mirror of https://github.com/makarcz/vm6502.git
263 lines
8.5 KiB
Plaintext
263 lines
8.5 KiB
Plaintext
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Project: MKBasic (VM6502).
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Author: Copyright (C) Marek Karcz 2016. All rights reserved.
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Free for personal and non-commercial use.
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Code can be distributed and included in derivative work under
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condition that the original copyright notice is preserved.
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For use in commercial product, please contact me to obtain
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permission and discuss possible fees, at: makarcz@yahoo.com
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This software is provided with no warranty.
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Purpose:
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MOS 6502 emulator, Virtual CPU/Machine and potentially retro-style 8-bit
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computer emulator.
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MOS-6502-compatible virtual computer featuring BASIC interpreter, machine code
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monitor, input/output device emulation etc.
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Program works in DOS/shell console (text mode) only.
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Makefile are included to build under Windows 32/64 (mingw compiler required)
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and under Linux Ubuntu or Ubuntu based.
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To build under Windows 32/64:
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* Install MINGW64 under C:\mingw-w64\x86_64-5.3.0 folder.
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* Run mingw terminal.
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* Change current directory to that of this project.
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* Run: makeming.bat
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To build under Linux:
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* Make sure C++11 compliant version of GCC compiler is installed.
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* Change current directory to that of this project.
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* Run: make clean all
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Program passed following tests:
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* 6502 functional test by Klaus Dormann
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* AllSuiteA.asm from project hmc-6502
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1. Credits/attributions:
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Parts of this project is based on or contains 3-rd party work:
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- Tiny Basic.
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- Enhanced Basic by Lee Davison.
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- Microchess by Peter Jennings (http://www.benlo.com/microchess/index.html).
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- 6502 functional test by Klaus Dormann.
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- All Suite test from project hmc-6502.
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2. Format of the memory image definition file.
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Program can load raw binary image of MOS 6502 opcodes.
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Binary image is always loaded from address $0000 and can be up to 64 kB long,
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so the code must be properly located inside that image.
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Depending on your favorite 6502 assembler, you may need to use proper command
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line arguments or configuration to achieve properly formatted binary file.
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E.g.: if using CL65 from CC65 package, create configuration file that defines
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memory segments that your 6502 code uses, then all of the segments (except the
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last one) must have attribute 'fill' set to 'yes' so the unsused areas are
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filled with 0-s.
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Two CFG files, one for microchess and one for All Suite from hmc-6502 project
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are supplied with this project and assembler source code adapted to be
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compiled with CL65.
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Other assemblers may need a different approach or may not be able to generate
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binary images that are required for this emulator.
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Program can also load memory image definition file (plain text), which is
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a format developed especially for this project.
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The format of the plain text memory image definition file is described below:
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; comments
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ADDR
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address
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data
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ORG
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address
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data
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IOADDR
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address
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ROMBEGIN
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address
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ROMEND
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address
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ENROM
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ENIO
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EXEC
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address
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Where:
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ADDR - label indicating that starting and run address will follow in
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the next line
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ORG - label indicating that the address counter will change to the
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value provided in next line
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IOADDR - label indicating that character I/O emulation trap address will
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follow in the next line
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ROMBEGIN - label indicating that the emulated read-only memory start address
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will follow in the next line
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ROMEND - label indicating that the emulated read-only memory end address
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will follow in the next line
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ENROM - enable read-only memory emulation
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ENIO - enable character I/O emulation
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EXEC - label indicating that the auto-execute address will follow
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in the next line
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address - decimal or hexadecimal (prefix $) address in memory
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E.g:
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ADDR
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$0200
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or
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ADDR
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512
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changes the default start address (256) to 512.
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ORG
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49152
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moves address counter to address 49152, following data will be
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loaded from that address forward
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data - the multi-line stream of decimal of hexadecimal ($xx) values
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of size unsigned char (byte: 0-255) separated with spaces
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or commas.
