50.002 Computation Structures
Information Systems Technology and Design
Singapore University of Technology and Design

Lab 5: Assembly Language

Starter Code

The following files inside your /50002/ folder are what you’re going to use for this lab:

lab5_submit.uasm

Please submit all *_submit.jsim files by the due date to our Telegram bot (see course calendar), and do the lab questionnaire in eDimension as usual.

The lecture notes on Stack and Procedures is closely related for this lab. Related sections include:

Background on procedure linkage and stack:
- Managing resources for each procedure
- The Stack implementation (BP, SP convention)
- Handling arguments and return value in a procedure (function)
Implementation of Procedure Linkage Contract:
- Converting C code into Beta assembly with procedure contract
- In particular, to implement callee entry sequence and exit sequence

By the end of this lab, you should have a deeper understanding in how compilers work in transforming functions in high-level languages into procedures in assembly, and linking them together during runtime.

Introduction

In this lab you will have the opportunity to write your first Beta assembly language program. Your task is to write a scoring subroutine for the game of “Moo”, a numeric version of Mastermind®.

In Moo, you will try to guess the secret 4-digit number.

Each guess is scored with a count of “bulls” and “cows”.
Each “bull” means that one of the digits in the guess matches both the value and position of a digit in the secret number.
Each “cow” is a correctly guessed digit but its position in the guess doesn’t match the position in the secret.
Once a digit in the secret has been used to score a digit in the guess (e.g: counted as bull) it won’t be used in the scoring for other digits in the guess (e.g: wont be double counted as cow).
The count of bulls should be determined BEFORE scoring any cows.

Here are two examples:

Secret word: 1234	
Guess: 1379		Bulls=1, Cows=1
Guess: 4321		Bulls=0, Cows=4
Guess: 1344		Bulls=2, Cows=1
Guess: 1234		Bulls=4, Cows=0 

Secret word: 2270
Guess: 2208     Bulls=2, Cows=1
Guess: 0227     Bulls=1, Cows=3
Guess: 0000     Bulls=1, Cows=0
Guess: 2207     Bulls=2, Cows=2

In addition to this handout, there are some other useful documents that will help you:

BSim documentation. Describes how to use BSim, our Beta simulator with built-in assembler. Includes a brief introduction to the syntax and structure of Beta assembly language programs.
Beta Documentation: A detailed description of each instruction in the Beta instruction set. Also documents our convention for subroutine entry and exit sequences.
Beta ISA Summary of Instruction Formats: A one-page quick reference for Beta instructions.

Making Moo

The expected subroutine (function) should take two arguments:

A secret word: a (32 bits)
A test word: b (32 bits)

The secret and test words contain four 4-bit digits packed into the LOW order 16 bits of the word.

For example, if secret word was 1234, it would be encoded as a = 0x00001234 where “0x” indicates hexadecimal (base 16) notation. If the guess word was 1344, it will be encoded as b = 0x00001344. Even though 4 bits are used to encode each digit (0 to F), the words will only contain the digits 0 through 9 per digit.

It should return an integer encoding the number of bulls and cows as (16*bulls) + cows.

For instance if a = 0x00001234 and b = 0x00001344,
This results in 2 bulls and 1 cow
Hence, the return value is 0x00000021
The last four bits represents the value of cows, and bit 4 to 8 represents the value of bulls.

An Approach in C

You are welcome to compute the score however you would like. In case you would like a head start, here is one approach, written in C:

// Test two MOO words, report Bulls & Cows...
// Each word contains four 4-bit digits, packed into low order.
// Each digit ranges from 0 to 9.
// Returns a word whose two low-order 4-bit digits are Bulls & Cows.

int count_bull_cows(int a, int b) {
  int bulls;                  // number of bulls
  int cows;                   // number of cows
  int i, j, btemp, atry, btry, mask; //temp vars

  // Compute Bulls: check each of the four 4-bit digits in turn
  bulls = 0;
  mask = 0xF;                 // mask chooses which 4-bit digit we check
  for (i = 0; i < 4; i = i + 1) {
    // if the 4-bit digits match, we have a bull
    if ((a & mask) == (b & mask)) {
      bulls = bulls + 1;
      // turn matching 4-bit digits to 0xF so we don't
      // count them again when computing number of cows
      a = a | mask;
      b = b | mask;
    }
    // shift mask to check next 4-bit digit
    mask = mask << 4;
  }

  // Compute Cows: check each non-0xF digit of A against all the
  // non-0xF digits of B to see if we have a match
  cows = 0;
  for (i = 0; i < 4; i = i + 1) {
    atry = a & 0xF;	     // this is the next digit from A 
    a = a >> 4;		     // next time around check the next digit
    if (atry != 0xF) {	     // if this digit wasn’t a bull
      // check the A digit against each of the four B digits
      btemp = b;		     // make a copy of the B digits
      mask = 0xF;            // mask chooses which 4-bit digit we check
      for (j = 0; j < 4; j = j + 1) {
        btry = btemp & 0xF;  // this is the next digit from B
        btemp = btemp >> 4;  // next time around check the next digit
        if (btry == atry) {  // if the digits match, we've found a cow
	    cows = cows + 1;
	    b = b | mask;	     // remember that we matched this B digit	
	    break;		     // move on to next A digit
        }
        mask = mask << 4;
      }
    }
  }

  // encode result and return to caller
  return (bulls << 4) + cows;
}

Testing Moo

The test jig uses our usual convention for subroutine calls:

The two arguments are pushed on the stack in REVERSE order (i.e., the first argument is the last one pushed on the stack) and,
Control is transferred to the beginning of the Moo subroutine, leaving the return address in register LP.
The result (bulls and cows values) should be returned in R0.

