present is a constant area, and, below that, the heap growing down towards the stack. The NP (the new pointer) register points to the top (lowest used address) of the heap. When EP gets greater than NP, the machine's memory is exhausted. The fifth register, PC, points at the current instruction in the code area. Calling conventions Stack frames look like this: EP -> local stack SP -> ... locals ... parameters ... return address (previous PC) previous EP dynamic link (previous MP) static link (MP of surrounding procedure) MP -> function return value The procedure calling sequence works as follows: The call is introduced with mst n where n specifies the difference in nesting levels (remember that Pascal supports nested procedures). This instruction will mark the stack, i.e. reserve the first five cells of the above stack frame, and initialise previous EP, dynamic, and static link. The caller then computes and pushes any parameters for the procedure, and then issues cup n, p to call a user procedure (n being the number of parameters, p the procedure's address). This will save the PC in the return address cell, and set the procedure's address as the new PC. User procedures begin with the two instructions ent 1, i ent 2, j The first sets SP to MP + i, the second sets EP to SP + j. So i essentially specifies the space reserved for locals (plus the number of parameters plus 5), and j gives the number of entries needed locally for the stack. Memory exhaustion is checked at this point. Returning to the caller is accomplished via retC with C giving the return type (i, r, c, b, a as above, and p for no return value). The return value has to be stored in the appropriate cell previously. On all types except p, returning will leave this value on the stack. Instead of calling a user procedure (cup), standard procedure q can be called with csp q These standard procedures are Pascal procedures like readln() (csp rln), sin() (csp sin), etc. Peculiarly eof() is a p-Code instruction instead. Example machine Niklaus Wirth specified a simple p-code machine in the 1976 book Algorithms + Data Structures = Programs. The machine had 3 registers - a program counter p, a base register b, and a top-of-stack register t. There were 8 instructions: lit 0, a : load constant a