Editing P-code machine (section)

===Architecture===
Like many other P-code machines, the UCSD P-Machine is a [[stack machine]], which means that most instructions take their operands from a [[Stack (abstract data type)|stack]], and place results back on the stack. Thus, the <code>add</code> instruction replaces the two topmost elements of the stack with their sum. A few instructions take an immediate argument. Like Pascal, the P-code is [[Strong and weak typing|strongly typed]], supporting Boolean (b), character (c), integer (i), real (r), set (s), and pointer (a) [[data type]]s natively.

Some simple instructions:

 Insn.   Stack   Stack   Description
         before  after
 &nbsp;
 adi     i1 i2   i1+i2   add two integers
 adr     r1 r2   r1+r2   add two reals
 inn     i1 s1   b1      set membership; b1 = whether i1 is a member of s1
 ldi     i1 i1   i1      load integer constant
 mov     a1 a2   a2      move
 not     b1 b1   -b1     Boolean negation