1.
SYSTEM
SOFTWARE
PCODE
R.
V.
COLLEGE
OF
ENGINEERING
DEPARTMENT
OF
COMPUTER
SCIENCE
AND
ENGINEERING
(Autonomous
Institution
Affiliated
to
VTU)
Bangalore
–
560
059
Assignment
Report
on
P-­‐Code
Compiler
Bachelor
of
Engineering
in
Computer Science & Engineering
By
Sandeep
R.V
1RV10CS089
(Academic
Year
:
2012-­‐13
)

2.
SYSTEM
SOFTWARE
PCODE
1. INTRODUCTION
Interpretive compilers are translators for high-level languages,
Such translators produce (as
output) intermediate code (P-code for example) which is intrinsically simple enough to satisfy the
constraints imposed by a practical interpreter, even though it may still be quite a long way from
the machine code of the system on which it is desired to execute the original program. Rather than
continue translation to the level of machine code, an alternative approach that may perform
acceptably well is to use the intermediate code as part of the input to a specially written
interpreter. This in turn "executes" the original algorithm, by simulating a virtual machine for
which the intermediate code effectively is the machine code.
Machine
Independent
Machine
Dependent
Fig.
1.1
2.
P-­‐CODE
P-Code (Portable Code) is an assembly language for a hypothetical stack machine.
One Such kind is Bytecode, bytecodes are compact numeric codes, constants, and references
(normally numeric addresses), which encode the result of parsing and semantic analysis of things
like type, scope, and nesting depths of, program objects. They therefore allow much better
performance than direct interpretation of source code.
The name bytecode stems from instruction sets, which have one byte opcodes, followed by
optional parameters.
Example: Java bytecode
3.
P-­‐CODE
COMPILER
n P-code compilers (also called bytecode compilers) are very similar in concept to
interpreter
n The source program is analyzed and converted into an intermediate form, which is
then executed interpretively
n With a P-code compiler, this intermediate forms is the machine language for a
hypothetical machine, often called pseudo-machine
n P-code object programs can be executed on any machine that has a P-code
interpreter

3.
SYSTEM
SOFTWARE
PCODE
n The P-code object program is often much smaller than a corresponding machine
code (native code) program would be
Fig 3.1 Translation and Execution Using a P-code Compiler
4. P-CODE MACHINE
Portable code machine is a virtual machine designed to execute p-code.
This term is applied both generically to all such machines (such as the Java Virtual Machine
and MATLAB precompiled code), and to specific implementations, the most famous being the p-
Machine of the Pascal-P system, particularly the UCSD Pascal implementation.
4.1. Machine architecture
The P-code machine is similar to a conventional computer in that it consists of a processor
and a memory. A major difference is that many operations performed by the processor involve the
stack, which is part of memory. For example a procedure call entails manipulating various factors
such as parameters and return addresses and these are held on the stack. The machine instructions,
called P-code, are stored in the memory and accessed in the normal manner.
The processor has a defined instruction set as well as five registers, which have distinct
functions for controlling the instructions and the stack areas within the memory.

4.
SYSTEM
SOFTWARE
PCODE
The registers are:
. PC the program counter;
. SP the stack pointer
. MP the mark stack pointer;
. NP the new pointer;
. EP the extreme stack pointer.
The program counter, PC, is a pointer to the current instruction being executed. Other
registers are used to control the remaining area of memory - the data store.
The data store is divided into three areas as shown in Fig. 4.1.1
Fig 4.1.1
The constants area is generated by the assembler and accessed by the code.
The heap grows towards the low-numbered locations of store and the NP pointer points at
free heap space. Data is put onto the heap during a call to the new procedure and removed by
using mark and release. The stack area has a further internal structure and is used to hold stack
frames. A stack frame is generated and placed on the stack each time a procedure or function is

5.
SYSTEM
SOFTWARE
PCODE
called (in the following section, except where explicitly stated, references to procedures also
include functions.) The first stack frame is the exception in that this belongs to the program block.
The stack frames on the stack are shown in Fig. 4.1.2.