🚀 go-pugleaf

Charles Richmond <nume...@aquaporin4.com> writes:
>On 1/27/2018 7:22 PM, Charlie Gibbs wrote:
>> On 2018-01-28, Dan Espen <dan1...@gmail.com> wrote:
>> 
>>> John Levine <jo...@taugh.com> writes:
>>>
>>>> In article <b204534e-f56c-4b29...@googlegroups.com>,
>>>>
>>>>> Memory addressing was a big issue in the original concept of
>>>>> System/360.  Small machines had simple addresses which saved
>>>>> memory, big machines had more complex addresses.  Providing space
>>>>> for a large address was wasteful in a small machine, most
>>>>> S/360 users were getting small ones.  As a compromise, IBM came
>>>>> up with the base+displacement method.
>>>>
>>>> All 360s other than the /20 used the same addressing, full 24 bit
>>>> addresses, instructions used base register plus 12 bit displacement
>>>> sometimes plus index register.  Base register 0 meant direct
>>>> addressing with the 12 bit displacement, but as far as I know that was
>>>> used only in little bits of startup and trap handler code, not in
>>>> applications.
>>>
>>> MVS applications often get system information
>>> by using an address at a fixed low storage location (CVT at 16).
>> 
>> It also lets you use Load Address to load a constant from 0 to
>> 4095 inclusive into a register (although if you're setting the
>> register to zero it's faster to subtract it from itself).
>> 
>
>How does that compare with the time for exclusive-oring the resister 
>with itself???  On the Intel 8080, that was the fastest way to zero the 
>accumulator...

That doesn't necessarily mean that the 8080 (and successors through Haswell)
actually do the xor.   Simply recognizing that the two registers are identical
would be sufficient to drive the result to zero.