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21 July 2011, 22:28
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#1 |
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Dazed and Confused
Join Date: Dec 2001
Location: portsmouth/uk
Posts: 242
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C to assembler
A basic question, but it's confusing me!
 here's a bit of C code: screen->RastPort.Flags=DBUFFER is this equivalent? movea.l my_screen,a0 movea.l sc_RastPort(a0),a0 lea rp_Flags(a0),a0 move.w #2,a0 Thanks... |
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21 July 2011, 22:35
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#2 |
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Registered User
Join Date: Jan 2002
Location: Germany
Posts: 6,987
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move.l screen,a0
move.w #2,sc_RastPort+rp_Flags(a0) To correct your code, it should be move.l screen,a0 lea sc_RastPort(a0),a0 lea rp_Flags(a0),a0 move.w #2,(a0) It does the same with more instructions. |
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22 July 2011, 00:26
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#3 | |
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Computer Nerd
Join Date: Sep 2007
Location: Rotterdam/Netherlands
Age: 47
Posts: 3,753
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Quote:
 You can also do it in three instructions Code:
move.l my_Screen,a0 lea sc_RastPort(a0),a0 move.w #2,rp_Flags(a0) |
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22 July 2011, 20:45
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#4 | |
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Dazed and Confused
Join Date: Dec 2001
Location: portsmouth/uk
Posts: 242
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Quote:
 This is how my somewhat limited understanding sees this. sc_RastPort is the offset to the pointer in the screen structure of the RastPort's address. rp_Flags is the offset in the RastPort to the Flags. If I add the two offsets together, I would have thought that I would get an offset somewhere past the RastPort into the screen structure; I don't understand how I get the address of the Flags in the RastPort... Can you explain to my dim brain, how this works? 
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22 July 2011, 23:21
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#5 |
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Computer Nerd
Join Date: Sep 2007
Location: Rotterdam/Netherlands
Age: 47
Posts: 3,753
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It's actually quite simple: The RastPort structure is part of the Screen structure, which means that sc_RastPort is an offset that points to the RastPort structure instance inside the Screen structure and not to a pointer.
There are some other structures that are part of the Screen structure as well, so just make sure that you know exactly when an offset points to a pointer or a structure instance. |
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23 July 2011, 20:22
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#6 |
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Dazed and Confused
Join Date: Dec 2001
Location: portsmouth/uk
Posts: 242
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Doh! What an idiot! It's so easy when you stop and think!
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