Slide 4.0: Programming Exercise I: Showing a snakes game

Slide 3.14: SPEC power benchmark
Slide 4.1: Programming exercise guidelines
Home Print version

Programming Exercise I: Showing a Snakes Game

Absolutely no copying others’ works
(Unlike other languages, using others’ assembly code is obvious.)
Development Requirements
When start developing the exercise, follow the two requirements below:

Have to use the MIPS (Microprocessor without Interlocked Pipeline Stages) assembly language to develop the exercise.
Use the MARS (MIPS Assembler and Runtime Simulator) to test your exercise. The MARS will also be used by the instructor to grade exercises.

Soft Due Date^† and Submission Method
On or before Thursday, February 19, 2026 and upload the source code (no documentation needed) to the section of “COVID-19 Exams, Homeworks, & Programming Exercises” of Blackboard.

^†Since related topics may not be covered completely by the due date, no penalty will be applied if submitted after the due date. However, you may lag behind if you are not able to submit it by then. In addition, the Exam I will cover the materials from the Programming Exercise I.

Objective
Design and implement a MIPS assembly program, which displays a 6×6 snakes game of one player. The final program is less than 200 lines. The purpose of this exercise is to get students ready for assembly programming and the second exercise, playing a snakes game.

The first player, Blue (or top), can not move, and so Red (or bottom) wins.

Rules of a Snakes Game
It is a board game for two players who take turns in drawing segments of a snake. The last player able to move wins. The game is played on a grid; 6×6 is a good size. The above game is played on a matrix of 5×5 dots. The first player, Blue (top), starts in the second row and column, and the other player, Red (bottom), starts in the last but one row and column, as shown by the blue and red squares. The players take turns in growing a snake, extending it a segment at a time by drawing a horizontal or vertical line from the previous dot to an adjacent dot. Each player must avoid touching either his/her opponent’s snake or his/her own snake. The first player unable to move loses.

Requirements
The 6×6 snakes game includes the following requirements:

(Picking a piece: 10%) Ask the user to pick a piece (X or O) to start the game.

(Taking turns: 15%) The players (X and O) take turns making moves.

(Next move: 10%) Ask for the next move [/0-z].

(New game if the next move is ‘/’: 15%) Start a new game after asking the user to pick y or n.

(Marking the move if the next move is [0-z]: 20%) For example, if the first move of the player X is k, the boards are displayed as follows:

      . . . . . .     ⇒     0 1 2 3 4 5
      . . . . . .     ⇒     6 7 8 9 a b
      . . . . . .     ⇒     c d e f g h
      . . . . . .     ⇒     i j k l m n
      . . . . . .     ⇒     o p q r s t
      . . . . . .     ⇒     u v w x y z

                    ⇓ X at k

      . . . . . .     ⇒     0 1 2 3 4 5
      . . . . . .     ⇒     6 7 8 9 a b
      . . . . . .     ⇒     c d e f g h
      . . X . . .     ⇒     i j k l m n
      . . . . . .     ⇒     o p q r s t
      . . . . . .     ⇒     u v w x y z

(Changing the markers after marking the move: 30%) For example, if the next move of the player X is g, the boards are displayed as follows:

      . . . . . .     ⇒     0 1 2 3 4 5
      . . . . . .     ⇒     6 7 8 9 a b
      . $ $ X . .     ⇒     c d e f g h
      . . O . . .     ⇒     i j k l m n
      . . + . . .     ⇒     o p q r s t
      . . + . . .     ⇒     u v w x y z

                    ⇓ X at g

      . . . . . .     ⇒     0 1 2 3 4 5
      . . . . . .     ⇒     6 7 8 9 a b
      . $ $ $ X .     ⇒     c d e f g h
      . . O . . .     ⇒     i j k l m n
      . . + . . .     ⇒     o p q r s t
      . . + . . .     ⇒     u v w x y z

Programming Hints
The four essential components of software are (i) algorithms, (ii) data structures, (iii) programming languages, and (iv) code, where algorithms are the most critical one. In addition, using appropriate data structures could save a great deal of coding work, especially for assembly coding. The following hints are from the instructor and you do not necessarily have to use them:

To represent the game board, a 6×6 grid:

