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�ASSEMBLY LANGUAGE COURS PART IX �by Mark van den Boer

�System Control instructions�

In this part of the course the last group of instructions will be 
explained.  This group of instructions deals with the supervisor-
mode  and are therefore sometimes referred to as  system  control 
instructions.  To  remind  you:  the  S-bit  in  the  SR  (status 
register)  of  the  68000  determines whether  the  68000  is  in 
supervisor-mode or not.  Many of these instructions deal with  so 
called  exceptions.  Exception is another word for interrupt  and 
these are used to force program control immediately to a specific 
routint exception handler routine.  Exceptions are used to detect 
situations  that  are  urgent and need to  be  handled  directly. 
Therefore  every  exception has a vector  assigned  to  it.  This 
vector is a pointer to a routine which performs some action which 
should  be  taken when such an exception  occurs.  The  exception 
vectors are located in the first 256 longwords of memory.




Instruction:   CHK
Syntax:        CHK <ea>,Dn
Data sizes:    word
Condition codes affected:
               X    not affected
               N    Set if Dn is less than zero,  cleared if <ea> 
                    less than Dn, in all other cases undefined
               Z
               V
               C    always undefined












Addressing modes allowed:
Source:
          Dn
          (An)
          (An)+
          -(An)
          w(An)
          b(An,Rn)
          w
          l
          w(PC)
          b(PC,Rn)
          #
Destination:
          Dn







Function: Compares the contents of the effective address  operand 
          with  the data-register.  If the data register is  less 
          than zero (the data register is always considered to be 
          a  signed word) or greater than the contents  of  <ea>, 
          then   an  exception  occurs.   The  pointer  to   this 
          exception-routine  is  located  at  address  $18.  This 
          instruction  is  used to check if a  data  register  is 
          within  a  range.   It  is  often  used  by  high-level 
          languages   such  as  PASCAL  to  perform   array-bound 
          checking.
Examples:
Instruction              Before              After
CHK #50,D0               D0=45               D0=45
                         No exception occured, if D0 had been 51
                         or greater then an exception would have
                         occured.


Instruction:   MOVE USP       �(privileged instruction)�
Syntax:        MOVE USP,An    or   MOVE An,USP
Data sizes:    long

Condition codes affected:
               X
               N
               Z
               V
               C    not affected
Addressing modes allowed:
     See syntax
Function: As  you all should know,  the 68000 has in fact two  A7 
          registers.  One A7 register is used when in supervisor-
          mode,  the  other  when in usermode (this  is:  not  in 
          supervisor-mode.   It  is  sometimes  desirable  for  a 
          program  which is executing in supervisor mode to  know 
          the value of the usermode A7-register. This instruction 
          provides  a  way to obtain and change the value  of  A7 
          usermode-register.
Example:
Instruction              Before              After
MOVE USP,A6              A7user=$12345678    A7user=$12345678
                         A6    =$00000000    A6    =$12345678
                         A7sup =$87654321    A7sup =$87654321

Instruction:   RESET
Syntax:        RESET          �(privileged instruction)�
Data sizes:    none
Condition codes affected:
               X
               N
               Z
               V
               C    not affected
Function: Reset all external devices. A device can be a chip like 
          the 6850.


Instruction:   RTE            �(privileged instruction)�
Syntax:        RTE
Data sizes:    none
Condition codes affected:
               none




Function: Every exception is terminated by this  instruction.  It 
          can be compared to RTS. The only difference is that RTE 
          will  restore  the  SR  in  addition.   Note  that   an 
          exception-routine   has  the  responsibility  to   save 
          registers if this is important.


Instruction:   STOP           �(privileged instruction)�
Syntax:        STOP #
Data sizes:    word
Condition codes affected:
          All set as a direct result of the operand
Addressing modes allowed:
Source:
          #
Function: Stop execution of a program until an exception  occurs. 
          The  operand  stored  in the SR.  Note  that  with  the 
          operand  a minimum interrupt level can  be  determined. 
          With  this  instruction it is possible to  wait  for  a 
          videochip interrupt to occur.


Example:
          STOP #%0010011000011111
          Wait  for  an exception with a priority of 6  or  7  to 
          occur and set the XNZVC-bits.


Instruction:   TRAP
Syntax:        TRAP #
Data sizes:    # must be >=0 and <=15
Condition codes affected:
               X
               N
               Z
               V
               C    not affected







Function: This  instruction generates an exception.  The  operand 
          indicates  an  exception number.  The vectors  for  the 
          exceptions  are located at addresses $80 to  $BC.  This 
          instruction  is  mainly  used to  allow  programs  that 
          execute in user-mode to call supervisor-mode  routines. 
          This  way  a  user can be given a  number  of  specific 
          functions. In the ST trap vectors 2, 14 and 15 are used 
          for GEM,  BIOS and XBIOS functions.  In the case of the 
          ST  the TRAP-instructions is preceded  by  instructions 
          that  put  function numbers and  parameters  for  these 
          functions  on  the stack.  This way it is  possible  to 
          assign �groups� of functions to one trap-vector.
          Note  that  when  calling  a  TRAP  in  user-mode   the 
          stackpointers  change (supervisor-mode  and  user-mode, 
          remember??). Thus, the MOVE USP instruction can be used 
          to retrieve parameters that had been put on the stack.


Instruction:   TRAPV
Syntax:        TRAPV
Data sizes:    none

Condition codes affected:
               none
Function: When  the  V-bit  is  set  an  exception  occurs.   The 
          exception vector is located at address $1C. When the V-
          bit is clear nothing happens.  This instruction can  be 
          used by high-level languages to inform the user that an 
          overflow error has occured.