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mhindsbo
Registered: Dec 2014
Posts: 51 |
ROL'in a byte onto itself
What is the fastest and/or shortest code for rolling a pattern through a byte? I use it to trigger an action following a certain pattern. I came up with (18 cycles worst case / 12 bytes):
clc
lda pattern
bpl no_carry
sec
no_carry rol pattern
bcs take_action
if your pattern ends on a 1 (bit 0 = 1) then you could also do (17 cycles worst case / 13 bytes)
lda pattern
bne @cont
lda #%00110101 ; restore pattern
@cont asl
sta pattern
bcs take_action
any other variations? |
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mhindsbo
Registered: Dec 2014
Posts: 51 |
i like Hoogo's for both its speed and size and Groepaz if you have the mem and need the long patterns. this was good input. thanks all. |
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JackAsser
Registered: Jun 2002
Posts: 2014 |
Quote: asl pattern
bcc no_action
inc pattern
This is excellent. I never thought of it.
1) Smaller
2) Faster
3) Doesn't clobber any registers except Status.
=> Perfect optimization. 10/10 rating. |
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Oswald
Registered: Apr 2002
Posts: 5094 |
yup. +1 |
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Frantic
Registered: Mar 2003
Posts: 1648 |
Yes, very nice.
@Hoogo: I'd like to add this to codebase, but before doing so I just want to ask if you would rather prefer to do it yourself?
(A possible modification of this might be to use 0 as "take action flag" instead of 1, so that the inc is only performed when no particular action is to be taken, which might smoothen the averate cpu cycle consumtion each time the code is executed including the execution of the code for the "action".) |
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Oswald
Registered: Apr 2002
Posts: 5094 |
btw never thought of this, but RMW instructions work so, that the 6510 reads the byte into an 'inner' alu, so it can leave A alone ? |
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JackAsser
Registered: Jun 2002
Posts: 2014 |
Quote: btw never thought of this, but RMW instructions work so, that the 6510 reads the byte into an 'inner' alu, so it can leave A alone ?
Not quite. There is not a single ALU, there are increment unit, shift units etc. Each unit can be used by different sources. Check http://vignette1.wikia.nocookie.net/microchip/images/c/c9/MOS-6.. for more in-depth. |
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Bitbreaker
Registered: Oct 2002
Posts: 508 |
For a more readable version try this :-)
http://vignette1.wikia.nocookie.net/microchip/images/c/c9/MOS-6..
or:
http://www.weihenstephan.org/~michaste/pagetable/6502/6502.jpg |
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Oswald
Registered: Apr 2002
Posts: 5094 |
Quote: Not quite. There is not a single ALU, there are increment unit, shift units etc. Each unit can be used by different sources. Check http://vignette1.wikia.nocookie.net/microchip/images/c/c9/MOS-6.. for more in-depth.
;) |
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Hoogo
Registered: Jun 2002
Posts: 105 |
Quoting Frantic@Hoogo: I'd like to add this to codebase, but before doing so I just want to ask if you would rather prefer to do it yourself? Don't bother, just add it. I'm a little surprised by all this reaction... |
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Frantic
Registered: Mar 2003
Posts: 1648 |
Quote: Quoting Frantic@Hoogo: I'd like to add this to codebase, but before doing so I just want to ask if you would rather prefer to do it yourself? Don't bother, just add it. I'm a little surprised by all this reaction...
I guess nerdy coders like us find the simplicity of such small and simple tricks beautiful.
https://codebase64.org/doku.php?id=base:rotate_byte_and_act_on_..
(Couldn't come up with a sensible name for the article really, but hopefully it is comprehensible.) |
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