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BigEd
PostPosted: Mon Dec 20, 2010 4:37 pm 
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Joined: Thu Dec 11, 2008 1:28 pm
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Location: England
This is old news, but there's a little programming quiz in this 2006 blog posting: what does the following do (by way of reading the value in BYTE and producing a result in A)?
Code:
 LDA BYTE
 LSR
 LSR
 LSR
 ADC BYTE
 ROR
 LSR
 LSR
 ADC BYTE
 ROR
 LSR
 LSR
 RTS

The code is taken from an early issue of Bob Sander-Cederlof's newsletter Apple Assembly Line - which is handy to search using a technique like this:
http://www.google.com/search?q=site:web ... gs+monitor
http://www.google.com/search?q=site:web ... quare+root

There's also a downloadable archive here courtesy of Scott Alfter, as noted nearby in the Source Code Repository.
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8BIT
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PostPosted: Mon Dec 20, 2010 5:21 pm 
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Posts: 1748
Location: Sacramento, CA
It appears to be a divide by 7 routine. Very clever!
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dclxvi
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PostPosted: Wed Dec 22, 2010 1:59 am 
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Posts: 362
Somewhere I've got a routine that generates object code for that type of division routine. You pass it N, and you get a divide-by-N routine. It was inspired by that AAL article/routine.
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teamtempest
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PostPosted: Wed Dec 22, 2010 4:41 am 
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Posts: 411
Location: Minnesota
Code:
 LDA BYTE  ; byte
 LSR       ; byte/2
 LSR       ; byte/4
 LSR       ; byte/8
 ADC BYTE  ; byte + byte/8 = 9/8 byte
 ROR       ; (9/8 byte)/2
 LSR       ; (9/8 byte)/4
 LSR       ; (9/8 byte)/8 = 9/64 byte
 ADC BYTE  ; byte + 9/64 byte = 73/64 byte
 ROR       ; (73/64 byte)/2
 LSR       ; (73/64 byte)/4
 LSR       ; (73/64 byte)/8 = 73/512 byte
 RTS
 


- 73 * 7 = 511
- 512/73 = 7.01369863, where everything after the decimal point repeats indefinitely

Not that I would have recognized this without it already having been pointed out (really, uncommented code that does obscure things...). Then I had to see how it worked.
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BigEd
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PostPosted: Thu Dec 23, 2010 8:50 pm 
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Location: England
Thanks for the annotation - it helped me realise why (exactly) two RORs were in there: the additions produce a 9 bit result, because we multiplied by a little more than one.
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kc5tja
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PostPosted: Fri Dec 24, 2010 6:20 am 
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Posts: 1706
I never would have figured this out just by looking at the code. Wow, that is slick. :)
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