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‘?dup’ and ‘drop’
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‘?dup’ and ‘drop’
by Jason Kemp-2
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Hi,
I understand that ?dup only compiles a dup if the previous instruction was not a drop. If it was a drop then the drop is removed and no dup is compiled either. Karig, http://www.karig.net/os/cf/c.htm#cdrop explains this. But how can it be assumed that the TOS (that was to have been dropped) and NOS are identical so that we don't need a drop followed by a dup? Thanks, Jason --------------------------------------------------------------------- To unsubscribe, e-mail: colorforth-unsubscribe@... For additional commands, e-mail: colorforth-help@... Main web page - http://www.colorforth.com |
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Re: ‘?dup’ and ‘drop’
by Jason Kemp-2
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Sorry: ignore that one. Kristopher Johnson had asked it before and got
an answer. http://www.strangegizmo.com/forth/ColorForth/msg00193.html I've just spent quite a while trying to work it out but should have looked at the archive first. Jason Kemp said the following on 17/10/2008 17:42: > Hi, > > I understand that ?dup only compiles a dup if the previous instruction > was not a drop. If it was a drop then the drop is removed and no dup > is compiled either. Karig, http://www.karig.net/os/cf/c.htm#cdrop > explains this. > > But how can it be assumed that the TOS (that was to have been dropped) > and NOS are identical so that we don't need a drop followed by a dup? > > Thanks, > Jason > > > > > > --------------------------------------------------------------------- > To unsubscribe, e-mail: colorforth-unsubscribe@... > For additional commands, e-mail: colorforth-help@... > Main web page - http://www.colorforth.com > > > --------------------------------------------------------------------- To unsubscribe, e-mail: colorforth-unsubscribe@... For additional commands, e-mail: colorforth-help@... Main web page - http://www.colorforth.com |
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Re: ‘?dup’ and ‘drop’
by Jason Kemp-2
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Oh, I get it now. Block 24 (CF05) has a perfect example:
0: ?dup c031 2, ; The ‘c031’ is ‘31 0c’ which is ‘xor eax, eax’ You want a zero put on the stack and the flags all reinitialised, so you use a ?dup just to put any old value on the stack—you don’t actually care that a dup really is dup-ing, so you’re happy to remove the ‘drop’ if there was a ‘drop’ in the word before—and then you xor it with itself to do the work. You use ‘?dup’ when you want to create a new value on the stack and you don’t care what value because you are going to modify eax directly; you’re effectively just creating a space on top of the stack. If you cared that it really was a duplicate then you would use the un-optimized ‘dup’. Jason Jason Kemp said the following on 17/10/2008 17:55: > Sorry: ignore that one. Kristopher Johnson had asked it before and > got an answer. http://www.strangegizmo.com/forth/ColorForth/msg00193.html > > I've just spent quite a while trying to work it out but should have > looked at the archive first. > > Jason Kemp said the following on 17/10/2008 17:42: >> Hi, >> >> I understand that ?dup only compiles a dup if the previous >> instruction was not a drop. If it was a drop then the drop is >> removed and no dup is compiled either. Karig, >> http://www.karig.net/os/cf/c.htm#cdrop explains this. >> >> But how can it be assumed that the TOS (that was to have been >> dropped) and NOS are identical so that we don't need a drop followed >> by a dup? >> >> Thanks, >> Jason >> >> >> >> >> >> --------------------------------------------------------------------- >> To unsubscribe, e-mail: colorforth-unsubscribe@... >> For additional commands, e-mail: colorforth-help@... >> Main web page - http://www.colorforth.com >> >> >> > > > --------------------------------------------------------------------- > To unsubscribe, e-mail: colorforth-unsubscribe@... > For additional commands, e-mail: colorforth-help@... > Main web page - http://www.colorforth.com > > > --------------------------------------------------------------------- To unsubscribe, e-mail: colorforth-unsubscribe@... For additional commands, e-mail: colorforth-help@... Main web page - http://www.colorforth.com |
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