Page Comparison

IN:
     A = 0, always (call level, other values reserved for system use)
     B = 0
     HL = address of new error handler
     HL = 0, use default system error handler (in lower 8K of segment 0)

OUT:
     Fc = 0, always
     A = old call level
     HL = address of old error handler

Registers changed after return:
     ...CDE../IXIY same
     AFB...HL/.... different