Long Length Data

The procedure P has three local arrays. The storage for these arrays is not part of the activation record for P; only a pointer to the beginning of each array appears in the activation record. The relative addresses of these pointers are known at the compile time, so the target code can access array elements through the pointers. Also shown is the procedure Q called by P . The activation record for Q begins after the arrays of P. Access to data on the stack is through two pointers, top and stack top. The first of these marks the actual top of the stack; it points to the position at which the next activation record begins. The second is used to find the local data. For consistency with the organization of the figure in slide 16, suppose the stack top points to the end of the machine status field. In this figure the stack top points to the end of this field in the activation record for Q. Within the field is a control link to the previous value of stack top when control was in calling activation of P. The code that repositions top and stack top can be generated at compile time, using the sizes of the fields in the activation record. When q returns, the new value of top is stack top minus the length of the machine status and the parameter fields in Q's activation record. This length is known at the compile time, at least to the caller. After adjusting top, the new value of stack top can be copied from the control link of Q.