Clearing the X-bit

oRBIT · 20 April 2012, 12:29

Is there a way of clearing the X-bit in CCR without manipulating the CCR-register directly?
The 6502 has the "clc"-instruction, 680x0 doesn't have anything similar?

pmc · 20 April 2012, 13:16

Performing an add that results in no carry would clear the x flag as x == c under those circumstances.

Obviously other flags might be set or cleared in the process so this might not suit your purpose...

oRBIT · 20 April 2012, 13:22

If I recall correctly, manipulating the CCR directly wasn't very fast in terms of cycles but I guess that's the way to go then..

StingRay · 20 April 2012, 14:07

If you have a spare data register just do addx dx,dx this will clear the x flag too.

Photon · 21 April 2012, 19:56

You can get it for free if you can move the instruction that needs it to after an instruction after which the state is known, such as some addq. It doesn't even need to be immediately after, as long as it's before an instruction that could set x (usually math/shift instructions that could set carry). You can also reorder instructions whose order is not important to make that happen, so you might not even have to move the instruction.

Thorham · 21 April 2012, 22:16

Quote:

Originally Posted by StingRay

If you have a spare data register just do addx dx,dx this will clear the x flag too.

But only if the result doesn't overflow, which can still happen. Just use sub dx,dx. No overflow or underflow possible.

Leffmann · 22 April 2012, 02:52

Just do "add.w #0, d0" and you don't have to spare a register for it. Clearing the extend flag alone is something like 10/12 cycles in best case/cache case on 020 and up, so it's not too bad.

20 April 2012, 12:29	#1
oRBIT Zone Friend Join Date: Apr 2006 Location: Gothenburg/Sweden Age: 48 Posts: 339	Clearing the X-bit Is there a way of clearing the X-bit in CCR without manipulating the CCR-register directly? The 6502 has the "clc"-instruction, 680x0 doesn't have anything similar?

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20 April 2012, 13:16	#2
pmc gone Join Date: Apr 2007 Location: completely gone Posts: 1,596	Performing an add that results in no carry would clear the x flag as x == c under those circumstances. Obviously other flags might be set or cleared in the process so this might not suit your purpose...

20 April 2012, 13:22	#3
oRBIT Zone Friend Join Date: Apr 2006 Location: Gothenburg/Sweden Age: 48 Posts: 339	If I recall correctly, manipulating the CCR directly wasn't very fast in terms of cycles but I guess that's the way to go then..

20 April 2012, 14:07	#4
StingRay move.l #$c0ff33,throat Join Date: Dec 2005 Location: Berlin/Joymoney Posts: 6,863	If you have a spare data register just do addx dx,dx this will clear the x flag too.

21 April 2012, 19:56	#5
Photon Moderator Join Date: Nov 2004 Location: Eksjö / Sweden Posts: 5,602	You can get it for free if you can move the instruction that needs it to after an instruction after which the state is known, such as some addq. It doesn't even need to be immediately after, as long as it's before an instruction that could set x (usually math/shift instructions that could set carry). You can also reorder instructions whose order is not important to make that happen, so you might not even have to move the instruction.

22 April 2012, 02:52	#7
Leffmann Join Date: Jul 2008 Location: Sweden Posts: 2,269	Just do "add.w #0, d0" and you don't have to spare a register for it. Clearing the extend flag alone is something like 10/12 cycles in best case/cache case on 020 and up, so it's not too bad.

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