Memory allocation: Difference between revisions
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Shared memory between the CPU and connected hardware is accessed via memory-mapped ports. These appear as memory locations in the CPU's address space. However, they do not necessarily have the same properties as regular memory. Some are read-only, some are write-only, others have unusual behavior.
=={{header|68000 Assembly}}==
The <code>LINK</code> and <code>UNLK</code> instructions are designed to create [[C]]-style stack frames. The operands of the <code>LINK</code> instruction are an address register (other than A7) and a displacement (must be even and either 0 or negative.)
<lang 68000devpac>MyFunction:
LINK A6,#-16 ;create a stack frame of 16 bytes. Now you can safely write to (SP+0) thru (SP+15).
;your code goes here.
UNLK A6 ;free the stack frame
RTS</lang>
<code>LINK An, #-disp</code> is effectively equivalent to the following:
<lang 68000devpac>MOVE.L An,-(SP)
MOVEA.L SP,An
SUBA.L #disp,SP</lang>
You can use negative offsets of <code>An</code> or positive offsets of <code>SP</code> to refer to the same memory region.
=={{header|Action!}}==
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