Description
A.Why? • Implementing a computer really helps you understand its structure. B.Outcomes After this project, you should • Know how to implement the major parts of a computer in a higher-level language. • Know how to access structures via pointers in C. C.Project • You are to implement of a simple simulator for the LC-3 in C. As in the simulator you did for the Simple Decimal Computer, you will be given a skeleton of the program; your job is to complete the implementation. • Once again, the user interface will be console-oriented. Instead of typing in the program, however, the user will be able to read the program to be executed. • This is an individual project. • Your program should: • Prompt for and read the name of an input file and then read the input file to initialize memory (see below). • Run a command loop to execute the program or dump registers/memory. D.Input File • You should prompt the user for a file name (complain if the file doesn’t exist). Use input.hex as the default filename. (If you can’t get the prompting to work correctly, at least read from input.hex.) • The file should be a *.hex file (the kind produced by the LC-3 editor on assembling a *.asm file). A *.hex file is a sequence of lines with a four-digit hex number on each line (no leading x or 0x). The first line specifies the .ORIG to load the program to. The remaining lines comprise the program. E.g., Assembler File Hex file ORIG x3000 3000 LD R1, X 2203 ADD R1, R1, 1 1261 ST R1, X 3201 HALT F025 X .FILL 17 0011 .END • The rest of memory should be initialized to zeros except for the last location, which should be initialized to a HALT instruction. (The textbook’s LC3 simulator causes an error if you execute that location.) E.The Command Loop • Once the program is read into memory, initialize the program counter (to the .ORIG value), set the IR to all 0’s, the condition code to Z, and start a command loop. There are five commands. The user should enter a carriage return after each command (so h <cr, for example). • h for help: Print a summary of the simulator commands. • d for dump control unit: Print out the program counter, instruction register, condition code, and data registers. • m n₁ n₂ for memory: Print out the values of memory at locations n₁, n₁+1, …, n₂. (If n₁ n₂, then no values get printed out.) The values n₁ and n₂ should be in hex with a leading x. Don’t print out the value of a location if the value is zero. If n₁ or n₂ is an illegal address, print an error message and skip the memory dump.
• An integer: Run that number of instruction cycles (but stop early if you execute a HALT). If the machine is already halted, say so but don’t run any instruction cycles. • No input, just • For LD reg, offset, you say which register you’re loading, the offset value, the PC + offset address, and the value at M[PC + offset] you’re copying to the register. • The output contains all the same information as the sample solution’s output. Minor differences like spacing and case and what order you print things on a line aren’t relevant. • The code should be commented, readable, with good-sized routines (don’t have just a main program hundreds of lines long; each routine should do one conceptual thing). • For a grade of “B” • The program compiles to an executable, though there may be warnings. • The simulator commands h, r, m, <integer, and q should work, though the m command might not do everything it should (check for bad addresses, avoid printing zeros, etc). • The simulator has words of the right length, but it may get confused about overflow and truncation from int to short int etc. • The simulator always reads its hex input from the file input.hex (it doesn’t prompt for a name, or it handles the name incorrectly and reads from input.hex instead). • 75% of the opcodes should be implemented. (This can range from 75% of the opcodes completely working to all the opcodes 75% working.) Each TRAP counts as a different opcode. • The code is mostly commented, mostly readable, and there may be a bit of imbalance in routine sizes (but again, not just a main program hundreds of lines long). • For a grade of “C” • As for a grade of “B” but only 33% – 74% of the opcodes are implemented. • The m command doesn’t support (or ignores) the memory limit values. (But m