      . . . . . .     ⇒     0 1 2 3 4 5
      . . . . . .     ⇒     6 7 8 9 a b
      . . . . . .     ⇒     c d e f g h
      . . . . . .     ⇒     i j k l m n
      . . . . . .     ⇒     o p q r s t
      . . . . . .     ⇒     u v w x y z

the instructor uses the following string:

     board:  .ascii   "\n\n    . . . . . .      0 1 2 3 4 5"
             .ascii     "\n    . . . . . .      6 7 8 9 a b"
             .ascii     "\n    . . . . . .      c d e f g h"
             .ascii     "\n    . . . . . .      i j k l m n"
             .ascii     "\n    . . . . . .      o p q r s t"
             .asciiz    "\n    . . . . . .      u v w x y z\n"

Also, the instructor uses the following command to represent the offsets:

     offset: .half     6,   8,  10,  12,  14,  16  
             .half    39,  41,  43,  45,  47,  49
             .half    72,  74,  76,  78,  80,  82
             .half   105, 107, 109, 111, 113, 115
             .half   138, 140, 142, 144, 146, 148
             .half   171, 173, 175, 177, 179, 181

where

the numbers are offsets (not assembly code),
the new line, ‘\n’, is one byte long, and
each offset uses 2 bytes because the range of 1 byte, [-128 – 127], is not enough for a number ≥ 128.

Convert the character indexes 0–9a–z to integer indexes 0–35, respectively. Assuming the converted index is in $t0, the following code then puts the piece X at that location:

     mul  $t0, $t0, 2        # Each offset is two-byte long.
     lh   $t1, offset($t0)   # Load $t1 with the offset of the index $t0.
     li   $t2, 'X'           # Put the piece ‘X’ in $t2.
     sb   $t2, board($t1)    # Put the piece at the location, board+offset.

Using subroutines can reduce repeated code. An example of how to call the subroutine routine1 is given as follows:
```
          jal   routine1      # Jump and link to routine1
   next:  
          ...
```
where the label next is not required. The following command in the subroutine routine1 will return the control back to the next, the next command after the “jal routine1”:
```
   routine1:
          ...
          jr    $ra           # Jump to the contents of $ra
```
where the return address is saved in the register $31 or $ra

When a subroutine calls another subroutine, the return address, $31 or $ra, has to be saved before making the call. The return address is restored after the called subroutine returns. The instructor uses the runtime stack to save the return addresses, so deep subroutine calls are supported. Use the following two commands to push the $ra to the runtime stack:
```
       subu  $sp, $sp, 4      # Decrement the $sp to make space for $ra.
       sw    $ra, ($sp)       # Push the return address, $ra.
```
and use the following two commands to pop up the $ra from the runtime stack:
```
       lw    $ra, ($sp)       # Pop the return address, $ra.
       addu  $sp, $sp, 4      # Increment the $sp.
```
You may also use the runtime stack to save the register and variable values.

Sample code of marking a game board is given below for your reference:

MarkBoard.asm

# 
######################### Marking the Game Board #########################
#

        .data
pr1:    .asciiz  "\nStart Marking a Snakes Game Board."
pr2:    .asciiz  "\nEnter X's next move [/0..z]: "
pr3:    .asciiz  "\nEnter O's next move [/0..z]: "

player: .byte    'X'       # current player
mov:    .byte    ' '       # current move
last_X: .byte    ' '       # last move of X
last_O: .byte    ' '       # last move of O

board:  .ascii   "\n\n    . . . . . .      0 1 2 3 4 5"
        .ascii     "\n    . . . . . .      6 7 8 9 a b"
        .ascii     "\n    . . . . . .      c d e f g h"
        .ascii     "\n    . . . . . .      i j k l m n"
        .ascii     "\n    . . . . . .      o p q r s t"
        .asciiz    "\n    . . . . . .      u v w x y z\n"

offset: .half     6,   8,  10,  12,  14,  16
        .half    39,  41,  43,  45,  47,  49
        .half    72,  74,  76,  78,  80,  82
        .half   105, 107, 109, 111, 113, 115
        .half   138, 140, 142, 144, 146, 148
        .half   171, 173, 175, 177, 179, 181

        .text
        .globl main
#
####################### Main Program #######################
#
main:
      la    $a0, pr1          # Greeting
      li    $v0, 4
      syscall
      la    $a0, board        # Game board
      li    $v0, 4 
      syscall

L0:   lb    $v0, player       # $v0 = current player
      beq   $v0, 'O', L1
      la    $a0, pr2          # X's next move
      li    $v0, 4
      syscall
      li    $t0, 'O'          # $t0 = 'O'
      j     L2
      
L1:   la    $a0, pr3          # Y's next move
      li    $v0, 4
      syscall
      li    $t0, 'X'          # $t0 = 'X'
      
