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545 lines
16 KiB
ArmAsm
545 lines
16 KiB
ArmAsm
;
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; Ullrich von Bassewitz, 19.03.2000
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;
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; Free a block on the heap.
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;
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; void __fastcall__ free (void* block);
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;
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;
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; C implementation was:
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;
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; void free (void* block)
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; /* Release an allocated memory block. The function will accept NULL pointers
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; ** (and do nothing in this case).
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; */
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; {
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; unsigned* b;
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; unsigned size;
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; struct freeblock* f;
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;
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;
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; /* Allow NULL arguments */
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; if (block == 0) {
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; return;
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; }
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;
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; /* Get a pointer to the real memory block, then get the size */
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; b = (unsigned*) block;
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; size = *--b;
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;
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; /* Check if the block is at the top of the heap */
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; if (((int) b) + size == (int) _hptr) {
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;
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; /* Decrease _hptr to release the block */
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; _hptr = (unsigned*) (((int) _hptr) - size);
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;
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; /* Check if the last block in the freelist is now at heap top. If so,
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; ** remove this block from the freelist.
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; */
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; if (f = _hlast) {
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; if (((int) f) + f->size == (int) _hptr) {
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; /* Remove the last block */
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; _hptr = (unsigned*) (((int) _hptr) - f->size);
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; if (_hlast = f->prev) {
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; /* Block before is now last block */
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; f->prev->next = 0;
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; } else {
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; /* The freelist is empty now */
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; _hfirst = 0;
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; }
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; }
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; }
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;
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; } else {
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;
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; /* Not at heap top, enter the block into the free list */
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; _hadd (b, size);
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;
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; }
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; }
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;
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.importzp ptr1, ptr2, ptr3, ptr4
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.export _free, heapadd
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.include "_heap.inc"
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.macpack generic
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;-----------------------------------------------------------------------------
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; Code
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_free: sta ptr2
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stx ptr2+1 ; Save block
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; Is the argument NULL? If so, bail out.
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ora ptr2+1 ; Is the argument NULL?
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bne @L1 ; Jump if no
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rts ; Bail out if yes
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; There's a pointer below the user space that points to the real start of the
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; raw block. We will decrement the high pointer byte and use an offset of 254
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; to save some code. The first word of the raw block is the total size of the
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; block. Remember the block size in ptr1.
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@L1: dec ptr2+1 ; Decrement high pointer byte
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ldy #$FF
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lda (ptr2),y ; High byte of real block address
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tax
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dey
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lda (ptr2),y
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stx ptr2+1
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sta ptr2 ; Set ptr2 to start of real block
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ldy #usedblock::size+1
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lda (ptr2),y ; High byte of size
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sta ptr1+1 ; Save it
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dey
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lda (ptr2),y
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sta ptr1
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; Check if the block is on top of the heap
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add ptr2
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tay
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lda ptr2+1
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adc ptr1+1
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cpy __heapptr
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bne heapadd ; Add to free list
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cmp __heapptr+1
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bne heapadd
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; The pointer is located at the heap top. Lower the heap top pointer to
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; release the block.
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@L3: lda ptr2
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sta __heapptr
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lda ptr2+1
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sta __heapptr+1
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; Check if the last block in the freelist is now at heap top. If so, remove
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; this block from the freelist.
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lda __heaplast
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sta ptr1
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ora __heaplast+1
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beq @L9 ; Jump if free list empty
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lda __heaplast+1
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sta ptr1+1 ; Pointer to last block now in ptr1
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ldy #freeblock::size
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lda (ptr1),y ; Low byte of block size
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add ptr1
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tax
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iny ; High byte of block size
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lda (ptr1),y
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adc ptr1+1
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cmp __heapptr+1
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bne @L9 ; Jump if last block not on top of heap
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cpx __heapptr
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bne @L9 ; Jump if last block not on top of heap
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; Remove the last block
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lda ptr1
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sta __heapptr
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lda ptr1+1
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sta __heapptr+1
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; Correct the next pointer of the now last block
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ldy #freeblock::prev+1 ; Offset of ->prev field
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lda (ptr1),y
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sta ptr2+1 ; Remember f->prev in ptr2
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sta __heaplast+1
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dey
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lda (ptr1),y
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sta ptr2 ; Remember f->prev in ptr2
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sta __heaplast
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ora __heaplast+1 ; -> prev == 0?
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bne @L8 ; Jump if free list not empty
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; Free list is now empty (A = 0)
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sta __heapfirst
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sta __heapfirst+1
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; Done
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@L9: rts
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; Block before is now last block. ptr2 points to f->prev.
