The "Is VDU 1 in effect?" during the call to VDUCHR should be "Is VDU 21 in effect?" (not VDU 1!).
Or "Is VDU disabled by VDU 21?" to be clearer. [Jgharston, 9 September 2021]
- There isn't really a clean way to draw VDUCHR as it's both a character sink and part of the flow control. The 'Screen' terminal is the sink part of VDUCHR dragged down to sit alongside the others, whereas it's actually part of the VDUCHR process.
- The only reasons the two parts are separated are (a) to show OSWRCH forwarding to the printer under VDUCHR's control and (b) VDUCHR's own diversion to the printer under VDU 1. (The presence of two distinct paths leads to the double-printing bug in OS 1.20.) The 'Screen' encapsulates all the other display functions and VDU codes including VDU 21; the effect of the 'Screen' on VDUCHR used to be represented by an arrow which I removed long ago for clarity.