58 lines
1.4 KiB
Text
58 lines
1.4 KiB
Text
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PICK and ROLL are problematic in that they require the ability
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to address the stack as if it were an array. The implementations
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here are not efficient as RETRO's stacks are *not* addressable.
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These will never be added to the standard image, but are provided
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here as an aid in porting ANS FORTH code or for those curious as
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to how such things could be added.
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# PICK
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6.2.2030 PICK
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CORE EXT
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( xu ... x1 x0 u -- xu ... x1 x0 xu )
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Remove u. Copy the xu to the top of the stack. An ambiguous
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condition exists if there are less than u+2 items on the stack
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before PICK is executed.
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~~~
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{{
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:save-stack (...-a)
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here [ depth &, times ] dip ;
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:fetch-prior (a-n[a-1])
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dup fetch swap n:dec ;
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:restore-stack (a-...)
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here swap - here n:dec swap [ fetch-prior ] times drop ;
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---reveal---
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:PICK (...n-...m)
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&Heap [ [ save-stack ] dip
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over + fetch [ restore-stack ] dip ] v:preserve ;
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}}
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~~~
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# ROLL
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6.2.2150 ROLL
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CORE EXT
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( xu xu-1 ... x0 u -- xu-1 ... x0 xu )
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Remove u. Rotate u+1 items on the top of the stack. An ambiguous
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condition exists if there are less than u+2 items on the stack
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before ROLL is executed.
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~~~
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{{
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:save-values (...n-a)
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[ , ] times here ;
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:restore-values (a-...)
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here - here swap [ fetch-next swap ] times drop ;
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---reveal---
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:ROLL (...n-...m)
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&Heap [ save-values ] v:preserve swap [ restore-values ] dip ;
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}}
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~~~
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