Author: Eugene Nalimov
Date: 23:54:42 03/09/00
Go up one level in this thread
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
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