Your code should use the following template. It is already given in lab5_submit.uasm. Be sure to include the last TWO lines since they allocate space for the stack used by the test jig when calling your program:

.include beta.uasm
.include lab5checkoff.uasm

count_bull_cows: | your subroutine must have this name
	| standard subroutine entry sequence
	PUSH(LP)	
	PUSH(BP)	
	MOVE(SP,BP)	
	| PUSH all used registers
	PUSH(R1)		| bulls
	PUSH(R2)		| cows
	PUSH(R3)		| i
	PUSH(R4)		| j
	PUSH(R5)		| btemp
	PUSH(R6)		| atry
	PUSH(R7)		| btry
	PUSH(R8)		| mask
	PUSH(R9)		| temp reg
	PUSH(R10)		| temp reg
	PUSH(R11)		| a
	PUSH(R12)		| b
	LD(BP,-12,R11)		| load the arg value of constant a to R11
	LD(BP,-16,R12)		| load the arg value of constant b to R12
	CMOVE(0,R1)		| set initial val of var bulls = 0
	CMOVE(0xF,R8)		| set initial val of var mask = 0xF
	CMOVE(0,R3)		| set initial val of var i = 0
  	CMOVE(0,R4)   | set initial val of var j = 0
	
	|||||||||||||||||||||||||||||||||||||||||||||||
	|||| … your code here, leave score (return value) in R0 …
	|||||||||||||||||||||||||||||||||||||||||||||||

	| … POP saved registers above in reverse order…
	MOVE(BP,SP)
	POP(BP)
	POP(LP)
	RTN()


StackBase: 
	LONG(.+4)		| Pointer to the bottom of stack
	.=.+0x1000		| Reserve space for stack

You are recommended to use the registers above for each variables.

Using BSim, assemble your subroutine using the assemble tool.

If the assembly completes without errors, BSim will bring up the display window and you can execute the test jig (which will call your subroutine) using the run or single-step tools:

The test jig will try 32 different test values and type out any error messages on the tty console at the bottom of the display window. Successful execution will result in the following printout at the BSim console:

Unlike in JSim, DO NOT press the green tick. The success checkoff message is sufficient to know that your program works fine for submission. If there exist an error, such message will be printed out:

The error message will give you a clue about your bug. The example above shows that the output should be 4 bulls and 0 cows, but your code computes 0 bulls instead. You can trace your code by adding .breakpoint along your computation of bulls so that the execution will pause at that line. For instance, adding such .breakpoint will pause the beta execution there. You can then inspect each register content and stack content slowly by running each instructions thereafter step by step:

	SHLC(R1,4,R1)		|bulls = bulls << 4
	.breakpoint
	ADD(R1,R2,R0)		|bulls + cow = R0

Implementation and Debug Notes

`.breakpoint`

If you want to examine the execution state of the Beta at a particular point in your program, insert the assembly directive .breakpoint at the point where you want the simulation to halt. You can restart your program by clicking the run button, or you can click single-step button to step through your program instruction-by-instruction.

You can insert as many .breakpoints in your program as you would like. This works the same way as breakpoints in regular debuggers.

Usage of Registers

If your subroutine uses registers other than R0, remember that they have to be restored to their ORIGINAL values before returning to the caller. The usual technique is to PUSH their value onto the stack right after the instructions of the entry sequence and POP those values back into the registers in the REVERSE order just before starting the exit sequence.

Allocate a register to hold each of the variables in the C code.

For example, load a into R1, b into R2,
Assign current bulls count to R3, etc
Reserve R15 to R20 for temporary variables like i and j to track loops.

You will eliminate a lot of LDs and STs by keeping your variables in registers instead of in memory locations on the stack. There are more than enough registers to hold all variables in Moo.

Loading arguments `a` and `b`

Assuming you have used the subroutine entry sequence shown above, the FIRST argument can be loaded into a register using the instruction LD(BP,-12,Rx). Similarly the SECOND argument can be loaded using LD(BP,-16,Ry), where Rx and Ry are two arbitrary registers that you can choose to hold the initial a and b values.

Beta Macros

The instruction macro CMOVE(constant,Rx) is useful for loading small numeric constants into a register. For example, assuming that the variable mask has been assigned to R11, the C statement mask = 0xF can be implemented in a single instruction: CMOVE(0xF,R11).

If Statements

The CMP instructions and BEQ/BNE are useful for compiling C “if” statements. For example, assuming atry has been assigned to R7, the C fragment:

if (atry != 0xF) { 
    ... statements ...
    }

It can be compiled into the following instruction sequence:

      CMPEQC(R7,0xF,R0)     | R0 is 1 if atry==0xF, 0 otherwise
      BNE(R0,endifatry)             | so branch if R0 is not zero
      … statements if the condition is true…

endifatry:                                 | need a unique label for each if
      … statements if the condition is false (outside if-clause)

For-loops

Here is the template for compiling the a for statement. Given the C fragment,

for (inits; tests; afters) { 
    // body
    ... statements ...
    }

Note that the body of the loop is executed as long as the tests are true. The above can be compiled into the following instruction sequence:

      … code for inits …
      BR(endfor32) | go to the test 
for32:
      … code for for-loop statement body …
      … code for afters …
endfor32:
      … code for tests, Rx is 1 if tests are true 
      …
      BNE(Rx,for32) | if tests are true, execute the loop body

C Operators

Here’s a brief summary of C operators:

=	assignment
==	equality test (use CMPEQ, CMPEQC)
!=	inequality test (use CMPEQ, CMPEQC, reverse sense of branch)
<	less than (use CMPLT, CMPLTC)
<<	left shift (use SHL)
>>	right shift (use SRA)
&	bit-wise logical and (use AND, ANDC)
|	bit-wise logical or (use OR, ORC)
+	addition (doh!, use ADD, ADDC)