1 A Simple but Realistic Assembly Language for a Course in Computer Organization Eric Larson Moon Ok Kim Seattle University October 25, 2008

Problem A few years ago we had two computer hardware courses:  Assembly Language Programming  Computer Organization Now we only have one course that combines the two courses. Result: Too much material to cover in one course. 2

Solution Use a simple, yet realistic, assembly language (ANNA) throughout the course. Focus on key concepts of assembly language programming. Does not burden students with the complexities of large instruction sets. ANNA can be used in illustrating the datapath and pipelining units. 3

Outline Background about the course ANNA assembly language and tools How ANNA was used in the course Feedback and Observations 4

Computer Organization Course 5 credit (quarter) sophomore level class  requirement for CS majors Main topics:  Data representation  Digital logic  Assembly language programming  Datapath and control  Memory hierarchy Material from other classes:  OS: virtual memory, some I/O  Discrete math: Boolean algebra Taught using ANNA three times 5

Assembly Language Concepts Key concepts in assembly language in our course: Registers and memory Conditional branches and jumps Translating high-level languages constructs into assembly:  if-else statements  loops  procedure calls  arrays and pointers Most importantly – experience actually writing assembly programs 6

ANNA Assembly Language v1 Memory is word addressable, words are 16 bits. Memory is shared by instructions or data. Data in memory / registers is either an integer or an address.  Can actually store anything – no instruction support for other data types. Load / store architecture 16 general purpose registers, register r0 is 0.  Wise to introduce a calling convention that reserves a register for the stack pointer. 16 instructions (next slide). 7

ANNA Instruction Set v1 8 OpcodeOperandsDescription add Rd Rs 1 Rs 2 Addition sub Rd Rs 1 Rs 2 Subtraction lli Rd Imm8Set lower bits using immediate lui Rd Imm8Set upper bits using immediate and Rd Rs 1 Rs 2 Bitwise and or Rd Rs 1 Rs 2 Bitwise or not Rd Rs 1 Bitwise not shf Rd Rs 1 Rs 2 Bit shift lw Rd Rs 1 Imm4Load word from memory sw Rd Rs 1 Imm4Store word from memory in RdGet a word from user input out RdSend a word to output bez Rd Imm8Branch if equal to zero bgz Rd Imm8Branch if greater than zero jalr Rd Rs 1 Jump and link register halt Halt the program

ANNA Instruction Set v2 Added a new add immediate “ addi ” instruction.  Stack pointer updates  Loop control variables Removed halt : made it a special case of the output instruction ( out r0 ).  Input / output / halt already greatly simplified Reduced the number of registers from 16 to 8.  Restricts register usage (function calls in particular).  Increases immediate sizes in the machine code format. 9

Tools Assembler: translates assembly language files (.ac) into machine language files (.mc). Simulator: loads and runs machine language files.  Debugging support: breakpoints, stepping, etc. First two quarters: used command-line tools on Linux.  Worked fine – students already know Linux and familiar with command line tools. Last quarter: students could use Windows GUI- based tools or the Linux command line tools.  Most, but not all, chose to use the Windows tools. 10

Assembler 11

Simulator 12

Computer Organization: Outline Topic Lecture Hours Data representation4 Digital design10 CPU components3 Assembly programming15 Datapath and pipelining8 Memory and caching5 Input / output2 Review / exams3 13 I did not require the students to purchase a textbook.

Computer Organization: Outline Topic Lecture Hours Data representation4 Digital design10 CPU components3 Assembly programming15 Datapath and pipelining8 Memory and caching5 Input / output2 Review / exams3 14 I did not require the students to purchase a textbook.

Course Information: Lectures Assembly Language  Introduction to assembly  Converting C/C++ into assembly  Comparing ANNA to IA-32 instruction set  Overview of the compilation / linking process Datapath / control  Single cycle datapath (omits I/O instructions)  Control ROM implementation  Pipelining  We no longer cover multiple cycle datapath 15

Course Information: Assignments Assembly Language Programming  Find the highest number entered by a user.  Compute the log 2 of a number.  Find the mode of a set of numbers entered by the user.  Implement linked list functions. Control & Datapath  Add instruction X to the datapath and control ROM.  Create a control ROM for a different datapath / instruction set.  Trace the execution of a program in a pipelined implementation. 16

Students Feedback Most students felt…  they learned a lot when completing the ANNA programming assignments.  the ANNA tools were easy to use.  there was adequate documentation. There were several suggestions for improving the Windows tools:  line numbers in the assembler  knowing what addresses labels map to in the simulator 17

Course Outcomes Students were asked to judge how well a particular topic was covered. Scale: 1(not covered) to 5 (very well covered) Most topics received a 4. No topic received a 2 or lower. 18

My Observations Using the ANNA assembly language helped students learn the important concepts. No differences between the classes that use the Windows tools and Linux tools except for implementing function calls.  Could be due to improvements in lecture ANNA, a simple toy assembly language, did not dampen student motivation. The datapath and design went fairly well.  Some “hand-waving” at times (pun intended) Some students would benefit from having a textbook.  Most textbooks are tied to particular language. 19

Questions? More info? Contact me: Eric Larson 20