Demonstration of a simple stack virtual machine & assembler

vm.c

/*
	Demonstration of a simple stack virtual machine & assembler

	Input:

		lit 10
		:loop
		cpy
		cpy
		put
		lit 1
		sub
		cpy
		jpc loop
		put

	Output:

		10
		9
		8
		7
		6
		5
		4
		3
		2
		1
		0
*/

#include <stdlib.h>
#include <stdio.h>
#include <string.h>

typedef enum {
	LIT,
	CPY,
	ADD,
	SUB,
	MUL,
	DIV,
	JMP,
	JPC,
	EQU,
	NEQ,
	GRT,
	LOT,
	GET,
	PUT,
	CHR
} OPCODE;

int stack[ 1024 ];
int tos = -1;

struct cmd
{
	OPCODE 	c;
	int 	p;
};

struct cmd prg[ 1024 ];

int prg_len = 0;


int pop( void )
{
	return stack[ tos-- ];
}

void push( int x )
{
	stack[ ++tos ] = x;
}

int assemble()
{
	char l[ 1024 ];
	char t[ 32 ];
	char p[ 32 ];
	int i;

	struct LTAB
	{
		char l[32];
		int	  a;
	} ltab[1024];
	int ltab_cnt = 0;

	while( fgets( l, 1024, stdin ) && strcmp( l, "\n" ) )
	{
		switch( *l )
		{
			case ';':
			case '\0':
				break;

			case ':':
				sscanf( l, ":%s", t );

				for( i = 0; i < ltab_cnt; i++ )
					if( strcmp( ltab[ i ].l, t ) == 0 )
					{
						printf( "Label \"%s\" is already defined!\n", t );
						return 0;
					}

				strcpy( ltab[ ltab_cnt ].l, t );
				ltab[ ltab_cnt ].a = prg_len;

				ltab_cnt++;
				break;

			default:
				sscanf( l, "%s %s", t, p );

				#define SET_IF_OPCODE( op, set ) \
					if( !strcasecmp( t, op ) ) \
						prg[ prg_len ].c = set

				SET_IF_OPCODE( "LIT", LIT );
				else SET_IF_OPCODE( "CPY", CPY );
				else SET_IF_OPCODE( "ADD", ADD );
				else SET_IF_OPCODE( "SUB", SUB );
				else SET_IF_OPCODE( "MUL", MUL );
				else SET_IF_OPCODE( "DIV", DIV );
				else SET_IF_OPCODE( "JMP", JMP );
				else SET_IF_OPCODE( "JPC", JPC );
				else SET_IF_OPCODE( "EQU", EQU );
				else SET_IF_OPCODE( "NEQ", NEQ );
				else SET_IF_OPCODE( "GRT", GRT );
				else SET_IF_OPCODE( "LOT", LOT );
				else SET_IF_OPCODE( "GET", GET );
				else SET_IF_OPCODE( "PUT", PUT );
				else SET_IF_OPCODE( "CHR", CHR );
				else
				{
					printf( "Unknown command: %s\n", t );
					return 0;
				}

				if( prg[ prg_len ].c == JPC )
				{
					for( i = 0; i < ltab_cnt; i++ )
						if( strcmp( ltab[ i ].l, p ) == 0 )
							break;

					if( i == ltab_cnt )
						strcpy( ltab[ ltab_cnt++ ].l, t );

					sprintf( p, "%d", i );
				}

				prg[ prg_len ].p = atoi( p );
				prg_len++;
				break;
		}
	}

	for( i = 0; i < prg_len; i++ )
		if( prg[ i ].c == JPC )
			prg[i].p = ltab[prg[i].p].a;

	printf( "Program assembled with %d lines of code!!\n", prg_len );

	return 1;
}

int main( int argc, char** argv )
{
	int x;
	int i;

	if( !assemble() )
		return -1;

	printf( "Fine! Now we will execute the program... *g*\n\n" );

	for( i = 0; i < prg_len; i++ )
	{
		/*
		int j;
		printf( "Cmd = %d Parm = %d, Stack: ", prg[i].c, prg[i].p );
		for( j = 0; j <= tos; j++ )
			printf( "%d ", stack[ j ] );
		printf( "\n" );
		*/

		switch( prg[i].c )
		{
			case LIT:
				push( prg[i].p );
				break;
			case CPY:
				push( stack[tos] );
				break;
			case ADD:
				push( pop() + pop() );
				break;
			case SUB:
				x = pop();
				push( pop() - x );
				break;
			case MUL:
				push( pop() * pop() );
				break;
			case DIV:
				x = pop();
				push( pop() / x );
				break;
			case JMP:
				i = prg[i].p - 1;
				break;
			case JPC:
				if( pop() )
					i = prg[i].p - 1;
				break;
			case EQU:
				push( pop() == pop() );
				break;
			case NEQ:
				push( !( pop() == pop() ) );
				break;
			case GRT:
				push( !( pop() < pop() ) );
				break;
			case LOT:
				push( !( pop() > pop() ) );
				break;
			case GET:
				scanf( "%d", &x );
				push( x );
				break;
			case PUT:
				printf( "%d\n", pop() );
				break;
			case CHR:
				printf( "%c", pop() );
				break;
		}
	}

	return 0;
}