CS 3220 Processor Design
Fall 2002

Instructor: Pete Manolios

iJVM2 Instructions

Below we list the instructions of the iJVM2 and their semantics.  We first introduce some notation.  If x is a 32-bit word and b is a byte then: Given an iJVM2 state s where  C[pc] is equal to the opcode of inst, the semantics (meaning) of the iJVM2 instructions specify how we obtain the next state, the state obtained by executing inst, by providing a list of commands that applied to the current state will result in the next state.  Each list of commands starts by incrementing the pc by 1 and for conciseness, we will not show this instruction explicitly.  In addition, all of the arithmetic and logical operations return 32 bit words. As per the discussion on bit vectors, we represent bit vectors as numbers. The semantics of the iJVM2 closely mimicks the semantics of the corresponding JVM instructions; the official specification of the JVM is maintained by Sun at http://java.sun.com/docs/books/vmspec/.
 
Instruction Opcode Meaning (how to obtain the next state)
NOP 0  
ILOAD 21
  • Push[L[C[pc]]]
  • pc:=pc+1
ISTORE 54
  • Pop[]
  • val := S[sp]
  • L[C[pc]] := val
  • pc := pc+1 
BIPUSH 16
  • Push[C[pc]], where C[pc] is treated as a signed number and extended to a 32-bit word.
  • pc := pc+1
POP 87
  • Pop[]
DUP 89
  • Push[S[sp-4]]
SWAP 95
  • val1 := S[sp-4]
  • val2 := S[sp-8]
  • S[sp-8] := val1
  • S[sp-4] := val2
    •
ISUB 100
  • val1 := S[sp-4]
  • val2 := S[sp-8]
  • S[sp-8] := val2-val1
  • sp := sp-4
IADD  96
  • val1 := S[sp-4]
  • val2 := S[sp-8]
  • S[sp-8] := val2+val1
  • sp := sp-4
IMUL 104
  • val1 := S[sp-4]
  • val2 := S[sp-8]
  • S[sp-8] := val2*val1
  • sp := sp-4
IAND 126
  • val1 := S[sp-4]
  • val2 := S[sp-8]
  • S[sp-8] := val2 AND val1
  • sp := sp-4
IOR 128
  • val1 := S[sp-4]
  • val2 := S[sp-8]
  • S[sp-8] := val2 OR val1
  • sp := sp-4
GOTO 167
  • pc :=  (pc-1)+ Cs[pc]
IFEQ 153
  • Pop[]
  • If S[sp] = 0
    then pc := (pc-1) + Cs[pc] 
    else pc := pc+2
IFNE 154
  • Pop[]
  • If !(S[sp] = 0)
    then pc := (pc-1) + Cs[pc] 
    else pc := pc+2
IFGT 157
  • Pop[]
  • If S[sp] > 0
    then pc := (pc-1) + Cs[pc] 
    else pc := pc+2
    •
INVOKESTATIC1  184
  • old-pc :=  pc+2
  • old-fp := fp
  • old-ap := ap
  • pc :=  (pc-1)+ Cs[pc]
  • nargs := C[pc]
  • pc := pc+1
  • nlocs := C[pc]
  • pc := pc+1
  • ap := sp - (4*nargs)
  • fp := sp + (4* nlocs)
  • sp := fp
  • Push[old-ap]
  • Push[old-fp]
  • Push[old-pc]
    •
IRETURN 172
  • val := S[sp-4]
  • sp := ap
  • ap := S[fp]
  • pc := S[fp+8]
  • fp := S[fp+4]
  • push[val]
    •

1: The part that differs signficantly from the iJVM is the call/return mechanism.  iJVM2 does not have classes and there is no need to resolve method calls.  An invokestatic instruction specifies the beginning address of the method.  At that address, the first byte indicates the number of arguments to the method  and the second byte is the number of local variables in the method.  The method can access the arguments and locals using iload and istore.  The arguments are numbered 0, ..., nargs-1 and the locals are numbered nargs, ..., nargs+nlocs-1.   An ireturn instruction saves the top of the stack, restores the previous frame and pushes the saved value onto the new stack.