=> R0 = pointer to error block
<= only returns (with V flag on) if X was set
This generates an error and invokes the error handler. The X form may appear to be completely useless as it is effectively
=> no parameters passed in registers
<= all registers preserved
V flag set
however, it has at least 3 uses
1 0 terminated string conversion in BASIC
2 detecting whether OS_CallASWI is available by calling it via OS_CallASWI and seeing what comes back in R0
3 force a callback to occur by calling its X form
See PRM 1, page 45.
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