L2:   li    $v0, 12
      syscall
      sb    $v0, mov          # mov = $v0, next move
      beq   $v0, '/', L3
      move  $a0, $v0
      jal   MarkBoard         # Mark the move.
      sb    $t0, player
      j     L0
      
L3:   li    $v0, 10           # End of program
      syscall

#
####################### Mark the board. #######################
#
# Input:  $a0 (the char move [0-z])  
# Output: the updated board

MarkBoard:
      subu  $sp, $sp, 4
      sw    $ra, ($sp)         # Push the return address, $ra.
      subu  $sp, $sp, 4        # Decrement the $sp to make space for $t0.
      sw    $t0, ($sp)         # Push the $t0.
      
      # Mark the board.
      jal   FindIndex
      lh    $t1, offset($t0)
      lb    $t2, player        # $t2 = current player
      sb    $t2, board($t1)

      # Change the marker.
      beq   $t2, 'O', L21
      lb    $t0, last_X
      sb    $a0, last_X
      li    $t2, '$'
      j     L22
      
L21:  lb    $t0, last_O
      sb    $a0, last_O
      li    $t2, '+'

L22:  move  $a0, $t0
      jal   FindIndex
      lh    $t1, offset($t0)
      sb    $t2, board($t1)
      la    $a0, board
      li    $v0, 4
      syscall
  
      lw    $t0, ($sp)           # Pop $t0.
      addu  $sp, $sp, 4          # Increment the $sp.
      lw    $ra, ($sp)           # Pop the return address, $ra.
      addu  $sp, $sp, 4
      jr    $ra
          
#
####################### Find the Index of Offset. #######################
#
# Input:  $a0 (the char move [0-z])  
# Output: $t0 (the integer index)

FindIndex:
      subu  $sp, $sp, 4
      sw    $ra, ($sp)           # Push the return address, $ra.
      subu  $sp, $sp, 4          # Decrement the $sp to make space for $a0.
      sw    $a0, ($sp)           # Push the $a0.
      
      bgt   $a0, '9', L31
      sub   $a0, $a0, '0'
      j     L32
L31:  sub   $a0, $a0, 'a'
      add   $a0, $a0, 10
L32:  mul   $t0, $a0, 2
      
      lw    $a0, ($sp)           # Pop $a0.
      addu  $sp, $sp, 4          # Increment the $sp.
      lw    $ra, ($sp)           # Pop the return address, $ra.
      addu  $sp, $sp, 4
      jr    $ra                  # Return.

An Example of MarkBoard.asm Execution

Start Marking a Snakes Game Board.

    . . . . . .      0 1 2 3 4 5
    . . . . . .      6 7 8 9 a b
    . . . . . .      c d e f g h
    . . . . . .      i j k l m n
    . . . . . .      o p q r s t
    . . . . . .      u v w x y z

Enter X's next move [/0..z]:  8 

    . . . . . .      0 1 2 3 4 5
    . . X . . .      6 7 8 9 a b
    . . . . . .      c d e f g h
    . . . . . .      i j k l m n
    . . . . . .      o p q r s t
    . . . . . .      u v w x y z

Enter O's next move [/0..z]:  r 

    . . . . . .      0 1 2 3 4 5
    . . X . . .      6 7 8 9 a b
    . . . . . .      c d e f g h
    . . . . . .      i j k l m n
    . . . O . .      o p q r s t
    . . . . . .      u v w x y z

Enter X's next move [/0..z]:  9 

    . . . . . .      0 1 2 3 4 5
    . . $ X . .      6 7 8 9 a b
    . . . . . .      c d e f g h
    . . . . . .      i j k l m n
    . . . O . .      o p q r s t
    . . . . . .      u v w x y z

Enter O's next move [/0..z]:  q 

    . . . . . .      0 1 2 3 4 5
    . . $ X . .      6 7 8 9 a b
    . . . . . .      c d e f g h
    . . . . . .      i j k l m n
    . . O + . .      o p q r s t
    . . . . . .      u v w x y z

Enter X's next move [/0..z]:  a 

    . . . . . .      0 1 2 3 4 5
    . . $ $ X .      6 7 8 9 a b
    . . . . . .      c d e f g h
    . . . . . .      i j k l m n
    . . O + . .      o p q r s t
    . . . . . .      u v w x y z

Enter O's next move [/0..z]:  k 

    . . . . . .      0 1 2 3 4 5
    . . $ $ X .      6 7 8 9 a b
    . . . . . .      c d e f g h
    . . O . . .      i j k l m n
    . . + + . .      o p q r s t
    . . . . . .      u v w x y z

Enter X's next move [/0..z]:  / 

-- program is finished running --

^†The italic, white text with a navy background color is entered by users.

