protected void decode_first_half() {
int opcode;
int optype, optypes;
int optypes2 = 0; /* It takes a kludge to handle a kludge */
int optypebytes;
int isextended = 0;
opcode = zm.get_code_byte() & 0xFF;
if (opcode == OP_EXTENDED) {
isextended = 0xC0;
}
switch (isextended | opcode & 0xC0) {
case 0xC0: /* variable form */
if (isextended != 0xC0) {
opnum = opcode;
} else {
opnum = (0x100 | (zm.get_code_byte() & 0xFF));
}
if ((opcode & 0x20) == 0) {
/* 2OP */
/*
* note that variable-form je can have more than two arguments, though
* it is a 2OP
*/
opnum = opcode & 0x1F;
}
count = 0;
optypes = zm.get_code_byte();
if ((opnum == OP_CALL_VS2) || (opnum == OP_CALL_VN2)) {
optypebytes = 2;
optypes2 = zm.get_code_byte();
} else optypebytes = 1;
while (optypebytes-- != 0) {
optype = (optypes & 0xC0) >> 6;
if (optype == zm.OP_OMITTED) break;
operands[count++] = zm.get_operand(optype);
optype = (optypes & 0x30) >> 4;
if (optype == zm.OP_OMITTED) break;
operands[count++] = zm.get_operand(optype);
optype = (optypes & 0x0C) >> 2;
if (optype == zm.OP_OMITTED) break;
operands[count++] = zm.get_operand(optype);
optype = (optypes & 0x03);
if (optype == zm.OP_OMITTED) break;
operands[count++] = zm.get_operand(optype);
optypes = optypes2;
}
break;
case 0x80: /* short form */
optype = (opcode & 0x30) >> 4;
if (optype == zm.OP_OMITTED) {
/* 0OP */
opnum = opcode;
count = 0;
} else {
/* 1OP */
opnum = opcode & 0x8F;
count = 1;
operands[0] = zm.get_operand(optype);
}
break;
default: /* long form */
/* always 2OP */
opnum = opcode & 0x1F;
count = 2;
optype = ((opcode & 0x40) >> 6) + 1;
operands[0] = zm.get_operand(optype);
optype = ((opcode & 0x20) >> 5) + 1;
operands[1] = zm.get_operand(optype);
}
}
