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OS_ChangedBox
Programmer's Reference Manuals
» Part 15 – SWI Calls
» OS
» OS_ChangedBox
OS_ChangedBox
(SWI &5A)
| Entry | |
|---|---|
| R0 | 0 = Disable changed box calculations |
| 1 = Enable changed box calculations | |
| 2 = Reset changed box to null rectangle | |
| -1 = Read changed box information |
| Exit | |
|---|---|
| R0 | Previous enable state in bit 0 |
| R1 | Pointer to a fixed block of 5 words |
Use
The purpose of this call is to determine which areas of the screen have changed.
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.