CPMBIOS.S Bug Report
                             CPMBIOS.S Bug Report
                             CPMBIOS.S Bug Report




                                 Robert Heller
                                 Robert Heller
                                 Robert Heller


                               Fri May 16, 1986
                               Fri May 16, 1986
                               Fri May 16, 1986



























































                            Chapter 1: The Problem



               I  encountered  the  problem  while  working  with  the


          Special BIOS  function  (number  17)  to  change  the  shell
          Special
          Special


          address.   The  problem  was, the shell address would not be


          set to the address  I  passed  in  (via  A0).  Instead,  the


          system would   effectively   crash.     I  checked  my  code


          carefully  and  after  being  satisfied  that  my  code  was


          correct,  I  looked  at  CPMBIOS.S  to  see  if  there was a


          problem there.  There was.  Here is the code at the  special
                                                               special
                                                               special


          label:



******************************************************************************


*


*       Special Entry Point.


*


*       This BIOS function performs special functions.  Register D1 is


*       further decoded into additional functions.


*


*       Function codes:


*


*               0       set terminal control characters


*               1       set shell entry address


*               2       unused


*               3       turn on BIOS Xon/Xoff stalling


*               4       turn off BIOS Xon/Xoff stalling


*               5       return pointer to environment space


*



special:


        cmp.w   #5,d1           ; check the range of the BIOS function code


        bhi     badcode         ; jump if the code is out of range


        lsl.w   #2,d1           ; convert the code to a table offset


        lea     spectbl,a0      ; point to the base of the special table


        move.l  0(a0,d1.w),a0   ; fetch the address of the routine


        jsr     (a0)            ; go execute the routine



badcode:


        rts                     ; return to caller




spectbl:


        .dc.l   setterm         ; set the terminal control characters


        .dc.l   setshell        ; set the shell entry address


        .dc.l   badcode         ; unused


        .dc.l   stallon         ; turn on BIOS Xon/Xoff stalling


        .dc.l   stalloff        ; turn off BIOS Xon/Xoff stalling


        .dc.l   getenvp         ; return a pointer to the environment space




               This   code   uses   a   jump   table   to  decode  the


          sub-functions.  Register A0  is  used  to  access  the  jump


          table.  Thus  any  shell  address passed in A0 is invariably


          clobbered.


















                              Chapter 2: The Fix



               To fix this bug, I modified CPMBIOS.S to  use  register


          D2 to  pass  in  the  new  shell address instead of A0.  The


          code is as follows:



******************************************************************************


*


*       Set Shell Entry Address.


*


*       This is a Special Function.  It allows you to set the entry


*       address of a shell.  The entry address is jumped to on warm boot.


*       The default shell entry address is _ccp.


*


********This is wrong.  it must never have been tested.


*       A0 contains the new shell entry address.  The old shell entry


*       address is returned in D0.


********


**      changed to use D2 to hold new shell address. RPH Thr Feb 6, 1986


*



setshell:


        move.l  shell,d0        ; return the old shell address in D0


*       move.l  a0,shell        ; set the new shell entry address


        move.l  D2,shell        ; set the new shell entry address (patch RPH)


        rts                     ; return to caller




               After making this change to  CPMBIOS.S  and  rebuilding


          and  reinstalling  CPM.SYS,  my  program  which  changed the


          shell address worked.