컴파일러 구성론 - VSM

Virtual stack machine 만들기

 

< instruction set >

opcode	mnemonic	instruction	arg?	function
Arithmetic
1	ADD	Add	n	Add the top two values on the stack
2	SUB	Subtract	n	Subtract the top of stack from the value below it.
3	MUL	Multiply	n	Multiply the top two values on the stack.
4	DIV	Divide	n	Divide the second value on the stack by the top one. Truncate and discard the remainder(like Pascal's DIV operation).
5	NEG	Negate	n	Negate the top of the stack.
Boolean
6	CMPNE	Not Equal	n	if the top two items on the stack are not equal, push a 1 else push a 0.
7	CMPEQ	Equal	n	if the top two items on the stack are equal, push a 1 else push a 0.
8	CMPLT	Less	n	if the top of the stack is less than the value below it, push a 1 else push a 0.
9	CMPGT	Greater	n	if the top of the stack is greater than the value below it, push a 1 else push a 0.
10	CMPGE	Greater or equal	n	if the top of the stack is greater than or equal to the value below it, push a 1 else push a 0.
11	CMPLE	Less or equal	n	if the top of the stack is less than or equal to the value below it, push a 1, else push a 0.
12	CMPZR	Zero	n	if the top of the stack is 0, replace it with 1 else replace it with 0
Stack Manipulation
13	PUSHC	Push constant	y	Put the argument on the stack
14	PUSH	Push addr	y	Put the contents of the location specified by the argument on the stack
15	POPC	Pop constant	y	Put the top of the stack into the location specified by the argument
16	POP	Pop	n	Put the second value on the stack into the location specified by the top of the stack
Control
17	BRANCH	Branch	y	Set the program counter to the location specified by the argument
18	JUMP	Jump	n	Set the program counter to the top of stack
19	BREQ	Branch if equal	y	if the top of the stack is 0, branch to the argument
20	BRLT	Branch if less	y	if the top of the stack is less than 0, branch to the argument
21	BRGT	Branch if greater	y	if the top of the stack is greater than 0, branch to the argument
Input / Output
22	RDCHR	Read char	n	Read a character from the keyboard and put its ASCII value on the stack
23	RDINT	Read int	n	Read an integer from the keyboard and put it on the stack
24	WRCHR	Write char	n	Write the top of the stack to the screen treating it as an ASCII value.
25	WRINT	Write int	n	Write the top of the stack to the screen treating if as a signed integer.
Miscellaneous
26	CONTS	Contents	n	Replace the top of stack with the contents of the location specified by the top of stack
27	HALT	Halt	n	Stop the machine

 

< op code 형식 >

Header Record, Text Record, End Record 세 종류
Header Record 형식:
H 프로그램_이름
Text Record 형식( Text Record는 여러 줄이 될 수 있음):
T Op_코드의_시작_주소 Text_Record의_길이 코드_Text
    , 여기서 Text는 10진수로 구성된다.
End Record 형식:
E 명령어의_시작_주소

 

ex)

H TEST1
T 0 9 5 10 14 0 14 1 1 25 27
E 2

 

 

< 구현 >

                                   vm.7z

 

/* 코드실행의 기본인 fetch - decode - execute 대로 구현함 */

public class MyVSM {

private VSMMemory memory;

private int programCtr;

private int stackPtr;

private int stackLimit;

...

public MyVSM(int memorySize){
     memory = new VSMMemory(memorySize);
}
 
public void loadProgram(String filename) {
    VSMOpcodeLoader loader = new VSMOpcodeLoader(filename);
    loader.loadTo(memory);
  

    programCtr = loader.getStartAddress();
    stackPtr = (memory.getSize()-1);
    stackLimit = loader.getEndOfProram();
 }
 
 public void startProgram() {
    short fetchedOpcode = 0;
    while( !isEnd ){
       // fetch
       fetchedOpcode = fetch();
   
       // decode (and execute)
       decodeAndExecute(fetchedOpcode);
     }
 }

 ...

}

 

/* 단순히 워드(구현상 short) 배열 관리클래스 */
public class VSMMemory {
    private int size;
 
    // memory type 'WORD' as short
    private short[] memorySpace;
 
    public VSMMemory(int size){
         this.size = size;
        memorySpace = new short[size];
    }
        ... 

public short accessAt(int address) {
    if( address < 0 || size <= address)
        throw new VSMMemAccessViolationException();
  
    return memorySpace[address];
}
 
public void assignTo(int address, short value) {
    if( address < 0 || size <= address)
        throw new VSMMemAccessViolationException();
 
    memorySpace[address] = value;
}
    ...

}

 

/* opcode를 읽어 타겟 메모리에 적재해주는 역할 */

public class VSMOpcodeLoader {
      private VSMMemory targetMemory;
      private FileReader reader;
      private int lookahead = 0;
          ...

public VSMOpcodeLoader(String fileName) {
    reader = new FileReader(fileName);
}
 
public void loadTo(VSMMemory memory) {
    targetMemory = memory;
 
    while((lookahead = reader.read()) != -1){
         if(lookahead == '\r' || lookahead == '\n' || lookahead == '\t' || lookahead == ' ')

         continue;
  
  if(lookahead == 'H')
      parseName();
  else if(lookahead == 'T')
      parseTextRecord();
  else if(lookahead == 'E')
      parseEndRecord();
}

}

   ...

}

 

물론 실제 vm은 무진장 복잡하겠지만,, 기본은 똑같지않을까 싶다.