^‡Notice the subroutine is not necessary. It is to show how to use subroutines and runtime stack.

Execution Examples
The following list shows some execution examples:

Examples of Exercise I Execution

Start Showing a Snakes Game.

    . . . . . .      0 1 2 3 4 5
    . . . . . .      6 7 8 9 a b
    . . . . . .      c d e f g h
    . . . . . .      i j k l m n
    . . . . . .      o p q r s t
    . . . . . .      u v w x y z

Pick a piece (X|O):  X 
Enter X's next move [/0..z]: 8

    . . . . . .      0 1 2 3 4 5
    . . X . . .      6 7 8 9 a b
    . . . . . .      c d e f g h
    . . . . . .      i j k l m n
    . . . . . .      o p q r s t
    . . . . . .      u v w x y z

Enter O's next move [/0..z]:  v 

    . . . . . .      0 1 2 3 4 5
    . . X . . .      6 7 8 9 a b
    . . . . . .      c d e f g h
    . . . . . .      i j k l m n
    . . . . . .      o p q r s t
    . O . . . .      u v w x y z

Enter X's next move [/0..z]:  / 
New game (y/n)?  y 

Start Showing a Snakes Game.

    . . . . . .      0 1 2 3 4 5
    . . . . . .      6 7 8 9 a b
    . . . . . .      c d e f g h
    . . . . . .      i j k l m n
    . . . . . .      o p q r s t
    . . . . . .      u v w x y z

Pick a piece (X|O):  O 
Enter O's next move [/0..z]:  p 

    . . . . . .      0 1 2 3 4 5
    . . . . . .      6 7 8 9 a b
    . . . . . .      c d e f g h
    . . . . . .      i j k l m n
    . O . . . .      o p q r s t
    . . . . . .      u v w x y z

Enter X's next move [/0..z]:  a 

    . . . . . .      0 1 2 3 4 5
    . . . . X .      6 7 8 9 a b
    . . . . . .      c d e f g h
    . . . . . .      i j k l m n
    . O . . . .      o p q r s t
    . . . . . .      u v w x y z

Enter O's next move [/0..z]:  j 

    . . . . . .      0 1 2 3 4 5
    . . . . X .      6 7 8 9 a b
    . . . . . .      c d e f g h
    . O . . . .      i j k l m n
    . + . . . .      o p q r s t
    . . . . . .      u v w x y z

Enter X's next move [/0..z]:  g 

    . . . . . .      0 1 2 3 4 5
    . . . . $ .      6 7 8 9 a b
    . . . . X .      c d e f g h
    . O . . . .      i j k l m n
    . + . . . .      o p q r s t
    . . . . . .      u v w x y z

Enter O's next move [/0..z]:  k 

    . . . . . .      0 1 2 3 4 5
    . . . . $ .      6 7 8 9 a b
    . . . . X .      c d e f g h
    . + O . . .      i j k l m n
    . + . . . .      o p q r s t
    . . . . . .      u v w x y z

Enter X's next move [/0..z]:  f 

    . . . . . .      0 1 2 3 4 5
    . . . . $ .      6 7 8 9 a b
    . . . X $ .      c d e f g h
    . + O . . .      i j k l m n
    . + . . . .      o p q r s t
    . . . . . .      u v w x y z

Enter O's next move [/0..z]:  l 

    . . . . . .      0 1 2 3 4 5
    . . . . $ .      6 7 8 9 a b
    . . . X $ .      c d e f g h
    . + + O . .      i j k l m n
    . + . . . .      o p q r s t
    . . . . . .      u v w x y z

Enter X's next move [/0..z]:  e 

    . . . . . .      0 1 2 3 4 5
    . . . . $ .      6 7 8 9 a b
    . . X $ $ .      c d e f g h
    . + + O . .      i j k l m n
    . + . . . .      o p q r s t
    . . . . . .      u v w x y z

Enter O's next move [/0..z]:  / 
New game (y/n)?  n 
       
-- program is finished running --

Possible Instructions to Be Used
The following directives and instructions may be used in this exercise, but you are not limited to them. For instruction syntax, check MIPS Instruction Reference.

