Acorn Arcade forums: Programming: conditional execution with non-CMP instructions
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conditional execution with non-CMP instructions |
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(16:39 6/12/2000)
johnstlr (19:11 6/12/2000)
andrew (00:15 7/12/2000)
johnstlr (11:59 7/12/2000)
Phlamethrower (13:52 7/12/2000)
andrew (14:02 7/12/2000)
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andrew |
Message #4692, posted at 16:39, 6/12/2000 |
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How are the status registers affected after an instruction other than a CMP? For example a bitwise operator using GT: does the result of the instruction affect the flags or is a comparison done between the operands? Thanks Andrew |
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johnstlr |
Message #4693, posted at 19:11, 6/12/2000, in reply to message #4692 |
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Do you mean something like CMP R0,R1
LSLGT R0,R0,#2
LSLLE R1,R1,#2 ?? If so then IIRC the flags aren't effected by the LSL instructions. After the LSL the flags will still hold the result from the CMP. Of course you could use the "S" version (LSLGTS) and I'll admit to not knowing precisely what this would do.
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andrew |
Message #4694, posted at 00:15, 7/12/2000, in reply to message #4693 |
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No I mean: ANDS R0,R1,#26
ADDGT R2,R0,#1 Does the second instruction execute only if R1 was > 26 or does the value in R0 determine this? |
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johnstlr |
Message #4695, posted at 11:59, 7/12/2000, in reply to message #4694 |
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I would guess that the second instruction would only be executed if after the ANDS R0 was greater than zero. Dunno if that's right though - it's been a while since I've done any ARM. I think you might be better knocking up a quick test program.
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Phlamethrower |
Message #4696, posted at 13:52, 7/12/2000, in reply to message #4694 |
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OK.... From what I understand, adding the 'S' suffix will set the Z flag if the result is zero, the C flag if a carry occurs, and maybe the oVerflow flag as well. I'm not sure about the negative flag off-hand, but from your code: ANDS R0,R1,#26
ADDGT R2,R0,#1 Looks like it will add 1 to R0 if R1 has either the 16,8 or 2 bit's set (since R1 AND 26 would be greater than zero if any of those bits were set, and 16+8+2=26). If you want it to add 1 to (R1 AND 26) if R1 is greater than 26, then the only way I can think would be with three instructions AND R0,R1,#26
CMP R1,#26
ADDGT R2,R0,#1 |
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andrew |
Message #4697, posted at 14:02, 7/12/2000, in reply to message #4696 |
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Understood thanks.
The example was mostly arbritrary but it seems logical that the flags would be set if the result in the destination register was greater than 0.Thanks again Andrew |
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Acorn Arcade forums: Programming: conditional execution with non-CMP instructions |
