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Atari coding BBS
| Re: LINK + UNLK instructions |
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Posted by: ray
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Mar,01.2009-23:54
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hi there sunny,
make sure you understand what the link instruction does and what it's actually meant for... i.e. reserve a local stack-frame e.g. for a subroutine, or small chunks of dynamically allocated memory.
link will
1. stack the specified address-register
2. save the current stackpointer to the specified address-reg.
3. increment the stackpointer by the given displacement
since the stack grows into the lower address-range, it only makes sense to use link with negative displacement, allowing to put your new stack "on-top" (below, in terms of addresses) of your local-frame, now accessed by the address-reg. specified along with the link-instruction. also keep in mind your local stackpointer will point to the bottom (top in terms of addresses) of the allocated frame.
giving a positive displacement, would lead to your frame being overwritten by new return-addresses/whatsoever stacked by possibly executed iterrupt-service-routines since the "linking" an so on.
using a simple "lea.l 4(sp),a6" would be of more use for the situation posted above. since your point is skipping the stacked return-address in order to directly access the stacked parametes (which is fine and which is the prevalent method) rather then actually needing to allocate a stackframe, which would make sense in case you run out of registers in your subroutine, for example.
best regards,
ray//.tSCc.
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