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# Introduction

Parts 1, 2, and 3 of this text are the three programs which comprise the Model
Implementation of Standard Pascal. Throughout the text the term "standard Pascal" is used
to mean the programming language Pascal as defined by standard BS 6192 and ISO 7185, and
the term "the Pascal standard" refers to these definitions.

These programs are:

1 a model compiler which enforces all mandatory requirments of the Pascal standard on
each program input to it, and generates an equivalent "P-code" object program;

2 a P-machine which interprets a P-code program to simulate its executable effect,
including detection of all errors defined by the Pascal Standard;

3 a post-mortem generator which examines the "corpse" of an executed P-code program
to determine its cause of termination and to re-create the program's final state in source
language terms;

The Model Compiler ts a conventional one-pass Pascal compiler comprising a machine-independent
program analyser cleanly interfaced to a target machine-dependent code
generator. It accepts an ISO Standard Pascal source program and generates an equivalent
P-code object program for interpretive execution on the P-machine. A high degree of run-time
security is provided by generating error-checking code for each of the distinct errors
defined in the Pascal Standard.

The P-machine program should be regarded as an operational definition rather than a
production interpreter. Being written in Pascal, it relies on a host Pascal processor for all
arithmetic functions and fixed and floating-point I/O. If these host functions are deviant, the
Model Implementation may exhibit deviant behaviour also. Moreover, an implementation of
files and program parameters cannot be given a general expression in Standard Pascal.
Consequently, the P-machine as published here provides communication with the external
environment only via the program parameters Input and Output, together with an interface
for the implementation of file I/O in general.

The Postmortem Generator supplements each error report with a trace-back of all active
blocks, together with a symbolic variable dump of each. It is called whenever the P-machine
detects an error, but the means by which this is achieved is necessarily system-dependent.