| Re: zero latency context switch - The Best Methods !! |
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Group: comp.arch.embedded · Group Profile
Author: donalddonald Date: May 16, 2008 05:27
Vladimir Vassilevsky wrote:
>
>
> David Brown wrote:
>> Wim Lewis wrote:
>>
>>> In article giganews.com>,
>>> Bill Leary msn.com> wrote:
>>>>>>> "CBFalconer" yahoo.com> wrote in message
>>>> news:482BBFBC.71A71101@yahoo.com...
>>>>>>>>> Remember the Z80? One instruction flipped you to the
>>>>> alternate register set (ABCDEHL).
>>>>
>>>> Two instructions.
>>>>
>>>> 08 EX AF,AF' ; AF <-> AF'
>>>> D9 EXX ; BC/DE/HL <-> BC'/DE'/HL'
>>>
>>>
>>> I wonder why EX AF, AF' was made a distinct instruction?
>>>
>>
>> The EX instructions and the alternative registers can be used for more
>> than just a fast context switch. The EX AF,AF' in particular lets you
>> hold two accumulators in the air at a time. It's about twenty years
>> since I last programmed a Z80, so I can't remember any examples.
>
> What I disliked about Z80 is that there is no way to know which set of
> registers is current. BTW, the ADSP21xx and x51 provide for alternative
> sets, too.
The x51 has four register banks, however only one accumulator and only
one Xmem pointer register.
Make doing any kind of math a real pain.
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