Editing P-code machine (section)

===Calling conventions===
{{how-to|section|date=January 2024}}

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 <code>n</code> 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 initialize 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 (<code>n</code> being the number of parameters, <code>p</code> 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 + <code>i</code>, the second sets EP to SP + <code>j</code>. So <code>i</code> essentially specifies the space reserved for locals (plus the number of parameters plus 5), and <code>j</code> 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 <code>C</code> 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 <code>q</code> can be called with

  csp q

These standard procedures are Pascal procedures like <code>readln()</code> (<code>csp rln</code>), <code>sin()</code> (<code>csp sin</code>), etc. Peculiarly <code>eof()</code> is a p-Code instruction instead.