6. Digital simulation in Alecsis - LEDA

6. Digital simulation in Alecsis 876.1.4.1. Operator lengthofThis operator is syntactically very similar to the operator sizeof. It is a unary operator whose operandneeds to be a vector of enumeration type. (vector of signals or vector of variables). The vector can be a formalvariable, local variable, or a formal/local signal. In the case of a formal variable, AleC++ passes as a hiddenparameter the length of the actual argument extracted by the operator. The situation is much simpler with signals --a special instruction of the Alecsis virtual processor that returns the length of the signal-vector. Note that operand oflengthof operator cannot be a local pointer, because lengthof would return 0 in that case.If a vector is declared with inverse dimensionality, the lengthof returns the negative value of its length.Usually, for overloading vector-operands, you need the absolute value of the length. Direction can have animportant role with some specific operators (left shifting >).three_t v1[0:3], v2[3:0];int L1 = lengthof (v1); // L1 is 4int L2 = lengthof (v2); // L2 is -46.1.4.2. Buffers for temporary solutionsLogical operators often return a result, that can itself be an operand in a more complex expression. Since weare dealing with a vector, a temporary memory location needs to be reserved for the result, and another temporarymemory to prevent the overwrite of that result by the new operation. The following example illustrates this:# define Vreturn(v,size) {asm movq.l %d4, size; return v; }# define BUFF_SIZE 512three_t _op_buff[BUFF_SIZE], _res_buff[BUFF_SIZE];three_t *operator& (three_t *op1, three_t *op2) {int size1 = abs (lengthof op1);int size2 = abs (lengthof op2);if (size1 != size2) {printf("incompatible lengths - op &, type three_t\n");exit(1);}for (int i=0; i< size1; i++) _op_buff[i] = op1[i] & op2[i];memcpy(_res_buff, _op_buff, size1);Vreturn(_res_buff, size1)}three_t *operator= (three_t *op1, three_t *op2) {int size = abs(lengthof op1);memcpy(op1, op2, size);memcpy(_res_buff, op1, size);Vreturn(_res_buff, size)}foo () {static three_t v1[]="0100", v2[]="11x0", v3[4], v4[4];v4 = v3 = v1 & v2;}The example illustrates a concept of using two buffers -- one for calculations, the other for the return of theresult. In this particular example use of only one buffer would be a satisfactory solution. However, in some morecomplex operations it may happen for one of the operands to be a result of an overload operation, which may meanthat we are reading and changing the same vector simultaneously. That may produce errors that are very difficult to