7/17/2019 Extension Manual
1/2
Files included:
ExtensionReadMe.txt - this fileLC3Extension.h - A reusable
header defining all necessary functions
andconstantsExtensionSample.vcproj - A Microsoft Visual Studio .NET
project file for building the sampleExtensionSample.cpp - A sample
extension for the simulatorExtensionTest.asm - A sample LC-3
program that takes advantage of the sample extension
What is an extension?An extension lets you add new functionality
to the LC3 simulator. An extensionis a DLL that exports two
methods: FireInterrupt3 and ExecuteReservedOpcode3.The simulator
calls these methods at the appropriate time during the
instructioncycle to allow the extension to implement interrupts
and/or to provide animplementation for the reserved opcode in the
LC3 ISA.
What must you implement?Your DLL must export FireInterrupt3 and
ExecuteReservedOpcode3. The functionprototypes are defined in
LC3Extension.h along with other constants that may beuseful in
writing your extension. A sample project is also included
thatimplements both of these functions. LC3Extension.h is designed
to be resusedin your own extension. Each of these methods takes
callback methods to
read and write to the various memory addresses and registers in
the LC3 memoryspace at the time of the call.
ExecuteReservedOpcode3 takes a callback for reading and writing
values to theLC3 memory space. An implementation will typically use
these callbacks to getthe bits of the current instruction then
perform some custom operation giventhose bits.
FireInterrupt3 returns the interrupt vector for the interrupt
that should befired. If no interrupt should be fired, the method
returns NO_INTERRUPT.This method takes only the read callback and
therefore can not write to theLC3 memory space. Any modifications
should be performed by the executingprogram in the interrupt
handler. It is often the case that the interrupt
handler will make use of the reserved opcode to make further
calls into theextension.
The sample programThe included sample program can be compiled
with Microsoft Visual Studio .Net.It implements an interrupt,
x0081, as a one second timer. The simulatorcalls FireInterrupt3
each time through the instruction cycle in which nointerrupt
handler is executing. The sample extension keeps track of theamount
of time since it last fired the interrupt. If more than one
secondhas passed, it fires the interrupt again.
The accompanying LC3 application implements an interrupt handler
for interruptx0081. The handler executes an instruction that uses
the reserved opcode.
The extension has defined the opcode to return a random number
from 0 to 9in the register defined in bits 9 to 11 of the
instruction. The LC3 programthen writes that value to the console
in the interrupt handler.
Recommendations for your extensionsDon't cache the callback
pointers you receive during the extension calls.They should not be
saved and used in other threads after your extensionreturns. They
may also change from one call to the next.
The LC3 has been designed to operate on all 32 bit Windows
operating systems
