collapseos/blk/290

lbluflw BSET ( abortUnderflow )
    DE BIN( @ 0x06 ( uflw ) + LDd(i),
    JR, lblexec BWR
lblchkPS BSET ( chkPS )
( Note that you only need to call this in words that push
  back to PSP. If they don't, calling chkPS is redundant with
  check in next )
    EXX,
( We have the return address for this very call on the stack
  and protected registers. 2- is to compensate that. )
    HL PS_ADDR 2- LDdi,
    SP SUBHLd,
    EXX,
    CNC RETc, ( PS_ADDR >= SP? good )
    JR, lbluflw BWR