No.	Directive	Description
1	`.ascii "string"`	Allocating space for string
2	`.asciiz "string"`	Allocating space for string, NULL terminated
3	`.byte`	Allocating space for a byte
4	`.data`	Beginning of data section
5	`.globl name`	Making the following name be a global symbol
6	`.half`	Allocating space for a half word (two bytes)
7	`.space n`	Allocating `n` bytes of space
8	`.text`	Beginning of text section
No.	Instruction	Operation	Description
1	`add rd, rs, rt`	`rd = rs + rt`	Add; `rd`: destination register, `rs`: first source register, and `rt`: second source register or immediate value. Check MIPS Registers and Usage Convention.
2	`addu rd, rs, rt`	`rd = rs + rt` (no overflow)	Add unsigned
3	`beq rs, rt, label`	`if rs==rt then goto label`	Branch if equal to
4	`bgt rs, rt, label`	`if rs>rt then goto label`	Branch if greater than
5	`blt rs, rt, label`	`if rs<rt then goto label`	Branch if less than
6	`bne rs, rt, label`	`if rs≠rt then goto label`	Branch if not equal to
7	`j label`	`jump to label`	Jump
8	`jal label`	`jump to label` and save the return address in `$31` or `$ra`	Jump and link
9	`jr rs`	`jump to [rs]`; [ ]: contents of	Jump and link
10	`la rd, mem`	`rd = address( mem )`	Load address
11	`lb rd, mem`	`rd = mem`	Load byte
12	`lh rd, mem`	`rd = mem`	Load half word
13	`li rd, imm`	`rd = imm`	Load immediate
14	`lw rd, mem`	`rd = mem`	Load word
15	`mul rd, rs, rt`	`rd = rs × rt`	Multiply
16	`sb rs, mem`	`mem = rs`	Store byte
17	`sub rd, rs, rt`	`rd = rs - rt`	Subtract
18	`subu rd, rs, rt`	`rd = rs - rt` (no overflow)	Subtract unsigned
19	`sw rs, mem`	`mem = rs`	Store word
20	`syscall`		System call; check System Services.

The following table lists some System Services provided by the MARS:

Service	Code in `$v0`	Arguments	Result
print_int	1	`$a0` = integer to be printed
print_float	2	`$f12` = float to be printed
print_double	3	`$f12` = double to be printed
print_string	4	`$a0` = address of string in memory
read_int	5		integer returned in `$v0`
read_float	6		float returned in `$v0`
read_double	7		double returned in `$v0`
read_string	8	`$a0` = address of string input buffer `$a1` = length of string buffer (`n`)
malloc	9	`$a0` = amount	address in `$v0`
exit	10
print character	11	`$a0` = character to be printed
read character	12		character returned in `$v0`

Evaluations
The following features will be considered when grading:

Specifications:
- The instructor has given the exercise specifications as detailedly as possible. If you are confused about the specifications, you should ask in advance. Study the specifications very carefully. No excuses for misunderstanding or missing parts of the specifications after grading.
- The specifications may not be possible to cover every detail. You are free to implement the issues not mentioned in the specifications, but the implementations should make sense. Implemented functions lacking common sense may cause the instructor to grade your exercise mistakenly, and thus lower your grade.
- The exercise must meet the specifications. However, exercises with functions exceeding the specifications will not receive extra credits.

Grading:
- This exercise will be graded after the source code is submitted. Students take full responsibility if the code does not work.
- The MARS simulator will be used to grade exercises. Make sure your exercise works on MARS.
- Before submitting the exercise, test it comprehensively. Absolutely no extra points will be granted after grading.
- A set of test data will be used to test the exercises of all students. The grades are primarily based on the results of testing. Other factors such as performance, programming styles, algorithms, and data structures will be only considered minimally.
- Unless specified, no error checking is required.
- The total weight of exercises is 20% (10% each) of the final grade.
- Though Exercise II covers parts of Exercise I, you still have to submit both exercises separately. One exercise will not be counted twice.
- The instructor will inform you the exercise evaluations by emails or Blackboard after grading.

Comments:
- According to a study, students in computer-science courses learn much more by building large-scale exercises instead of many small-scale test programs, which give fragmented knowledge contrary to solid understanding of the language.
- Time management is critical for software development. If you are not able to complete the exercises, show whatever you have accomplished, so the instructor can give partial credit to your exercises.

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“We used to play spin the bottle when I was a kid,”
says comedy writer Gene Perret.
“A girl would spin the bottle pointed to you when it stopped,
the girl could either kiss you or give you a nickel.
By the time I was 14, I owned my own house.”