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E.g.:
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$00 $00 $00 $00
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$00 $00 $00 $00
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or
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$00,$00,$00,$00
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or
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0 0 0 0
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or
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0,0,0,0
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0 0 0 0
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Each described above element of the memory image definition file is optional.
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3. Character I/O emulation.
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Emulator has ability to simulate a 80x25 text output display device and
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rudimentary character I/O functions. The emulation is implemented by the means
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of trapping memory locations defined to be designated I/O emulation addresses.
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The default memory location is $E000 and also by default, the character I/O
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is disabled. It can be enabled from the debug console with 'I' command:
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I hexaddr
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E.g.:
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I E000
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or
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I FE00
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or by putting optional statements in the memory image dedinition file:
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ENIO
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or
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IOADDR
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address
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ENIO
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Where:
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address - decimal or hexadecimal (with prefix '$') address in memory
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$0000 - $FFFF.
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The same address is used for both, input and output operations.
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Reading from IOADDR inside the 6502 code invokes a blocking character
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input function from user's DOS/shell session.
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After user enters the character, the memory location contains the character
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code and also emulated CPU Acc register contains the same code.
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Reading from IOADDR+1 inside 6502 code invokes a non-blocking character
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input function from user's DOS/shell session.
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This function is different than blocking one in one respect.
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This function will return value 0 in the memory location and Acc register
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if there was no key pressed by the user (no character waiting in buffer).
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If there was a key typed, the function will act as the blocking counterpart.
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Writing to IOADDR inside the 6502 code will result in character code
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being put in the IOADDR memory location and also written to the character
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output buffer of the emulated display device. That character is not
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immediately transferred to the user's DOS/shell session. It is written to the
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emulated display's text memory instead. Depending on the mode in which
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emulator is currently working (continuous or step-by-step code execution),
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the emulated display device contents may or may not be updated on the user's
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screen in real time fashion. Remember that this is a DOS/shell console
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application. The user's console is shared among various functions of the
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program. In step-by-step mode, if the character I/O emulation is enabled, the
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current contents of the emulated display device can be displayed with
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corresponding debug console command: 'T'.
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4. ROM (read-only memory) emulation.
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This facility provides very basic means for memory mapping of the read-only
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area. This may be required by some 6502 programs that check for non-writable
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memory to establish the bounds of memory that can be used for data and code.
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One good example is Tiny Basic.
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By default the ROM emulation is disabled and the memory range of ROM is
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defined as $D000 - $DFFF.
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ROM emulation can be enabled (and the memory range defined) using debug
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console's command 'K':
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K [rombegin] [romend] - to enable
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or
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K - to disable
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The ROM emulation can also be defined and enabled in the memory image
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definition file with following statements:
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ROMBEGIN
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address
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ROMEND
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address
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ENROM
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5. Additional comments and remarks.
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IOADDR is permitted to be located in the emulated ROM memory range.
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The writing to IOADDR is trapped first before checking ROM range and writing
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to it is permitted when character I/O emulation and ROM are enabled at the
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same time. It is a good idea in fact to put the IOADDR inside ROM range,
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otherwise memory scanning routines like the one in Tiny Basic may trigger
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unexpected character input because of the reading from IOADDR during the scan.
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If you experience unexpected character input prompt while emulating
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6502 code, this may be the case. Reconfigure your IOADDR to be inside ROM in
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such case and try again.
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6. Warranty and License Agreement.
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This software is provided with No Warranty.
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I (The Author) will not be held responsible for any damage to computer
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systems, data or user's health resulting from using this software.
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Please use responsibly.
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This software is provided in hope that it will be be useful and free of
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charge for non-commercial and educational use.
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Distribution of this software in non-commercial and educational derivative
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work is permitted under condition that original copyright notices and
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comments are preserved. Some 3-rd party work included with this project
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may require separate application for permission from their respective
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authors/copyright owners.
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