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POReturning from ARM7 nested subroutine
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2012-11-15T21:18:55.363
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2012-11-16T02:12:19.267
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<p>Ok, I am trying to implement the selection sort algorithm in ARM7 Assembly. It starts with last element of array and works its way down to first element. It works for the first swap, but then exits before continuing on. I talked to my professor and he said to use the stack variables so that the lr doesnt get overwritten etc, but I still dont understand exactly how to do that. Ive searched for ways to do this, but cant seem to find anything that I understand how to implement in my code. </p> <p>My code is as follows: (a1 is pointer to char array of length a2. Both are passed in via C) </p> <pre><code>sort2: stmdb sp!, {v1-v8, lr} @ Copy registers to stack mov v1, a2 @store number of elements in v1,v2 (a2 is passed in via C) mov v2, a2 sub v1, v1,#1 @subtract 1 from a2 to get number last element in array sub v2, v1,#1 @subtract 1 from v1 to get element just below last element bl loop2 ldmia sp!, {v1-v8, pc} loop2: ldrb v4, [a1,v1] @store value of last element in array a1(passed in via C) into r4 ldrb v5, [a1,v2] @store value of next to last element in array a1(passed in via C) into r4 cmp v4, v5 blt swap2 @if last element is less than current element a[v2], swap them cmp v2, #0 @if v2 = 0, we cycled through one iteration subeq v1, v1,#1 @so now move on to a[v1-1] element in array moveq v2, v1 @if v2 = 0, move the value of v1 into v2 sub v2, v2,#1 @subtract 1 from v2 to get element below a[v1] cmp v1, #0 @if v1 = 0, were done bne loop2 mov pc, lr @return to sort2 swap2: mov v8, lr @store return address in v8 ldrb v6, [a1,v2] @store value of a[v2] into v6 strb v4, [a1,v2] @store v4 into a[v2] strb v6, [a1,v1] @store value of v6 into a[v1] mov lr, v8 @falls through and exits here instead of returning to loop2 </code></pre>
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<assembly><arm><isa><arm7>
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Returning from ARM7 nested subroutine
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