CP/M   V2.2
                              Application Note 05, 2/20/82
                              Improving the CTRL-S Function

                          Copyright   1982 by Digital Research
                   CP/M is a registered trademark of Digital Research.
                     DDT and SID are trademarks of Digital Research.
                                 Compiled November 1982

          Applicable products and version numbers:  CP/M   V2.2

          Program:  BDOS

               The CTRL-S function controls screen scrolling during CRT output.
          However,  the  system  does  not recognize CTRL-S if you type another
          character before it.

               In the following code segment procedures,  addresses  given  are
          hexadecimal  offsets  from  the  base of the CP/M system.  The CCP is
          usually located at 980H, but can be located at A00H if  a  two-sector
          boot is used.

               You can assemble the patch for your  size  memory  system.   The
          cpmbase  equals  the BDOS entry point address at locations 6 and 7 in
          the base page of memory minus 806H.  You must change this entry point
          address  when you load DDT   or SID   .  Under DDT or SID, follow the
          jump at location 5 until you find an address with a least significant
          digit  of  6.  In the following example, the cpmbase is E506H-806H or
          DD00H.

                  0005  JMP  CD00
                  CD00  JMP  D3A4
                  D3A4  XTHL
                  D3A5  SHLD E452
                  D3A8  XTHL
                  D3A9  JMP  E506

               Patch into the SYSGEN or MOVCPM image exactly as you would patch
          in  a  new  version of your BIOS, using the DDT i command followed by
          the DDT r command.  Use the same offset as  your  custom  BIOS.   The
          call at cpmbase + 950H should be cpmbase + 923H before installing the
          following code:

                  cpmbase equ     ?
                  ;
                  ;
                          org     cpmbase + 950h
                  ;
                          call    cpmbase + 92Ah
                          end


                                   1


          CP/M V2.2                                     Application Note 05


               Or, you can install the above procedure directly into MOVCPM, if
          MOVCPM.COM   is   on   your   system   disk.  The  patch  is  applied
          automatically to any size system that you build using MOVCPM.

               Make a back-up copy of MOVCPM.COM before using DDT to  make  the
          following changes:


                    A>ddt movcpm.com
                    DDT VERS 2.2
                    NEXT  PC
                    2700 0100
                    -l1350
                    1350  CALL 0923
                    1353  POP  B
                    1354  PUSH B
                    . . .

                    -a1350
                    1350 call 92a
                    1353 .
                    -g0

                    A>save 38 movcpm1.com


               Use the new program MOVCPM1.COM in  place  of  MOVCPM.COM.   Any
          CP/M  system  generated  with  MOVCPM1.COM  has  the  improved CTRL-S
          function.

               Licensed  users  are  granted  the  right   to   include   these
          modifications in CP/M V2.2 software.