## Display Device Driver Reference for OS/2

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### Overlapping BLTs

The following illustrations show possible overlaps that can occur when a BLT is performed. Since the source and target can overlap, the order in which bytes are processed must be chosen with care. If the order is wrong, a byte of the source might be overwritten (as a target) before the byte is used as a source. (You might remember propagating spaces through a field on the IBM* 360s and 370s).

In the following figures, S is the source, D is the target, and x is the corner from where the first byte will be fetched. The comments (on the right) tell in which directions (X and Y) the BLT must be performed to prevent the overwriting of a source byte before it is used as a source. The possible cases of overlap are as follows:

1.Disjointed (not special cased)
2.Identical (not special cased)
3.Overlapping

The following four examples are degenerate cases (along with identical and disjoint) and the directions are as shown: