IRSIM is a tool for simulating digital circuits. It is a "switch-level"
simulator; that is, it treats transistors as ideal switches. Extracted
capacitance and lumped resistance values are used to make the switch a little
bit more realistic than the ideal, using the RC time constants to predict the
relative timing of events.

IRSIM shares a history with magic, although it is an independent program. Magic
was designed to produce, and IRSIM to read, the ".sim" file format, which is
largely unused outside of these two programs. IRSIM was developed at Stanford,
while Magic was developed at Berkeley. Parts of Magic were developed especially
for use with IRSIM, allowing IRSIM to run a simulation in the "background"
(i.e., a forked process communicating through a pipe), while displaying
information about the values of signals directly on the VLSI layout.

For "quick" simulations of digital circuits, IRSIM is still quite useful for
confirming basic operation of digital circuit layouts. The addition of
scheduling commands ("at", "every", "when", and "whenever") put IRSIM into the
same class as Verilog simulators.

To run irsim, users should set CAD_HOME to the base installation directory
where magic was installed, e.g. /usr/local.  Alternatively, the system
administrator can create a dummy user named 'cad' with its home
directory set to the installation directory.