The LMC Simulator

Setting and Viewing Mailbox and Input Contents

• Scrolling Displayed Memory - only 10 mailboxes can be displayed at any time. Scroll controls above and below the mailboxes enable viewing of and access to mailboxes in blocks of 10. The "Edit | Edit Instructions" window allows more mailboxes to be viewed or edited in one long list.
• Modifying Memory Contents - clicking on a mailbox, or tabbing to a mailbox from the prior mailbox, and then typing in a new value will change the contents of that mailbox. You cannot enter more than three digits, so the old contents of a mailbox must be deleted first. The "Edit | Edit Instructions" window allows more mailboxes to be viewed or edited in one long list.
• Input Box - You can enter numbers onto the bottom of the list in the Input Box using the Add button or by pushing Enter after typing in each number. Every time the LMC executes an "IN" instruction, the top value in the Input Box list is moved to the Calculator and deleted from the top of the list. The remaining values in the Input Box list shift up, ready for the next "IN" instruction. If the LMC encounters an "IN" instruction with an empty Input Box list, the LMC stops and you must enter some value into the Input Box list and click "Go" to continue.

Run Controls

• Single Step - this control permits you to watch the LMC activity one instruction at a time. Note that execution of the HLT "halt" instruction by the little man has no effect if you are using single step mode - the LMC will move to execute the location after the HLT if you keep single-stepping.
• Go - this control permits you to run the LMC in a continuous fetch/increment/execute mode until a HLT "halt" occurs either because the "little man" performed a HLT "halt" instruction or because you hit the "Halt" control.
• Halt - the intent of this control is to enable you to interrupt a "run-away" or "infinite loop" program. Some infinite loops are hard to interrupt and you may have to bring up the Windows task manager to kill the runaway simulator!
• Reset - this is not technically a "run control" since it does not cause the "little man" to do anything immediately; this control simply sets the program counter to 00. You usually need to do this before you re-run a program.

Saving and Loading LMC Programs

• File menu - Submenus: New, Load, Save, Save As, and Exit
• File | New - Clears LMC memory. Does not affect the Input or Output lists or the Counter.
• File | Load - Opens a dialog window to specify the name of an existing file of machine code to load into the LMC memory. Does not affect the Input or Output lists or the Counter. Remember to Reset the Counter before you execute your loaded program!
• File | Save - Copies the current contents of all mailboxes back out to the file you just loaded.
• File | Save As - Opens a dialog window to specify the new name of a file to contain the LMC memory (with default extension ".lmc"), and then copies the contents of all mailboxes to this file. You may save in "old" format, one 300-digit record with no line breaks, or in the "new" format of 100 separate lines of 3-digit numbers.
• File | Exit - Exit the LMC. You are cautioned to save your mailboxes before you exit.

Editing LMC Programs in the Simulator

• Edit | Edit Instructions - Allows you to edit any mailbox in a long list.
• Edit | Edit Report - Allows you to edit a partial listing of your program that has an Assignment Label and the mnemonics for the opcodes already entered. You have to fill in the labels, operands, and comments.
• Edit | Terminal - Allows you to see the LMC output queue in another format.

Credits

The LMC simulator used is the Clone of Son of Little Man Computer simulator (re-)written by DAT2343 student Mark Aleksandrov in 2001. The simulator contains its own Help screens. It uses the "extended" LMC instruction set that includes SKNZ and CALL.