=> R0 = 218 (&DA)
R1 = EOR value
R2 = AND value
<= R0 preserved
R1 = previous count
R2 corrupted
This call reads or sets the count of bytes in the VDU queue. The value of the count is -(bytes needed to complete VDU sequence) held in 1 byte, so if two bytes are needed to complete the current sequence then count will be &FE.
Usually this call is used to set the count to 0, which abandons any current VDU sequence. This stops error messages being garbled by a partially completed VDU sequence.
See PRM 1, page 669
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