Assembly Code and Registers

• Register 16 starts with a value of 5

• Register 17 starts with a value of 1

• Because 5 is not equal to 0, Register 17 is set to 1 * 5 = 5 and Register 16 becomes 5 - 1 = 4

• Because 4 is not equal to 0, Register 17 is set to 5 * 4 = 20 (14 in hex), Register 16 becomes 4 - 1 = 3

• Because 3 is not equal to 0, Register 17 is set to 20 * 3 = 60 (3C in hex), Register 16 becomes 3 - 1 = 2

• Because 2 is not equal to 0, Register 17 is set to 60 * 2 = 120 (78 in hex), Register 16 becomes 2 - 1 = 1

• Because 1 is not equal to 0, Register 17 is set to 120 * 1 = 120 (78 in hex), Register 16 becomes 1 - 1 = 0

• Because 0 is equal to 0, the code goes to the "done" label

• Register 17 is set to 120 * 1 = 120 (78 in hex), Register 16 becomes 1 - 1 = 0

• The instruction sw $17, 0x1234($0) is executed

• The program counter gives the current address being read

• Registers are not reset because there is no explicit instruction to clear the data

• The final value of Register 17 is 78 in hex, which is 120 in decimal

• The assembly language represents a loop that multiplies two numbers and decrements one until that number reaches zero

Simulating Assembly in MIPS

• In a MIPS simulator, assembly language can be explored using a pre-written program to determine what the code does, and then writing the code using a high-level language (or pseudocode)
• The UNC miniMIPS Architecture simulator is a web-based tool for simulating MIPS, and can be found at http://www.csbio.unc.edu/mcmillan/miniMIPS.html

Steps for using the simulator

• Enter assembly code, noting that: the program uses the "mul" instruction (an R-type instruction), words followed by colons are labels used to reference memory locations, and instruction names should be aligned with instruction operands
• Assemble the program using the "Assemble" button, and watch for any error notifications
• Run the program using the "Step" button (recommended), the "Multistep" button, or setting a breakpoint with an asterisk (*) and using the "Run" button
• Observe that assembly instructions are translated to machine language, with memory addresses and hexadecimal codes shown
• The MIPS simulator tracks stored values in registers by their address number, and the PC (program counter) tracks the value stored
• Use the "Memory Dump" button to see the contents of memory by entering the address of memory, observing the final value held by $17 after the program finishes
• The "Reset" button reloads instructions into memory without resetting the contents of memory or the registers

High-Level Translation

• Write a short code snippet in a high-level language ( or pseudocode) which would result in the assembly language program, keeping in mind that registers are the processor's working storage for variables, using comments to show links between variables and the registers
• The final instruction being a sw instruction meaning it's not necessary to include high-level code, but could be added if wished

Lab Questions to Answer

• Include answers to questions posed in steps 6 and 8 of the assembly sim
• What is this assembly program doing, in plain English? Use high-level code if helpful to explain its function
• What are the MIPS names of the registers used in this program?

Grading and Appendix

• The document provides grading information, stating that grading is based on correctness and completeness of the solution
• There is an appendix that shows how to use the instruction set and mini MIPS by trying the given simple code and observing how the register ID change in the simulator