Lego RCX Assembler and a Case Study

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Lego RCX Assembler and a Case Study

• Luis Paris
• Dept. of Computer Science
• University of Mississippi

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Supported Languages/Firmwares
Language
Firmware

• Java
• C, C, Pascal
• Forth
• MindScript, NQC, Lego ASM

• TinyVM, leJOS
• brickOS
• pbForth
• Standard Lego RCX

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RCX Internals

• Hitachi H8 microcontroller
• On-chip 16K ROM
• External 32K RAM
• I/O devices
• Three motor ports
• Three sensor ports
• IR communications port

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RCX Virtual Machine

• Implemented by the Lego RCX firmware
• Virtual Machine Characteristics
• Byte Code Interpreter
• Sources and values
• Byte Code Command Set

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Case Study Lego RCX Assembler

• Motivation
• Project for CSCI-450 Programming Languages
• Lego Assembler hides output object file
• Result Byte code nor object format cant be
  studied
• Does not support definition list
• Result Programs hard to read and maintain
• Project Create a Lego RCX Assembler
• More robust
• Runs standalone

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Solution Strategy

• Two stages
• Lexical/Syntax analysis (Parsing)
• Finite State Machine
• Library functions (scanf, string.tokenizer)
• Code generation (one-to-one)
• No one-to-many semantic analysis (as in
  compilers)
• Mnemonic to Opcode conversion
• Parameters to Byte Code conversion

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Which language C/C or Java?

• Java is a powerful OOP language butnot really
  suitable for system programming
• No bit-fields support
• No operator overloading
• Both C/C support bit-fields
• However, C has unnecessary, confusing features
  (multiple inheritance)

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Bit-fields

• After parsing ASM command out 6, 23
• Variables contain instr out param1 6
  param2 23
• Problem encode out param1, param2 into above
  data structure
• C/C can declare P1 and P2 as bit-fields as
  follows
• Java can NOT declare bit-fields so it must merge
  P1 and P2
• Therefore, Java programmer must perform bit
  shifting and masking operationswhereas the
  C/C compiler does it for us.

then do
then do
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Part 2 A Lego RCX pathfinderin Assembler using
ScriptEd
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PreLab Instructions

• Part 1
• Click Start?Run
• Type \\luis\lego\
• Move workshop folder to your desktop
• Close \\luis\lego\ folder
• Part 2
• Open workshop folder in your desktop
• Install LEGOMindstormsSDK25.exe
• Install bricxcc_setup_3377.exe (if not
  installed)
• Download Lego RCX Firmware (if not installed)

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Open the ScriptEd application

• Click Start?LEGO SoftwareAll Programs?MindStorm
  s SDK?Tools?ScriptEd
• On the ScriptEd application,Click Open?Port
• Select your Tower Port (COM1, or USB1)
• On the toolbar select LASM (2nd combo box)

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Download pathfind.asm file to RCX

• Part 1
• On the ScriptEd application,Click File?Open
• Browse to workshop folder
• In File name, type .asm (instead of .rcx2)
• Select pathfind.asm and open it
• Part 2
• Click on Script?Download
• Test your RCX

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Part 3 A Lego RCX pathfinderin NQC using BricxCC
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Open the BricxCC application

• Click Start?All Programs?Bricx Command
  Center?Bricx Command Center
• On the Searching for the Brick window,Select
  your Tower Port (COM1, or USB1)

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Compile Download pathfind.nqc

• Part 1
• On the BricxCC application,Click File?Open
• Browse to workshop folder
• Select pathfind.nqc and open it
• Part 2
• Click on Compile?Download
• Test your RCX

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References

• The Bricx Command Center (BricxCC)http//bricxcc.
  sourceforge.net/
• The Lego Mindstorms SDK 2.5 (ScriptEd)http//mind
  storms.lego.com/eng/community/resources/