h6. [[OS SWI Calls]] h6(. » OS_ChangedBox h2. OS_ChangedBox h5. (SWI &5A) |_<^{width:3em}. Entry | | |/4<^. R0 |<^. 0 = Disable changed box calculations | |<^. 1 = Enable changed box calculations | |<^. 2 = Reset changed box to null rectangle | |<^. -1 = Read changed box information | |_<^{width:3em}. Exit | | |<^. R0|<^. Previous enable state in bit 0 | |<^. R1|<^. Pointer to a fixed [[OS_ChangedBox Block|block]] of 5 words | h4. Use The purpose of this call is to determine which areas of the screen have changed. h4. Notes The data pointed to by R1 on exit at offsets 4 - 16 are only valid if the change box calculations were in an enabled state immediately after the call; otherwise they are undefined. This call checks which areas of the screen were changed by VDU commands and returns the smallest rectangle which covers all changed pixels. h4. See also * [[OS_ChangedBox Block]]