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@L8: lda #$00
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dey ; Points to high byte of ->next
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sta (ptr2),y
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dey ; Low byte of f->prev->next
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sta (ptr2),y
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rts ; Done
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; The block is not on top of the heap. Add it to the free list. This was
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; formerly a separate function called __hadd that was implemented in C as
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; shown here:
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;
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; void _hadd (void* mem, size_t size)
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; /* Add an arbitrary memory block to the heap. This function is used by
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; ** free(), but it does also allow usage of otherwise unused memory
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; ** blocks as heap space. The given block is entered in the free list
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; ** without any checks, so beware!
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; */
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; {
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; struct freeblock* f;
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; struct freeblock* left;
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; struct freeblock* right;
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;
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; if (size >= sizeof (struct freeblock)) {
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;
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; /* Set the admin data */
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; f = (struct freeblock*) mem;
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; f->size = size;
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;
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; /* Check if the freelist is empty */
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; if (_hfirst == 0) {
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;
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; /* The freelist is empty until now, insert the block */
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; f->prev = 0;
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; f->next = 0;
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; _hfirst = f;
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; _hlast = f;
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;
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; } else {
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;
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; /* We have to search the free list. As we are doing so, we check
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; ** if it is possible to combine this block with another already
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; ** existing block. Beware: The block may be the "missing link"
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; ** between *two* other blocks.
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; */
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; left = 0;
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; right = _hfirst;
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; while (right && f > right) {
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; left = right;
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; right = right->next;
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; }
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;
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;
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; /* OK, the current block must be inserted between left and right (but
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; ** beware: one of the two may be zero!). Also check for the condition
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; ** that we have to merge two or three blocks.
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; */
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; if (right) {
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; /* Check if we must merge the block with the right one */
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; if (((unsigned) f) + size == (unsigned) right) {
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; /* Merge with the right block */
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; f->size += right->size;
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; if (f->next = right->next) {
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; f->next->prev = f;
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; } else {
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; /* This is now the last block */
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; _hlast = f;
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; }
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; } else {
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; /* No merge, just set the link */
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; f->next = right;
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; right->prev = f;
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; }
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; } else {
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; f->next = 0;
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; /* Special case: This is the new freelist end */
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; _hlast = f;
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; }
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; if (left) {
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; /* Check if we must merge the block with the left one */
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; if ((unsigned) f == ((unsigned) left) + left->size) {
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; /* Merge with the left block */
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; left->size += f->size;
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; if (left->next = f->next) {
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; left->next->prev = left;
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; } else {
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; /* This is now the last block */
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; _hlast = left;
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; }
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; } else {
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; /* No merge, just set the link */
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; left->next = f;
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; f->prev = left;
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; }
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; } else {
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; f->prev = 0;
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; /* Special case: This is the new freelist start */
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; _hfirst = f;
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; }
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; }
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; }
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; }
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;
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;
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; On entry, ptr2 must contain a pointer to the block, which must be at least
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; HEAP_MIN_BLOCKSIZE bytes in size, and ptr1 contains the total size of the
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; block.
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;
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; Check if the free list is empty, storing _hfirst into ptr3 for later
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heapadd:
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lda __heapfirst
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sta ptr3
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lda __heapfirst+1
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sta ptr3+1
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ora ptr3
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bne SearchFreeList
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; The free list is empty, so this is the first and only block. A contains
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; zero if we come here.
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ldy #freeblock::next-1
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@L2: iny ; f->next = f->prev = 0;
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sta (ptr2),y
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cpy #freeblock::prev+1 ; Done?
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bne @L2
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lda ptr2
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ldx ptr2+1
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sta __heapfirst
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stx __heapfirst+1 ; _heapfirst = f;
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sta __heaplast
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stx __heaplast+1 ; _heaplast = f;
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rts ; Done
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; We have to search the free list. As we are doing so, check if it is possible
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; to combine this block with another, already existing block. Beware: The
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; block may be the "missing link" between two blocks.
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; ptr3 contains _hfirst (the start value of the search) when execution reaches
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; this point, Y contains size+1. We do also know that _heapfirst (and therefore
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; ptr3) is not zero on entry.
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SearchFreeList:
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lda #0
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sta ptr4
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sta ptr4+1 ; left = 0;
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ldy #freeblock::next+1
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ldx ptr3
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@Loop: lda ptr3+1 ; High byte of right
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cmp ptr2+1
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bne @L1
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cpx ptr2
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beq @L2
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@L1: bcs CheckRightMerge
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@L2: stx ptr4 ; left = right;
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sta ptr4+1
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dey ; Points to next
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lda (ptr3),y ; right = right->next;
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tax
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iny ; Points to next+1
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lda (ptr3),y
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stx ptr3
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sta ptr3+1
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ora ptr3
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bne @Loop
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; If we come here, the right pointer is zero, so we don't need to check for
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; a merge. The new block is the new freelist end.
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; A is zero when we come here, Y points to next+1
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sta (ptr2),y ; Clear high byte of f->next
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dey
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sta (ptr2),y ; Clear low byte of f->next
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lda ptr2 ; _heaplast = f;
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sta __heaplast
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lda ptr2+1
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sta __heaplast+1
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; Since we have checked the case that the freelist is empty before, if the
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; right pointer is NULL, the left *cannot* be NULL here. So skip the
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; pointer check and jump right to the left block merge
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jmp CheckLeftMerge2
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; The given block must be inserted between left and right, and right is not
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; zero.
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CheckRightMerge:
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lda ptr2
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add ptr1 ; f + size
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tax
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lda ptr2+1
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adc ptr1+1
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cpx ptr3
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bne NoRightMerge
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cmp ptr3+1
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bne NoRightMerge
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; Merge with the right block. Do f->size += right->size;
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ldy #freeblock::size
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lda ptr1
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add (ptr3),y
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sta (ptr2),y
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iny ; Points to size+1
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lda ptr1+1
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adc (ptr3),y
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sta (ptr2),y
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; Set f->next = right->next and remember f->next in ptr1 (we don't need the
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; size stored there any longer)
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iny ; Points to next
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lda (ptr3),y ; Low byte of right->next
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sta (ptr2),y ; Store to low byte of f->next
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sta ptr1
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iny ; Points to next+1
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lda (ptr3),y ; High byte of right->next
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sta (ptr2),y ; Store to high byte of f->next
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sta ptr1+1
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ora ptr1
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beq @L1 ; Jump if f->next zero
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; f->next->prev = f;
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iny ; Points to prev
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lda ptr2 ; Low byte of f
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sta (ptr1),y ; Low byte of f->next->prev
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iny ; Points to prev+1
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lda ptr2+1 ; High byte of f
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sta (ptr1),y ; High byte of f->next->prev
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jmp CheckLeftMerge ; Done
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; f->next is zero, this is now the last block
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@L1: lda ptr2 ; _heaplast = f;
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sta __heaplast
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lda ptr2+1
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sta __heaplast+1
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jmp CheckLeftMerge
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; No right merge, just set the link.
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NoRightMerge:
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ldy #freeblock::next ; f->next = right;
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lda ptr3
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sta (ptr2),y
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iny ; Points to next+1
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lda ptr3+1
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sta (ptr2),y
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iny ; Points to prev
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lda ptr2 ; right->prev = f;
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sta (ptr3),y
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iny ; Points to prev+1
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lda ptr2+1
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sta (ptr3),y
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; Check if the left pointer is zero
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CheckLeftMerge:
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lda ptr4 ; left == NULL?
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ora ptr4+1
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bne CheckLeftMerge2 ; Jump if there is a left block
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; We don't have a left block, so f is actually the new freelist start
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ldy #freeblock::prev
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sta (ptr2),y ; f->prev = 0;
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iny
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sta (ptr2),y
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lda ptr2 ; _heapfirst = f;
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sta __heapfirst
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lda ptr2+1
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sta __heapfirst+1
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rts ; Done
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; Check if the left block is adjacent to the following one
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CheckLeftMerge2:
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ldy #freeblock::size ; Calculate left + left->size
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lda (ptr4),y ; Low byte of left->size
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add ptr4
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tax
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iny ; Points to size+1
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lda (ptr4),y ; High byte of left->size
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adc ptr4+1
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cpx ptr2
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bne NoLeftMerge
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cmp ptr2+1
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bne NoLeftMerge ; Jump if blocks not adjacent
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; Merge with the left block. Do left->size += f->size;
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dey ; Points to size
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lda (ptr4),y
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add (ptr2),y
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sta (ptr4),y
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iny ; Points to size+1
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lda (ptr4),y
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adc (ptr2),y
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sta (ptr4),y
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; Set left->next = f->next and remember left->next in ptr1.
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iny ; Points to next
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lda (ptr2),y ; Low byte of f->next
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sta (ptr4),y
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sta ptr1
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iny ; Points to next+1
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lda (ptr2),y ; High byte of f->next
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sta (ptr4),y
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sta ptr1+1
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ora ptr1 ; left->next == NULL?
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beq @L1
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; Do left->next->prev = left
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iny ; Points to prev
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lda ptr4 ; Low byte of left
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sta (ptr1),y
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iny
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lda ptr4+1 ; High byte of left
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sta (ptr1),y
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rts ; Done
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; This is now the last block, do _heaplast = left
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@L1: lda ptr4
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sta __heaplast
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lda ptr4+1
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sta __heaplast+1
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rts ; Done
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; No merge of the left block, just set the link. Y points to size+1 if
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; we come here. Do left->next = f.
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NoLeftMerge:
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iny ; Points to next
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lda ptr2 ; Low byte of left
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sta (ptr4),y
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iny
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lda ptr2+1 ; High byte of left
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sta (ptr4),y
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; Do f->prev = left
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iny ; Points to prev
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lda ptr4
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sta (ptr2),y
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iny
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lda ptr4+1
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sta (ptr2),y
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rts ; Done
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