ICode Instructions

For each ICode instruction its operation code, its parameters and its meaning are given in the following. The meaning is given as text and as formula which describe operations on the runtime stack. To simplify the description, within formulas it is not taken care about the types of the stack elements.

If not further mentioned, the operations apply to the top of the stack, which contains the actual element. The following shorthand notations are used:

Note: The types integer, real and boolean are encoded with 1, 2, and 3. The boolean values FALSE and TRUE are encoded by 0 and 1.

Load Instructions

load address with base and offset

   SP:=SP+1
   S[SP]:=base(l)+o;

load constant c of type t

   SP:=SP+1;
   S[SP]:=c;

   S[SP]:=S[S[SP]]

Store Instructions

store into address contained in the element below the top

   S[S[SP-1]]:=S[SP];
   SP:=SP-1;

Jump Instructions

unconditional jump

   PC:=a;

conditional jump

   if not S[SP] then PC:=a;
   SP:=SP-1;

Arithmetic Instructions

addition of type t

   SP:=SP-1;
   S[SP]:=S[SP]+S[SP+1];

integer subtraction

   SP:=SP-1;
   S[SP]:=S[SP]-S[SP+1];

multiplication of type t

   SP:=SP-1;
   S[SP]:=S[SP]*S[SP+1];

Logic Instructions

   S[SP]:=not S[SP];

less operation of type t

   SP:=SP-1;
   S[SP]:=S[SP]
Address Calculation Instructions

IXA c"

   compute indexed address


   SP:=SP-1;
   S[SP]:=c*S[SP+1]+S[SP];


Convert Instructions

FLT

   converts from integer to real


   S[SP]:=real(S[SP]);


Input-Output Instructions

WRI t


   write(S[SP]);
   SP:=SP-1;


REA t
   

   SP:=SP+1;
   read(S[SP]);


Subroutine Handling Instructions

MST l

   activation record initialization:


   S[SP+1]:=base(l);- store static predecessor
   S[SP+2]:=AP; - store dynamic predecessor
   SP:=SP+3; - return address (=S[SP+3]) is stored by JSR


JSR o a

   set AP to point to new activation record

   o = number of locations for parameters


   AP:=SP-(o+2);
   S[AP+2]:=PC; - store return address
   PC:=a ; - set PC to first instruction of subroutine


ENT o

   storage reservation for new block

   o = length of local data segment


   SP:=SP+o


RET

   return from subroutine:


   SP:=AP-1;
   PC:=S[SP+3]; - fetch return address to restore PC
   AP:=S[SP+2]; - restore activation record pointer AP


Check Instructions

CHK c1 c2

   check against upper and lower bounds


   if (S[SP]c2) then error














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