OS_Sci, Alter screen information

IN:
     A = reason code
          SC_LR0 ($01)   LORES0 (512 bytes granularity, 13 bits width)
          SC_LR1 ($02)   LORES1 (4K granularity, 10 bits width)
          SC_HR0 ($03)   HIRES0 (8K granularity, 9 bits width)
          SC_HR1 ($04)   HIRES1 (2K granularity, 11 bits width)
          SC_SBR ($05)   screen base (2K granularity, 11 bits width)

  B = 0, get pointer address (address is not set)
  B <> 0, set pointer address with BHL = new pointer address (L always = 0)
  and HL is truncated according to screen granularity

OUT: 
     Fc = 0
     BHL = old pointer address

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